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19-1563; Rev 3; 9/05
KIT ATION EVALU E AILABL AV
Low-Noise Step-Up DC-DC Converters
General Description Features
90% Efficiency Adjustable Output from VIN to 12V 1.6A, 0.21, 14V Power MOSFET (MAX1790) 2.4A, 0.15, 14V Power MOSFET (MAX8715) +2.6V to +5.5V Input Range Pin-Selectable 640kHz or 1.2MHz Switching Frequency 0.1A Shutdown Current Programmable Soft-Start Small 8-Pin MAX Package
MAX1790/MAX8715
The MAX1790/MAX8715 boost converters incorporate high-performance (at 1.2MHz), current-mode, fixed-frequency, pulse-width modulation (PWM) circuitry with a built-in 0.21/0.15 n-channel MOSFET to provide a highly efficient regulator with fast response. High switching frequency (640kHz or 1.2MHz selectable) allows easy filtering and faster loop performance. An external compensation pin provides the user flexibility in determining loop dynamics, allowing the use of small, low equivalent-series-resistance (ESR) ceramic output capacitors. The device can produce an output voltage as high as 12V from an input as low as 2.6V. Soft-start is programmed with an external capacitor, which sets the input-current ramp rate. In shutdown mode, current consumption is reduced to 0.1A. The MAX1790/ MAX8715 are available in a space-saving 8-pin MAX(R) package. The ultra-small package and high switching frequency allow the total solution to be less than 1.1mm high.
MAX is a registered trademark of Maxim Integrated Products, Inc.
Ordering Information
PART MAX1790EUA MAX1790EUA+ MAX8715EUA MAX8715EUA+ TEMP RANGE -40C to +85C -40C to +85C -40C to +85C -40C to +85C PIN-PACKAGE 8 MAX 8 MAX 8 MAX 8 MAX
Applications
LCD Displays PCMCIA Cards Portable Applications Hand-Held Devices
+ Denotes lead-free package.
Typical Operating Circuit
VIN 2.6V TO 5V
Pin Configuration
TOP VIEW
COMP
IN ON/OFF
SHDN
1 2 3
8 7
SS FREQ IN LX
VOUT LX
FB SHDN
MAX1790 MAX8715 FREQ GND
MAX1790 MAX8715
6 5
GND 4
MAX
SS COMP
FB
________________________________________________________________ Maxim Integrated Products
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For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
Low-Noise Step-Up DC-DC Converters MAX1790/MAX8715
ABSOLUTE MAXIMUM RATINGS
LX to GND ..............................................................-0.3V to +14V IN, SHDN, FREQ, FB to GND ................................-0.3V to +6.2V SS, COMP to GND .......................................-0.3V to (VIN + 0.3V) RMS LX Pin Current ..............................................................1.2A Continuous Power Dissipation (TA = +70C) 8-Pin MAX (derate 4.1mW/C above +70C) .............330mW Operating Temperature Range MAX1790EUA/MAX8715EUA ........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+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
