19-0143; Rev 1; 2/95 NUAL KIT MA ATION HEET S A EVALU T A WS D FOLLO Digitally Adjustable LCD Bias Supply ________________________Applications Notebook Computers ____________________________Features ♦ +2.0V to +6.0V Input Voltage Range ♦ Flexible Control of Output Voltage: Digital Control Potentiometer Adjustment PWM Control ♦ Output Voltage Range Set by One Resistor ♦ Low, 60µA Max Quiescent Current ♦ 15µA Max Shutdown Mode ♦ Small Size – 8-Pin SO and Plastic DIP Packages ______________Ordering Information PART TEMP. RANGE PIN-PACKAGE Laptop Computers MAX749CPA 0°C to +70°C 8 Plastic DIP Palmtop Computers MAX749CSA 0°C to +70°C 8 SO Personal Digital Assistants MAX749C/D 0°C to +70°C Dice* Communicating Computers MAX749EPA -40°C to +85°C 8 Plastic DIP Portable Data-Collection Terminals MAX749ESA -40°C to +85°C 8 SO * Contact factory for dice specifications. __________Typical Operating Circuit VIN +5V __________________Pin Configuration TOP VIEW RSENSE 0.1µF DIGITAL ADJUST ON/OFF 1 V+ CS 8 2 ADJ 7 MAX749 DHI 3 4 CTRL DLOW FB GND RFB 6 5 -VOUT V+ 1 8 CS ADJ 2 7 DHI 6 DLOW 5 GND CTRL 3 MAX749 FB 4 DIP/SO CCOMP _______________________________________________________________ Maxim Integrated Products Call toll free 1-800-998-8800 for free samples or literature. 1 MAX749 _______________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 60µA max and is reduced to under 15µA 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. MAX749 Digitally Adjustable LCD Bias Supply ABSOLUTE MAXIMUM RATINGS V+ ................................................................................-0.3V, +7V CTRL, ADJ, FB, DLOW, DHI, CS.....................-0.3V, (V+ + 0.3V) Continuous Power Dissipation (TA = +70°C) Plastic DIP (derate 9.09mW/°C above +70°C) ............727mW SO (derate 5.88mW/°C above +70°C) .........................471mW Operating Temperature Ranges: MAX749C_A........................................................0°C to +70°C MAX749E_A .....................................................-40°C to +85°C Storage Temperature Range .............................-65°C to +160°C Lead Temperature (soldering, 10sec) .............................+300°C 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 µA 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 µA Shutdown Current 15 µA 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 Ω 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 SYMBOL tR Minimum Reset Setup Minimum Reset Hold tRS tRH Minimum ADJ High Pulse Width tSH Minimum ADJ Low Pulse Width tSL Minimum ADJ Low to CTRL Low tSD 2 CONDITIONS V+ = 2V V+ = 5V Not tested Not tested V+ = 2V V+ = 5V V+ = 2V V+ = 5V V+ = 2V V+ = 5V MIN TA = +25°C TYP MAX 125 25 TA = TMIN to TMAX MIN MAX 300 85 0 0 400 100 0 0 15 10 170 60 70 20 85 85 400 150 200 85 ______________________________________________________________________________________ UNITS ns ns ns 100 100 500 200 250 100 ns ns ns Digitally Adjustable LCD Bias Supply EFFICIENCY vs. OUTPUT CURRENT – PNP EFFICIENCY (%) -5V 75 V+ = 3V RBASE = 470Ω RSENSE = 0.25Ω TRANSISTOR: ZTX750 70 -24V -12V -12V 76 -12V -24V 85 MAX749TOC2-B 78 80 EFFICIENCY (%) 80 -5V 74 -24V 72 V+ = 3V RBASE = 160Ω RSENSE = 0.25Ω TRANSISTOR = ZTX750 70 68 -5V 75 V+ = 5V RSENSE = 0.25Ω TRANSISTOR: SMD10P05L 70 66 64 65 40 50 65 0 60 10 20 30 40 50 60 70 80 90 100 OUTPUT CURRENT (mA) 5 10 15 20 25 30 35 