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19-0143; Rev 1; 2/95
NUAL KIT MA ATION EET EVALU ATA SH WS D FOLLO
Digitally Adjustable LCD Bias Supply
_______________General Description
The MAX749 generates negative LCD-bias contrast voltages from 2V to 6V inputs. Full-scale output voltage can be scaled to -100V or greater, and is digitally adjustable in 64 equal steps by an internal digital-toanalog converter (DAC). Only seven small surfacemount components are required to build a complete supply. The output voltage can also be adjusted using a PWM signal or a potentiometer. A unique current-limited control scheme reduces supply current and maximizes efficiency, while a high switching frequency (up to 500kHz) minimizes the size of external components. Quiescent current is only 60A max and is reduced to under 15A in shutdown mode. While shut down, the MAX749 retains the voltage set point, simplifying software control. The MAX749 drives either an external P-channel MOSFET or a PNP transistor.
____________________________Features
o +2.0V to +6.0V Input Voltage Range o Flexible Control of Output Voltage: Digital Control Potentiometer Adjustment PWM Control o Output Voltage Range Set by One Resistor o Low, 60A Max Quiescent Current o 15A Max Shutdown Mode o Small Size - 8-Pin SO and Plastic DIP Packages
MAX749
________________________Applications
Notebook Computers Laptop Computers Palmtop Computers Personal Digital Assistants Communicating Computers Portable Data-Collection Terminals
______________Ordering Information
PART MAX749CPA MAX749CSA MAX749C/D MAX749EPA MAX749ESA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO
* Contact factory for dice specifications.
__________Typical Operating Circuit
VIN +5V
__________________Pin Configuration
TOP VIEW
RSENSE 0.1F 1 V+ CS 8 V+ ADJ -VOUT 1 2 8 7 CS DHI DLOW GND
DIGITAL ADJUST ON/OFF
2 ADJ 7 MAX749 DHI 3 4 CTRL FB DLOW GND 6 5
CTRL 3 FB 4
MAX749
6 5
RFB
DIP/SO
CCOMP
_______________________________________________________________ Maxim Integrated Products
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Digitally Adjustable LCD Bias Supply MAX749
ABSOLUTE MAXIMUM RATINGS
V+ ................................................................................-0.3V, +7V CTRL, ADJ, FB, DLOW, DHI, CS.....................-0.3V, (V+ + 0.3V) Continuous Power Dissipation (TA = +70C) Plastic DIP (derate 9.09mW/C above +70C) ............727mW SO (derate 5.88mW/C above +70C) .........................471mW Operating Temperature Ranges: MAX749C_A........................................................0C to +70C MAX749E_A .....................................................-40C to +85C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(2V < V+ < 6V, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS V+ Voltage 2 6 V FB Source Current IFBS On power-up or reset, VFB = 0V (Note 1) 12.80 13.33 13.86 A Zero-Count FB Current VFB = 0V 0.45 0.55 IFBS Full-Count FB Current VFB = 0V 1.43 1.53 IFBS FB Offset Voltage 15 mV DAC Step Size (Note 2) Monotonicity guaranteed, VFB = 0V 1.00 1.56 2.12 %IFBS DAC Linearity (Note 2) VFB = 0V 1 %IFBS Supply Rejection V+ = 2V to 6V, full-count current 1.5 %IFBS Switching Frequency 100 to 500 kHz Logic Input Current 0V < VIN < V+, CTRL, ADJ 100 nA Logic High Threshold (Note 3) VIH CTRL, ADJ 1.6 V Logic Low Threshold (Note 3) VIL CTRL, ADJ 0.4 V Quiescent Current 60 A Shutdown Current 15 A V+ to CS Voltage Current-limit trip voltage 110 140 180 mV DHI Source Current V+ = 2V, VDHI = 1V 24 50 mA DHI Drive Level No load V+ - 50mV V+ V DLOW On Resistance V+ = 2V, VDLOW = 0.5V 5 10 Note 1: The device is in regulation when VFB = 0V (see Figures 3 - 6). Note 2: These tests performed at V+ = 3.3V. Operation over supply range is guaranteed by supply rejection test of full-count current. Note 3: VIH is guaranteed by design to be 1.8V min for V+ = 2V to 6V for TA = TMIN to TMAX. VIL is guaranteed by design from TA = TMIN to TMAX.