(VIN = SHDN = 3V, FREQ = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Input Supply Range VIN Undervoltage Lockout SYMBOL VIN UVLO VIN rising, typical hysteresis is 40mV, LX remains off below this level MAX1790 Quiescent Current IIN MAX8715 Shutdown Supply Current ERROR AMPLIFIER Feedback Voltage FB Input Bias Current Feedback-Voltage Line Regulation Transconductance Voltage Gain OSCILLATOR Frequency Maximum Duty Cycle N-CHANNEL SWITCH Current Limit ILIM VFB = 1V, duty cycle = 65% (Note 1) MAX1790 MAX8715 VLX = 12V MAX1790 MAX8715 MAX1790 MAX8715 1.2 1.8 1.6 2.4 0.21 0.15 0.01 5 2.3 A 3.4 0.5 0.35 20 30 A fOSC DC FREQ = GND FREQ = IN FREQ = GND FREQ = IN 540 1000 79 640 1220 85 84 740 1500 92 kHz % gm AV VFB IFB Level to produce VCOMP = 1.24V VFB = 1.24V MAX1790 MAX8715 1.222 1.24 0 125 0.05 70 70 140 160 700 1.258 40 190 0.15 240 240 V nA %/V S V/V IIN SHDN = GND VFB = 1.3V, not switching VFB = 1.0V, switching VFB = 1.3V, not switching VFB = 1.0V, switching CONDITIONS MIN 2.6 2.25 2.38 0.18 2 0.21 2.5 0.1 TYP MAX 5.5 2.52 0.35 5 0.35 5.0 10 A mA UNITS V V
Level to produce VCOMP = 1.24V, 2.6V < VIN < 5.5V I = 5A MAX1790 MAX8715
On-Resistance Leakage Current
RON ILXOFF
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Low-Noise Step-Up DC-DC Converters
ELECTRICAL CHARACTERISTICS (continued)
(VIN = SHDN = 3V, FREQ = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Current-Sense Transresistance SOFT-START Reset Switch Resistance Charge Current CONTROL INPUTS Input Low Voltage Input High Voltage Hysteresis FREQ Pulldown Current SHDN Input Current IFREQ ISHDN VIL VIH SHDN, FREQ SHDN, FREQ SHDN, FREQ 1.8 0.7 x VIN 0.1 x VIN 5 0.001 9.0 1 0.3 x VIN V V V A A VSS = 1.2V 1.5 4 100 7.0 A SYMBOL RCS MAX1790 MAX8715 CONDITIONS MIN 0.30 0.20 TYP 0.45 0.30 MAX 0.65 0.43 UNITS V/A
MAX1790/MAX8715
ELECTRICAL CHARACTERISTICS
(VIN = SHDN = 3V, FREQ = GND, TA = -40C to +85C, unless otherwise noted.) (Note 2)
PARAMETER Input Supply Range VIN Undervoltage Lockout SYMBOL VIN UVLO VIN rising, typical hysteresis is 40mV, LX remains off below this level MAX1790 Quiescent Current IIN MAX8715 Shutdown Supply Current ERROR AMPLIFIER Feedback Voltage FB Input Bias Current Feedback-Voltage Line Regulation Transconductance OSCILLATOR Frequency Maximum Duty Cycle fOSC DC FREQ = GND FREQ = IN FREQ = GND 490 900 78 770 1500 92 kHz % gm VFB IFB Level to produce VCOMP = 1.24V VFB = 1.24V MAX1790 MAX8715 1.215 1.24 1.260 40 190 0.15 70 70 260 260 V nA %/V S IIN SHDN = GND VFB = 1.3V, not switching VFB = 1.0V, switching VFB = 1.3V, not switching VFB = 1.0V, switching CONDITIONS MIN 2.6 2.25 TYP MAX 5.5 2.52 0.35 5 0.35 5 10 A mA UNITS V V
Level to produce VCOMP = 1.24V, 2.6V < VIN < 5.5V I = 5A MAX1790 MAX8715
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Low-Noise Step-Up DC-DC Converters MAX1790/MAX8715
ELECTRICAL CHARACTERISTICS (continued)
(VIN = SHDN = 3V, FREQ = GND, TA = -40C to +85C, unless otherwise noted.) (Note 2)
PARAMETER N-CHANNEL SWITCH Current Limit ILIM VFB = 1V, duty cycle = 65% (Note 1) MAX1790 MAX8715 MAX1790 MAX8715 SHDN, FREQ SHDN, FREQ 0.7 x VIN 0.30 0.20 MAX1790 MAX8715 1.2 1.8 2.3 A 3.0 0.5 0.35 0.65 0.43 0.3 x VIN V/A SYMBOL CONDITIONS MIN TYP MAX UNITS
On-Resistance Current-Sense Transresistance CONTROL INPUTS Input Low Voltage Input High Voltage
RON RCS
VIL VIH
V V
Note 1: Current limit varies with duty cycle due to slope compensation. See the Output-Current Capability section. Note 2: Specifications to -40C are guaranteed by design and not production tested.