40 45 50 OUTPUT CURRENT (mA) EFFICIENCY vs. OUTPUT VOLTAGE EFFICIENCY vs. OUTPUT VOLTAGE 85 MAX749-TOC1-A 85 -20mA 80 -20mA -5mA 80 EFFICIENCY (%) -40mA -5mA 75 V+ = 3V RBASE = 470Ω RSENSE = 0.25Ω TRANSISTOR : ZTX750 70 0 OUTPUT CURRENT (mA) MAX749-TOC1-B 30 -40mA 75 V+ = 5V RSENSE = 0.25Ω TRANSISTOR : SMD10P05L 70 65 65 -24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 LOAD CURRENT vs. INPUT VOLTAGE LOAD CURRENT vs. INPUT VOLTAGE 400 350 300 250 200 -12V 150 100 -24V 50 500 450 RBASE = 160Ω -5V RSENSE = 0.25Ω TRANSISTOR = ZTX750 400 LOAD CURRENT (mA) -5V RBASE = 470Ω RSENSE = 0.25Ω TRANSISTOR = ZTX750 -4 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) 350 300 250 200 -12V 150 -24V 100 -48V MAX749-TOC3-B 20 MAX749-TOC3-A 10 EFFICIENCY (%) 0 LOAD CURRENT (mA) EFFICIENCY (%) 80 MAX749TOC2-A 85 EFFICIENCY vs. OUTPUT CURRENT – MOSFET MAX749TOC2-C EFFICIENCY vs. OUTPUT CURRENT – PNP -48V 50 0 0 2 3 4 INPUT VOLTAGE (V) 5 6 2 3 4 5 6 INPUT VOLTAGE (V) _______________________________________________________________________________________ 3 MAX749 __________________________________________Typical Operating Characteristics (TA = +25°C, L = 47µH, unless otherwise noted.) MAX749 Digitally Adjustable LCD Bias Supply ____________________________Typical Operating Characteristics (continued) (TA = +25°C, L = 47µH, unless otherwise noted.) LINE-TRANSIENT RESPONSE LOAD-TRANSIENT RESPONSE 100mVAC/div OUTPUT VOLTAGE LOAD CURRENT 10mA/div 0mA OUTPUT VOLTAGE 100mVAC/div 1 V/div INPUT VOLTAGE VOUT = -15V TRANSISTOR = ZTX750 50ms/div 50µs/div VOUT = -15V TRANSISTOR = ZTX750 0V VOUT = -15V ILOAD = 5mA TRANSISTOR = ZTX750 ______________________________________________________________Pin Description PIN 4 NAME 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. 1 V+ 2 ADJ 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. 3 CTRL 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. 4 FB 5 GND 6 DLOW 7 DHI Output Driver High. Connect to the gate of the external P-channel transistor, or to the base of the external PNP transistor. 8 CS Current-Sense Input. The external transistor is turned off when current through the sense resistor, RSENSE, brings CS below V+ by 140mV (typ). Feedback Input for output full-scale voltage selection. -VOUT(MAX) = (RFB) x (20µA) 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. ______________________________________________________________________________________ Digitally Adjustable LCD Bias Supply 0.1µF MAX749 +2V TO +6V INPUT 22µF 6.2V V+ POWER-ON RESET RESET 6-BIT CURRENT-OUTPUT DAC 6-BIT COUNTER RSENSE REF CTRL ADJ 6.66µA TO 20µA LOGIC INCREMENT CS ON/OFF DHI SWITCHMODE POWER SUPPLY Q1 ZTX750 DLOW RBASE 470Ω BIAS RFB FB MAX749 D1 1N5819 L1 47µH VOUT (NEGATIVE) GND CCOMP 22µF 30V 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 (8µs later), or - the switch current reaches its limit (as determined by the current-sense resistor and the current comparator). _______________________________________________________________________________________ 5 MAX749 Digitally Adjustable LCD Bias Supply +2V TO +6V INPUT 0.1µF 22µF V+ 140mV Q TRIG MINIMUM OFF-TIME ONE-SHOT FLIP-FLOP DHI Q S R Q1 ZTX750 DLOW MAXIMUM ON-TIME ONE-SHOT TRIG RSENSE CURRENT COMPARATOR RBASE 470Ω VOLTAGE COMPARATOR Q FB REF D1 1N5819 VOUT (NEGATIVE) RFB 6-BIT CURRENT-OUTPUT DAC MAX749 L1 47µH 22µF 30V GND CCOMP Figure 