TIMING CHARACTERISTICS
PARAMETER Minimum Reset Pulse Width Minimum Reset Setup Minimum Reset Hold Minimum ADJ High Pulse Width Minimum ADJ Low Pulse Width Minimum ADJ Low to CTRL Low SYMBOL tR tRS tRH tSH tSL tSD CONDITIONS V+ = 2V V+ = 5V Not tested Not tested V+ = 2V V+ = 5V V+ = 2V V+ = 5V V+ = 2V V+ = 5V MIN TA = +25C TYP MAX 125 25 0 0 15 10 170 60 70 20 85 85 400 150 200 85 300 85 0 0 100 100 500 200 250 100 TA = TMIN to TMAX MIN MAX 400 100 UNITS ns ns ns ns ns ns
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Digitally Adjustable LCD Bias Supply
__________________________________________Typical Operating Characteristics
(TA = +25C, L = 47H, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT - PNP
MAX749TOC2-A
MAX749
EFFICIENCY vs. OUTPUT CURRENT - PNP
MAX749TOC2-B
EFFICIENCY vs. OUTPUT CURRENT - MOSFET
-24V -12V 80 EFFICIENCY (%)
MAX749TOC2-C
85
80 78 76 EFFICIENCY (%) 74 72 70 68 66 -24V V+ = 3V RBASE = 160 RSENSE = 0.25 TRANSISTOR = ZTX750 -12V -5V
85
80 EFFICIENCY (%) -24V
-12V -5V
-5V 75 V+ = 5V RSENSE = 0.25 TRANSISTOR: SMD10P05L
75 V+ = 3V RBASE = 470 RSENSE = 0.25 TRANSISTOR: ZTX750
70
70
65 0 10 20 30 40 50 60 OUTPUT CURRENT (mA)
64 0 10 20 30 40 50 60 70 80 90 100 OUTPUT CURRENT (mA)
65 0 5 10 15 20 25 30 35 40 45 50 OUTPUT CURRENT (mA)
EFFICIENCY vs. OUTPUT VOLTAGE
MAX749-TOC1-A
EFFICIENCY vs. OUTPUT VOLTAGE
-20mA -5mA
MAX749-TOC1-B
85 -20mA 80 EFFICIENCY (%) -5mA 75 V+ = 3V RBASE = 470 RSENSE = 0.25 TRANSISTOR : ZTX750 -40mA
85
80 EFFICIENCY (%) -40mA 75
70
70
V+ = 5V RSENSE = 0.25 TRANSISTOR : SMD10P05L
65 -24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 OUTPUT VOLTAGE (V)
65 -24 -22 -20 -18 -16 -14 -12 -10 -8 -6 OUTPUT VOLTAGE (V) -4
LOAD CURRENT vs. INPUT VOLTAGE
MAX749-TOC3-A
LOAD CURRENT vs. INPUT VOLTAGE
450 400 LOAD CURRENT (mA) 350 300 250 200 150 100 50 0 -24V -48V 2 3 4 INPUT VOLTAGE (V) 5 6 -12V RBASE = 160 -5V RSENSE = 0.25 TRANSISTOR = ZTX750
MAX749-TOC3-B
400 350 LOAD CURRENT (mA) 300 250 200 150 100 50 0 2 3 4 INPUT VOLTAGE (V) 5 6 -12V -24V -48V RBASE = 470 RSENSE = 0.25 TRANSISTOR = ZTX750 -5V
500
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Digitally Adjustable LCD Bias Supply MAX749
____________________________Typical Operating Characteristics (continued)
(TA = +25C, L = 47H, unless otherwise noted.)