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Low-Noise Step-Up DC-DC Converters MAX1790/MAX8715
Typical Operating Characteristics
(Circuit of Figure 1, VIN = 3.3V, fOSC = 640kHz, TA = +25C, unless otherwise noted.)
MAX1790 EFFICIENCY vs. OUTPUT CURRENT
MAX1790 toc01
MAX1790 EFFICIENCY vs. OUTPUT CURRENT
MAX1790 toc02
MAX1790 EFFICIENCY vs. OUTPUT CURRENT
90 85 EFFICIENCY (%) 80 75 70 65 60 fOSC = 640kHz L = 10H fOSC = 1.2MHz L = 5.4H
MAX1790 toc03
95 90 85 EFFICIENCY (%) 80 75 70 65 60 55 50 1 10 100 VIN = 3.3V VOUT = 5V fOSC = 1.2MHz L = 2.7H fOSC = 640kHz L = 5.4H
95 90 85 EFFICIENCY (%) 80 75 70 65 60 55 50 VIN = 3.3V VOUT = 12V 1 10 100 fOSC = 640kHz L = 10H fOSC = 1.2MHz L = 5.4H
95
55 50 1 10 100 OUTPUT CURRENT (mA)
VIN = 5V VOUT = 12V 1000
1000
1000
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE
MAX1790 toc04
MAX1790 OUTPUT VOLTAGE vs. OUTPUT CURRENT
MAX1790 toc05
MAX8715 EFFICIENCY vs. OUTPUT CURRENT
90 85 EFFICIENCY (%) 80 75 70 65 60 55 VIN = 3.3V 1 10 100 1000 VIN = 5.0V VOUT = 9V fOSC = 1.2MHz L = 6.8H
MAX1790 toc06
0.7 NO-LOAD SUPPLY CURRENT (mA) 0.6 0.5 0.4 0.3 0.2 0.1 0 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) VOUT = 12V 5.0 fOSC = 1.2MHz fOSC = 640kHz
12.10 12.05 12.00 OUTPUT VOLTAGE (V) 11.95 11.90 11.85 11.80 11.75 11.70 11.65 11.60 fOSC = 640kHz 0 TA = +25C TA = -40C TA = +85C
95
50 45
5.5
20 40 60 80 100 120 140 160 180 200 OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
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Low-Noise Step-Up DC-DC Converters MAX1790/MAX8715
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 3.3V, fOSC = 640kHz, TA = +25C, unless otherwise noted.)
MAX8715 PULSED LOAD-TRANSIENT RESPONSE
MAX1790 toc07 MAX1790 toc08
MAX8715 LOAD-TRANSIENT RESPONSE
200mA CH1 0 10mA CH2
MAX1790 LOAD-TRANSIENT RESPONSE
200mA CH1 10mA RCOMP = 120k CCOMP = 1200pF CCOMP2 = 56pF
MAX1790 toc09 MAX1790 toc12
RCOMP = 82k CCOMP = 750pF CCOMP2 = 10pF
1A CH1 40mA
CH2 CH2
CH3 CH3 40s/div CH1 = LOAD CURRENT, 200mA/div CH2 = OUTPUT VOLTAGE, AC-COUPLED, 100mV/div CH3 = INDUCTOR CURRENT, 500mA/div VIN = 3.3V, VOUT = 9.0V fOSC = 1.2MHz, L = 6.8H, COUT = 3 x 3.3F
CH3
10s/div CH1 = LOAD CURRENT, 1A/div CH2 = OUTPUT VOLTAGE, AC-COUPLED, 100mV/div CH3 = INDUCTOR CURRENT, 500mA/div VIN = 3.3V, VOUT = 9.0V fOSC = 1.2MHz, L = 6.8H, COUT = 3 x 3.3F
100s/div CH1 = LOAD CURRENT, 100mA/div CH2 = OUTPUT VOLTAGE, AC-COUPLED, 200mV/div CH3 = INDUCTOR CURRENT, 1A/div VIN = 3V VOUT = 12V, fOSC = 640kHz, COUT = 33F + 0.1F
MAX1790 LOAD-TRANSIENT RESPONSE
MAX1790 toc10
MAX1790 STARTUP WAVEFORM WITHOUT SOFT-START
MAX1790 toc11
STARTUP WAVEFORM WITH SOFT-START
500mA CH1 20mA
RCOMP = 62k CCOMP = 820pF CCOMP2 = 56pF
CH1
CH1
CH2
CH2
CH2
CH3
CH3 100s/div CH1 = LOAD CURRENT, 500mA/div CH2 = OUTPUT VOLTAGE, AC-COUPLED, 200mV/div CH3 = INDUCTOR CURRENT, 1A/div VOUT = 5V, fOSC = 640kHz, COUT = 47F + 0.1F