2. Switch-Mode Power-Supply Section Block Diagram Once turned off, a one-shot holds the switch off for a minimum of 1µs, 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 1µs, 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 CTRL Low Low Shut down RESULT High Low Reset counter to mid-range. The device is not shut down. X High On High Increment the counter ______________________________________________________________________________________ Digitally Adjustable LCD Bias Supply CTRL tR tSD ON SHUTDOWN RESET 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. 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 15µA. The MAX749 retains its DAC setting, simplifying software control. Reset Mode ADJ CTRL tSH HIGH tSL 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. Design Procedure _________and Component Selection Figure 4. Count-Up Operation 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 20µA (Figure 1). The device is in regulation when VFB = 0V. ADJ CTRL tRH tRS tR ON RESET ON 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 13.33µA. 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 INCREMENT CTRL CTRL tRH tR SHUTDOWN RESET ADJ ADJ RESET Figure 6. Reset Sequence with Shutdown ON SHUTDOWN ON SHUTDOWN Figure 7. Control Sequence Example (see Output Voltage Control section) _______________________________________________________________________________________ 7 MAX749 ADJ MAX749 Digitally Adjustable LCD Bias Supply Current-Sense Resistor +4.5V to +6V INPUT 22µF 0.1µF RSENSE V+ CS MAX749 DHI CTRL Q1 SMD10P05L DLOW L1 47µH ADJ R1 R2 D1 1N5819 VOUT (NEGATIVE) GND 22µF 30V VOUT(MIN) = -R1(13.33µA) VOUT(MAX) = -(R1+R2)(13.33µA) 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 6.66µA. 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 13.33µA. Choose R1 and R2 with the following equations: R1 = -VOUT(MIN)/13.33µA R2 = -VOUT(MAX)/13.33µA - R1. Where the potentiometer can be varied from 0 (producing VOUT(MIN)) to R2Ω (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. 8 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 0.3Ω RSENSE graph shows 25mA of output current with a 4.75V input, so we look next at the 0.25Ω RSENSE graph. It shows IOUT = 30mA for VIN = 4.75V and VOUT = -24V. Therefore select RSENSE = 0.25Ω. This provides a current limit in the range 440mA to 720mA. Alternatively, a 0.2Ω 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 0.2Ω resistor may be easier to obtain than an 0.25Ω 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: 0.1Ω, 0.2Ω, 0.3Ω, 0.5Ω, and 1.0Ω. ______________________________________________________________________________________ Digitally Adjustable LCD Bias Supply 0.2 VOUT = -15V L = 47µH 80 60 0.25 RSENSE (Ω) 100 MAX749-Fig 11 MAXIMUM OUTPUT CURRENT (mA) MAX749 Choosing an Inductor Practical inductor values range from 22µH to 100µH, and 47µH 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 300mΩ. To minimize radiated noise, use a toroid, pot-core, or shielded inductor. 