LOAD-TRANSIENT RESPONSE
LINE-TRANSIENT RESPONSE
OUTPUT VOLTAGE
100mVAC/div
OUTPUT VOLTAGE
100mVAC/div
LOAD CURRENT VOUT = -15V TRANSISTOR = ZTX750 50s/div VOUT = -15V TRANSISTOR = ZTX750
10mA/div 0mA
INPUT VOLTAGE
1 V/div
50ms/div VOUT = -15V ILOAD = 5mA TRANSISTOR = ZTX750
0V
______________________________________________________________Pin Description
PIN 1 NAME V+ FUNCTION +2V to +6V Input Voltage to power the MAX749 and external circuitry. When using an external P-channel MOSFET, V+ must exceed the MOSFET's gate threshold voltage. Logic Input. When CTRL is high, a rising edge on ADJ increments an internal counter. When CTRL is low, the counter is reset to mid-scale when ADJ is high. When ADJ is low, the counter does not change (regardless of activity on CTRL) as long as V+ is applied. Logic Input. When CTRL and ADJ are low, the MAX749 is shut down, but the counter is not reset. When CTRL is low, the counter is reset to mid-scale when ADJ is high. The device is always on when CTRL is high. Feedback Input for output full-scale voltage selection. -VOUT(MAX) = (RFB) x (20A) where RFB is connected from FB to -VOUT. The device is in regulation when VFB = 0V. Ground Output Driver Low. Connect to DHI when using an external P-channel MOSFET. When using an external PNP transistor, connect a resistor RBASE from DLOW to the base of the PNP to set the maximum base-drive current. Output Driver High. Connect to the gate of the external P-channel transistor, or to the base of the external PNP transistor. Current-Sense Input. The external transistor is turned off when current through the sense resistor, RSENSE, brings CS below V+ by 140mV (typ).
2
ADJ
3
CTRL
4 5 6
FB GND DLOW
7 8
DHI CS
4
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Digitally Adjustable LCD Bias Supply MAX749
+2V TO +6V INPUT 0.1F 22F 6.2V
V+ POWER-ON RESET RESET 6-BIT COUNTER 6-BIT CURRENT-OUTPUT DAC REF LOGIC INCREMENT CS DHI DLOW RBASE 470 Q1 ZTX750 D1 1N5819 L1 47H VOUT (NEGATIVE) 22F 30V 6.66A TO 20A RSENSE
CTRL ADJ
ON/OFF
SWITCHMODE POWER SUPPLY
BIAS
RFB FB
MAX749
GND CCOMP
Figure 1. Block Diagram, Showing External Circuitry Using a PNP Transistor
_______________Detailed Description
The MAX749 is a negative-output inverting power controller that can drive an external PNP transistor or Pchannel MOSFET. An external resistor and an internal DAC control the output voltage (Figure 1). The MAX749 is designed to operate from 2V to 6V inputs, ideal for operation from low-voltage batteries. In systems with higher-voltage batteries, such as notebook computers, the MAX749 may also be operated from the regulated +5V supply. A high-efficiency +5V regulator, such as the MAX782, is an ideal source for the MAX749. In this example, the MAX749 efficiency (80%) is compounded with the MAX782 efficiency (95%): 80% x 95% = 76%, which is still high.
Operating Principle
The MAX749 and the external components shown in the Typical Operating Circuit form a flyback converter. When the external transistor is on, current flows through the current-sense resistor, the transistor, and the coil. Energy is stored in the core of the coil during this phase, and the diode does not conduct. When the transistor
turns off, current flows from the output through the diode and the coil, driving the output negative. Feedback control adjusts the external transistor's timing to provide a regulated negative output voltage. The MAX749's unique control scheme combines the ultra-low supply current of pulse-skipping, pulse-frequency modulation (PFM) converters with the high fullload efficiency characteristic of pulse-width modulation (PWM) converters. This control scheme allows the device to achieve high efficiency over a wide range of loads. The current-sense function and high operating frequency allow the use of tiny external components. Switching control is accomplished through the combination of a current limit in the switch plus on- and offtime limits (Figure 2). Once turned on, the transistor stays on until either: - the maximum on-time one-shot turns it off (8s later), or - the switch current reaches its limit (as determined by the current-sense resistor and the current comparator).