CH3
100s/div CH1 = SHDN, 5V/div CH2 = OUTPUT VOLTAGE, 5V/div CH3 = INDUCTOR CURRENT, 1A/div VIN = 3.3V, VOUT = 12V, IOUT = 10mA, fOSC = 640kHz NO SOFT-START CAPACITOR, COUT = 33F
1ms/div CH1 = SHDN, 5V/div CH2 = OUTPUT VOLTAGE, 5V/div CH3 = INDUCTOR CURRENT, 200mA/div VOUT = 12V, IOUT = 10mA, fOSC = 640kHz, CSS = 0.027F, COUT = 33F
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Low-Noise Step-Up DC-DC Converters MAX1790/MAX8715
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 3.3V, fOSC = 640kHz, TA = +25C, unless otherwise noted.)
STARTUP WAVEFORM WITH SOFT-START
MAX1790 toc13
SWITCHING WAVEFORM
MAX1790 toc14
CH1 CH1 CH2 CH2
CH3 CH3 500ns/div CH1 = LX SWITCHING WAVEFORM, 5V/div CH2 = OUTPUT VOLTAGE, AC-COUPLED, 200mV/div CH3 = INDUCTOR CURRENT, 1A/div VOUT = 12V, IOUT = 200mA, fOSC = 640kHz, L = 10H; COUT = 33F + 0.1F
2ms/div CH1 = SHDN, 5V/div CH2 = VOUT, 5V/div CH3 = INDUCTOR CURRENT, 500mA/div VOUT = 12V, IOUT = 200mA, fOSC = 640kHz, CSS = 0.027F
MAX1790 MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE
MAX1790 toc15
MAX8715 MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE
1600 1400 1200 1000 800 600 400 200 VOUT = 9V fOSC = 1.2MHz L = 6.8H COUT = 3 x 3.3F
MAX1790 toc16
1800 MAXIMUM OUTPUT CURRENT (mA) 1600 1400 1200 1000 800 600 400 200 fOSC = 640kHz 0 VOUT = 12V VOUT = 5V
1800 MAXIMUM OUTPUT CURRENT (mA)
0 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 INPUT VOLTAGE (V)
3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 INPUT VOLTAGE (V)
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Low-Noise Step-Up DC-DC Converters MAX1790/MAX8715
Pin Description
PIN 1 2 3 4 5 6 7 NAME COMP FB SHDN GND LX IN FREQ FUNCTION Compensation Pin for Error Amplifier. Connect a series RC from COMP to ground. See the Loop Compensation section for component selection guidelines. Feedback Pin. Reference voltage is 1.24V nominal. Connect an external resistor-divider tap to FB and minimize the trace area. Set VOUT according to: VOUT = 1.24V (1 + R1 / R2). See Figure 1. Shutdown Control Input. Drive SHDN low to turn off the MAX1790/MAX8715. Ground Switch Pin. Connect the inductor/catch diode to LX and minimize the trace area for lowest EMI. Supply Pin. Bypass IN with at least a 1F ceramic capacitor directly to GND. Frequency Select Input. When FREQ is low, the oscillator frequency is set to 640kHz. When FREQ is high, the frequency is 1.2MHz. This input has a 5A pulldown current. Soft-Start Control Pin. Connect a soft-start capacitor (CSS) to this pin. Leave open for no soft-start. The softstart capacitor is charged with a constant current of 4A. Full current limit is reached after t = 2.5 x 105 CSS. The soft-start capacitor is discharged to ground when SHDN is low. When SHDN goes high, the soft-start capacitor is charged to 0.5V, after which soft-start begins.