0.3 40 0.5 20 1.0 0 2 4 3 5 6 INPUT VOLTAGE (V) Figure 9c. Maximum Output Current vs. Input Voltage, VOUT = -15V 0.2 0.3 150 100 0.5 50 1.0 0 2 3 4 5 40 0.25 0.3 30 20 0.5 10 1.0 0 6 2 INPUT VOLTAGE (V) 5 6 Figure 9d. Maximum Output Current vs. Input Voltage, VOUT = -24V 140 0.2 80 0.25 0.3 60 40 0.5 20 1.0 0 2 3 4 5 6 INPUT VOLTAGE (V) Figure 9b. Maximum Output Current vs. Input Voltage, VOUT = -12V MAX749-Fig 13 VOUT = -48V L = 47µH 20 0.25 0.3 15 RSENSE (Ω) 100 RSENSE (Ω) VOUT = -12V L = 47µH MAXIMUM OUTPUT CURRENT (mA) 25 0.2 MAX-749-Fig10 MAXIMUM OUTPUT CURRENT (mA) 4 3 INPUT VOLTAGE (V) Figure 9a. Maximum Output Current vs. Input Voltage, VOUT = -5V 120 RSENSE (Ω) 0.25 0.2 VOUT = -24V L = 47µH 50 MAX749-Fig 12 200 MAXIMUM OUTPUT CURRENT (mA) VOUT = -5V L = 47µH 60 RSENSE (Ω) MAX749-Fig 9 MAXIMUM OUTPUT CURRENT (mA) 250 10 0.5 5 1.0 0 2 3 4 5 6 INPUT VOLTAGE (V) Figure 9e. Maximum Output Current vs. Input Voltage, VOUT = -48V _______________________________________________________________________________________ 9 MAX749 Digitally Adjustable LCD Bias Supply The Sumida CD54-470N (47µH, 720mA, 370mΩ) is suitable for a wide range of applications, and the larger CD105-470N (47µH, 1.17A, 170mΩ) permits higher current levels and efficiencies. 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. 10 Table 2. Component Suppliers SUPPLIER PHONE FAX INDUCTORS Coiltronics (305) 781-8900 Gowanda (716) 532-2234 (305) 782-4163 (716) 532-2702 Sumida USA (708) 956-0666 (708) 956-0702 Sumida Japan 81-3-3607-511 81-3-3607-5428 Kemet (803) 963-6300 (803) 963-6322 Matsuo (714) 969-2491 (714) 960-6492 Nichicon (708) 843-7500 (708) 843-2798 Sprague (603) 224-1961 (603) 224-1430 Sanyo USA (619) 661-6322 Sanyo Japan 81-3-3837-6242 United Chemi-Con (714) 255-9500 CAPACITORS (714) 255-9400 DIODES Motorola (800) 521-6274 Nihon USA (805) 867-2555 (805) 867-2698 Nihon Japan 81-3-3494-7411 81-3-3494-7414 POWER TRANSISTORS - MOSFETS Harris (407) 724-3739 (407) 724-3937 International Rectifier (213) 772-2000 (213) 772-9028 Siliconix (408) 988-8000 (408) 727-5414 POWER TRANSISTORS - PNP TRANSISTORS Zetex USA (516) 543-7100 (516) 864-7630 Zetex UK 44 (61) 727 5105 44 (61) 627 5467 CURRENT-SENSE RESISTORS IRC (512) 992-7900 (512) 992-3377 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 150Ω to 510Ω 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. _____________________________________________________________________________________ Digitally Adjustable LCD Bias Supply Input Bypass Capacitor A 22µF tantalum capacitor in parallel with a 0.1µF ceramic normally provides sufficient bypassing. Mount the 0.1µF 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 0°C 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. 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. ______________________________________________________________________________________ 11 MAX749 Capacitors Output Filter Capacitor A 22µF, 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 10µF, 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. MAX749 Digitally Adjustable LCD Bias Supply _______________________________________________________Package Information D E DIM E1 A A1 A2 A3 B B1 C D1 E E1 e eA eB L A3 A A2 L A1 0° - 15° C e B1 eA B eB D1 Plastic DIP PLASTIC DUAL-IN-LINE PACKAGE (0.300 in.) 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 PKG. DIM PINS P P P P P N D D D D D D 8 14 16 18 20 24 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 DIM D 0°-8° A 0.101mm 0.004in. e B A1 E 12 C H L Narrow SO SMALL-OUTLINE PACKAGE (0.150 in.) 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 DIM PINS D D D 8 14 16 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 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