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Digitally Adjustable LCD Bias Supply MAX749
+2V TO +6V INPUT 0.1F 22F V+ 140mV Q TRIG MINIMUM OFF-TIME ONE-SHOT FLIP-FLOP S R MAXIMUM ON-TIME ONE-SHOT TRIG Q Q CURRENT COMPARATOR RSENSE
DHI DLOW RBASE 470 VOLTAGE COMPARATOR
Q1 ZTX750
L1 47H
D1 1N5819
FB REF 6-BIT CURRENT-OUTPUT DAC GND
RFB 22F 30V CCOMP
VOUT (NEGATIVE)
MAX749
Figure 2. Switch-Mode Power-Supply Section Block Diagram
Once turned off, a one-shot holds the switch off for a minimum of 1s, and the switch either stays off (if the output is in regulation), or turns on again (if the output is out of regulation). With light loads, the transistor switches for one or more cycles and then turns off, much like a traditional PFM converter. With heavy loads, the transistor stays on until the switch current reaches the current limit; it then shuts off for 1s, and immediately turns on again until the next time the switch current reaches its limit. This cycle repeats until the output is in regulation.
Output Voltage Control
The output voltage is set using a single external resistor and the internal current-output DAC (Figure 1). The fullscale output voltage is set by selecting the feedback resistor, RFB. The output voltage is controlled from 33% to 100% of the full-scale output by an internal 64-step DAC/counter. On power-up or after a reset, the counter sets the DAC output to mid-range. Each rising edge of ADJ incre6
ments the DAC output. When incremented beyond full scale, the counter rolls over and sets the DAC to the minimum value. In this way, a single pulse applied to ADJ increases the DAC set point by one step, and 63 pulses decrease the set point by one step. Table 1 is the logic table for the CTRL and ADJ inputs, which control the internal DAC and counter. Figures 3-7 show various timing specifications and different ways of incrementing and resetting the DAC, and of placing it in the low-power standby mode. As long as the timing specifications for ADJ and CTRL are observed, any sequence of operations can be implemented. Table 1. Input Truth Table
ADJ Low High X CTRL Low Low High High Shut down Reset counter to mid-range. The device is not shut down. On Increment the counter RESULT
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Digitally Adjustable LCD Bias Supply
ADJ CTRL
tR SHUTDOWN RESET ON
tSD SHUTDOWN
In Figure 3, the MAX749 is reset when it is taken out of shutdown, which sets the output at mid-scale. Figure 4 shows how to increment the counter. Figure 5 illustrates a reset without shutting the device down. Figure 7 provides an example of a sequence of operations: Starting from shutdown, the device is turned on, incremented, reset to mid-scale without being shut down, incremented again, and finally shut down.
MAX749
Shutdown Mode
Figure 3. Shutdown-Reset-On-Shutdown Sequence of Operation. The device is not shut down during reset.
When CTRL and ADJ are both low, the MAX749 is shut down (Table 1): The internal reference and biasing circuitry turn off, the output voltage drops to zero, and the supply current drops to 15A. The MAX749 retains its DAC setting, simplifying software control.
ADJ CTRL tSH tSL HIGH
Reset Mode
If ADJ is high when CTRL is low, the DAC set point is reset to mid-scale and the MAX749 is not shut down. Mid-scale is 32 steps from the minimum, 31 steps from the maximum.
Figure 4. Count-Up Operation
Design Procedure _________and Component Selection
Setting the Output Voltage
The MAX749's output voltage is set using an external resistor and the internal current-output DAC. The fullscale output voltage is set by selecting the feedback resistor RFB according to the formula: -VOUT(MAX) = RFB x 20A (Figure 1). The device is in regulation when VFB = 0V.
ON
ADJ CTRL tRS tR ON RESET tRH
Figure 5. Reset Sequence without Shutdown. The device is not shut down during reset.
DAC Adjustment On power-up or after a reset, the counter sets the DAC output to mid-range, and -VOUT = RFB x 13.33A. Each rising edge of ADJ increments the counter (and therefore the DAC output) in the direction of -VOUT(MAX) by one count. When incremented beyond -VOUT(MAX), the
INCREMENT ADJ CTRL tR SHUTDOWN RESET tRH ON SHUTDOWN ADJ CTRL
RESET
INCREMENT
ON
SHUTDOWN
Figure 6. Reset Sequence with Shutdown
Figure 7. Control Sequence Example (see Output Voltage Control section)
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Digitally Adjustable LCD Bias Supply MAX749
Current-Sense Resistor
+4.5V to +6V INPUT 0.1F 22F RSENSE V+ CS Q1 SMD10P05L
MAX749 DHI
CTRL DLOW L1 47H
ADJ R1 GND
D1 1N5819 VOUT (NEGATIVE)
R2
22F 30V VOUT(MIN) = -R1(13.33A) VOUT(MAX) = -(R1+R2)(13.33A) CCOMP
Figure 8. Using a Potentiometer to Adjust the Output Voltage
counter rolls over and sets the DAC to -V OUT(MIN) , where -VOUT(MIN) = RFB x 6.66A. In other words, a single rising edge of ADJ increments the DAC output by one, and 63 rising edges of ADJ decrement the DAC output by one.
Potentiometer Adjustment It is also possible to adjust the output voltage using a potentiometer instead of the internal DAC (Figure 8). On power-up (V+ applied), the internal current source is set to mid-scale, or 13.33A. Choose R1 and R2 with the following equations: R1 = -VOUT(MIN)/13.33A R2 = -VOUT(MAX)/13.33A - R1. Where the potentiometer can be varied from 0 (producing VOUT(MIN)) to R2 (producing VOUT(MAX)). Notice that ADJ is connected to ground, allowing the device to be shut down. PWM Adjustment A positive pulse-width modulated (PWM) logic signal (e.g., from a microcontroller) can control the MAX749's output voltage. Use the PWM signal to pull up the FB pin through a suitable resistor. An RC network on the PWM output would also be required. In this configuration, the longer the PWM signal remains high, the more negative the MAX749's output will be driven.
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The current-sense resistor limits the peak switch current to 140mV/RSENSE, where RSENSE is the value of the current-sense resistor, and 140mV is the typical current-sense comparator threshold (see V+ to CS Voltage in the Electrical Characteristics). To maximize efficiency and reduce the size and cost of the external components, minimize the peak current. However, since the output current is a function of the peak current (Figures 9a-9e), the limit should not be set too low. No calculations are required to choose the proper current-sense resistor; simply follow this two-step procedure: 1. Determine: - the minimum input voltage, VIN(MIN), - the maximum output voltage, VOUT(MAX), and - the maximum output current, IOUT(MAX). For example, assume that the output voltage must be adjustable to -24V (VOUT(MAX) = -24V) at up to 30mA (IOUT(MAX) = 30mA). The supply voltage ranges from 4.75V to 6V (VIN(MIN) = 4.75V). 2. In Figures 9a-9e, locate the graph drawn for the appropriate output voltage (which is either the desired output voltage or, if that is not shown, the graph for the nearest voltage more negative than the desired output). On this graph find the curve for the highest RSENSE (the lowest current limit) with an output current that is adequate at the lowest input voltage. In this example, select the -24V output graph, Figure 9d. We then want a curve where IOUT is 30mA with a 4.75V input. The 0.3 RSENSE graph shows 25mA of output current with a 4.75V input, so we look next at the 0.25 RSENSE graph. It shows IOUT = 30mA for VIN = 4.75V and VOUT = -24V. Therefore select RSENSE = 0.25. This provides a current limit in the range 440mA to 720mA. Alternatively, a 0.2 sense resistor can be used. This gives a current limit in the range 550mA to 900mA, but enables over 40mA to be generated at -24V with input voltages down to 4.5V. A 0.2 resistor may be easier to obtain than an 0.25 resistor. The theoretical design curves shown in Figures 9a-9e assume the minimum (worst-case) value for the currentlimit comparator threshold. Having selected the current-sense resistor, the maximum current limit is given by 180mV/RSENSE. Use the maximum current-limit figure when choosing the transistor, coil, and diode. IRC (see Table 2) makes surface-mount resistors with preferred values including: 0.1, 0.2, 0.3, 0.5, and 1.0.
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Digitally Adjustable LCD Bias Supply
Choosing an Inductor
Practical inductor values range from 22H to 100H, and 47H is normally a good choice. Inductors with a ferrite core or equivalent are recommended. The inductor's saturation current rating - the current at which the core begins to saturate and the inductance falls to 80% or 90% of its nominal value - should ideally equal the current limit (see Current-Sense Resistor section). However, because the current is limited by the MAX749, the inductor can safely be driven into saturation with only a slight impact on efficiency. For highest efficiency, use a coil with low resistance, preferably under 300m. To minimize radiated noise, use a toroid, pot-core, or shielded inductor.
MAXIMUM OUTPUT CURRENT (mA) 100 80 60 40 0.5 20 1.0 0 2 3 4 INPUT VOLTAGE (V) 5 6 0.2
MAX749-Fig 11
MAX749
VOUT = -15V L = 47H
0.25 0.3 RSENSE () RSENSE ()
Figure 9c. Maximum Output Current vs. Input Voltage, VOUT = -15V
250
MAX749-Fig 9
0.2 MAXIMUM OUTPUT CURRENT (mA) VOUT = -5V L = 47H
60 0.2
MAX749-Fig 12
MAXIMUM OUTPUT CURRENT (mA)
200
0.25 RSENSE () 0.3
50 40 30 20 10 0 2
VOUT = -24V L = 47H
150
0.25 0.3
100
0.5
0.5 1.0
50
1.0
0 2 3 4 INPUT VOLTAGE (V) 5 6
3
4 INPUT VOLTAGE (V)
5
6
Figure 9a. Maximum Output Current vs. Input Voltage, VOUT = -5V
140 MAXIMUM OUTPUT CURRENT (mA) 120 100 80 60 40 20 0 2 3 4 INPUT VOLTAGE (V) 5 6 0.5 1.0
MAX-749-Fig10
Figure 9d. Maximum Output Current vs. Input Voltage, VOUT = -24V
25 0.2 VOUT = -48V L = 47H
MAX749-Fig 13
VOUT = -12V L = 47H
MAXIMUM OUTPUT CURRENT (mA)
0.2
20
0.25 0.3
0.25 RSENSE () 0.3
10
0.5 5 1.0
0 2 3 4 INPUT VOLTAGE (V) 5 6
Figure 9b. Maximum Output Current vs. Input Voltage, VOUT = -12V
Figure 9e. Maximum Output Current vs. Input Voltage, VOUT = -48V
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RSENSE ()
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Digitally Adjustable LCD Bias Supply MAX749
The Sumida CD54-470N (47H, 720mA, 370m) is suitable for a wide range of applications, and the larger CD105-470N (47H, 1.17A, 170m) permits higher current levels and efficiencies. Table 2. Component Suppliers
SUPPLIER INDUCTORS Coiltronics Gowanda Sumida USA Sumida Japan CAPACITORS Kemet Matsuo Nichicon Sprague Sanyo USA Sanyo Japan United Chemi-Con DIODES Motorola Nihon USA Nihon Japan Harris International Rectifier Siliconix Zetex USA Zetex UK IRC (800) 521-6274 (805) 867-2555 81-3-3494-7411 (407) 724-3739 (213) 772-2000 (408) 988-8000 (516) 543-7100 44 (61) 727 5105 (512) 992-7900 (805) 867-2698 81-3-3494-7414 (407) 724-3937 (213) 772-9028 (408) 727-5414 (516) 864-7630 44 (61) 627 5467 (512) 992-3377 (803) 963-6300 (714) 969-2491 (708) 843-7500 (603) 224-1961 (619) 661-6322 81-3-3837-6242 (714) 255-9500 (714) 255-9400 (803) 963-6322 (714) 960-6492 (708) 843-2798 (603) 224-1430 (305) 781-8900 (716) 532-2234 (708) 956-0666 81-3-3607-511 (305) 782-4163 (716) 532-2702 (708) 956-0702 81-3-3607-5428 PHONE FAX
Diode Selection
The MAX749's high switching frequency demands a highspeed rectifier. Schottky diodes such as the 1N58171N5822 family are recommended. Choose a diode with an average current rating approximately equal to the peak current, as determined by 180mV/RSENSE and a breakdown voltage greater than V+ + I-VOUTMAXI.
External Switching Transistor
The MAX749 can drive a PNP transistor or a P-channel logic-level MOSFET. The choice of a power switch is dictated by the input voltage range, cost, and efficiency. MOSFETs provide the highest efficiency because they do not draw any DC gate-drive current (see Typical Operating Characteristics graphs). However, a gatesource voltage of several volts is needed to turn on a MOSFET, so a 5V or greater input supply is required (although this restriction may change as lower-threshold P-channel MOSFETs become available). PNP transistors, meanwhile, may be used over the entire 2V to 6V operating voltage range of the MAX749. When using a MOSFET, connect DHI and DLOW to its gate (see Typical Operating Circuit). When using a PNP transistor, connect DHI to its base, and connect a resistor between the base and DLOW (RBASE) (Figure 1). The PNP transistor is turned off quickly by the direct pull-up of DHI, and turned on by the base current provided through RBASE. This resistor limits the transistor's basedrive current to (VIN - 140mV - VBE)/RBASE, where VIN is the input voltage, 140mV is the drop across RSENSE, VBE is the transistor's base-emitter voltage, and RBASE is the current-limiting resistor. For maximum efficiency, make RBASE as large as possible, but small enough so that the transistor is always driven into saturation. Highest efficiency with a PNP transistor comes from using a device with a low collector-emitter saturation voltage and a high current gain. Use a fast-switching type. For example the Zetex ZTX792A has switching speeds of 40ns (tON) and 500ns (tOFF). The transistor must have a collector-to-emitter (PNP) or drain-to-source (MOSFET) voltage rating greater than the input-to-output voltage differential (VIN - VOUT). In either case the transistor must have a current rating that exceeds the peak current set by the current-sense resistor. PNP transistors are generally less expensive than Pchannel MOSFETs. Table 2 lists some suppliers of switching transistors suitable for use with the MAX749.
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POWER TRANSISTORS - MOSFETS
POWER TRANSISTORS - PNP TRANSISTORS
CURRENT-SENSE RESISTORS
Base Resistor
The base resistor, RBASE in Figure 1, controls the amount of base current in the PNP transistor. A low value for RBASE increases base drive, which provides higher output currents and compensates for lower input voltages, but decreases efficiency. Conversely, a high RBASE value increases efficiency but reduces the output capability, especially at low voltages. When using high-gain transistors, e.g. the Zetex ZTX750 or ZTX792, typical values for RBASE are in the 150 to 510 range, but will depend on the required input voltage range and output current (see Typical Operating Characteristics). Lower-gain transistors require lower values for RBASE and are less efficient. Larger RBASE values are suitable if less output power is required.
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Digitally Adjustable LCD Bias Supply
Capacitors
Output Filter Capacitor A 22F, 30V surface-mount (SMT) tantalum output filter capacitor typically maintains 100mVp-p output ripple when generating -24V at 40mA from a 5V input. Smaller capacitors, down to 10F, may be used for light loads in applications that can tolerate higher output ripple. Surface-mount capacitors are generally preferred because they lack the inductance and resistance of the leads of their through-hole equivalents. Input Bypass Capacitor A 22F tantalum capacitor in parallel with a 0.1F ceramic normally provides sufficient bypassing. Mount the 0.1F capacitor very close to the IC. Larger capacitors may be needed if the incoming supply has high impedance. Less bypass capacitance is acceptable if the circuit is run off a low-impedance supply. Begin prototyping with a large bypass capacitor; when the circuit is working, reduce the bypass to the smallest value that gives good results. Although bench power supplies have low impedance at DC, they often have high impedance at the frequencies used by switching DC-DC converters. The effective series resistance (ESR) of both the bypass and filter capacitors affects efficiency. Best performance is obtained by doubling up on the filter capacitors or using low-ESR types. The smallest low-ESR SMT capacitors currently available are Sprague 595D series, which are about half the size of competing products. Sanyo OS-CON organic semiconductor through-hole capacitors also exhibit low ESR, and are especially useful when operation below 0C is required. Table 2 lists the phone numbers of these and other manufacturers. Compensation Capacitor The high value of the feedback resistor makes the feedback loop susceptible to phase lag if parasitic capacitance is present at the FB pin. To compensate for this, it may be necessary to connect a capacitor, CCOMP, in parallel with R FB . Although C COMP is normally not required, the value of CCOMP depends upon the value of RFB and on the individual circuit layout--typical values range from 0pF to 220pF.
MAX749
PC Layout and Grounding
Due to high current levels and fast switching waveforms, proper PC board layout is essential. In particular, keep all leads short, especially the lead connected to the FB pin and those connecting Q1, L1, and D1 together. Mount the RFB resistor very close to the IC. Use a star ground configuration: Connect the ground lead of the input bypass capacitor, the output capacitor, and the inductor at a common point next to the GND pin of the MAX749. Additionally, connect the positive lead of the input bypass capacitor as close as possible to the V+ pin of the IC.
___________________Chip Topography
0.070" (0.1178mm) V+ V+ CS
ADJ CTRL DHI 0.808" (0.2032mm) DLOW
FB
GND
TRANSISTOR COUNT: 521; SUBSTRATE CONNECTED TO GND.
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11
Digitally Adjustable LCD Bias Supply MAX749
_______________________________________________________Package Information
E D A3 A A2 E1
DIM A A1 A2 A3 B B1 C D1 E E1 e eA eB L INCHES MAX MIN 0.200 - - 0.015 0.175 0.125 0.080 0.055 0.022 0.016 0.065 0.045 0.012 0.008 0.080 0.005 0.325 0.300 0.310 0.240 - 0.100 - 0.300 0.400 - 0.150 0.115 INCHES MIN MAX 0.348 0.390 0.735 0.765 0.745 0.765 0.885 0.915 1.015 1.045 1.14 1.265 MILLIMETERS MIN MAX - 5.08 0.38 - 3.18 4.45 1.40 2.03 0.41 0.56 1.14 1.65 0.20 0.30 0.13 2.03 7.62 8.26 6.10 7.87 2.54 - 7.62 - - 10.16 2.92 3.81 MILLIMETERS MIN MAX 8.84 9.91 18.67 19.43 18.92 19.43 22.48 23.24 25.78 26.54 28.96 32.13
21-0043A
L A1 e B D1
0 - 15 C B1 eA eB
Plastic DIP PLASTIC DUAL-IN-LINE PACKAGE (0.300 in.)
PKG. DIM PINS P P P P P N D D D D D D 8 14 16 18 20 24
DIM
D A e B
0.101mm 0.004in.
0-8
A1
C
L
A A1 B C E e H L
INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.157 0.150 0.050 0.244 0.228 0.050 0.016
MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 3.80 4.00 1.27 5.80 6.20 0.40 1.27
E
H
Narrow SO SMALL-OUTLINE PACKAGE (0.150 in.)
DIM PINS D D D 8 14 16
INCHES MILLIMETERS MIN MAX MIN MAX 0.189 0.197 4.80 5.00 0.337 0.344 8.55 8.75 0.386 0.394 9.80 10.00
21-0041A
12
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