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SS
Detailed Description
The MAX1790/MAX8715 are highly efficient power supplies that employ a current-mode, fixed-frequency PWM architecture for fast transient response and lownoise operation. The device regulates the output voltage through a combination of an error amplifier, two comparators, and several signal generators (Figure 2). The error amplifier compares the signal at FB to 1.24V and varies the COMP output. The voltage at COMP determines the current trip point each time the internal MOSFET turns on. As the load varies, the error amplifier sources or sinks current to the COMP output accordingly to produce the inductor peak current necessary to service the load. To maintain stability at high duty cycle, a slope-compensation signal is summed with the currentsense signal. At light loads, this architecture allows the ICs to "skip" cycles to prevent overcharging the output voltage. In this region of operation, the inductor ramps up to a fixed peak value (approximately 50mA, MAX1790 or 75mA, MAX8715), discharges to the output, and waits until another pulse is needed again.
VIN 2.6V TO 5.5V
CIN C1 10F 6.3V
L
IN ON/OFF VIN 1.2MHz SHDN LX D1 MBRS130LT1 0.1F*
VOUT
MAX1790 MAX8715
FREQ GND
COUT
640kHz
SS 0.027F COMP
FB R1 R2
CCOMP2
RCOMP CCOMP * OPTIONAL
Figure 1. Typical Application Circuit
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Low-Noise Step-Up DC-DC Converters MAX1790/MAX8715
SHDN BIAS
SKIP COMPARATOR SKIP SOFTSTART
4A
IN
SS
COMP ERROR AMPLIFIER FB ERROR COMPARATOR CONTROL AND DRIVER LOGIC CLOCK
LX N
1.24V
GND FREQ OSCILLATOR SLOPE COMPENSATION CURRENT SENSE
5A
MAX1790 MAX8715
Figure 2. Functional Diagram
Output-Current Capability
The output-current capability of the MAX1790/MAX8715 is a function of current limit, input voltage, operating frequency, and inductor value. Because of the slope compensation used to stabilize the feedback loop, the duty cycle affects the current limit. The output-current capability is governed by the following equation: IOUT(MAX) = [ILIM x (1.26 - 0.4 x Duty) 0.5 x Duty x VIN / (fOSC x L)] x x VIN / VOUT where: ILIM = current limit specified at 65% (see the Electrical Characteristics) Duty = duty cycle = (VOUT - VIN + VDIODE) / (VOUT - ILIM x RON + VDIODE) VDIODE = catch diode forward voltage at ILIM = conversion efficiency, 85% nominal
after the soft-start cycle is completed. When the shutdown pin is taken low, the soft-start capacitor is discharged to ground.
Frequency Selection
The MAX1790/MAX8715s' frequency can be user selected to operate at either 640kHz or 1.2MHz. Connect FREQ to GND for 640kHz operation. For a 1.2MHz switching frequency, connect FREQ to IN. This allows the use of small, minimum-height external components while maintaining low output noise. FREQ has an internal pulldown, allowing the user the option of leaving FREQ unconnected for 640kHz operation.
Shutdown
The MAX1790/MAX8715 are shut down to reduce the supply current to 0.1A when SHDN is low. In this mode, the internal reference, error amplifier, comparators, and biasing circuitry turn off while the n-channel MOSFET is turned off. The boost converter's output is connected to IN by the external inductor and catch diode.
Soft-Start
The MAX1790/MAX8715 can be programmed for softstart upon power-up with an external capacitor. When the shutdown pin is taken high, the soft-start capacitor (CSS) is immediately charged to 0.5V. Then the capacitor is charged at a constant current of 4A (typ). During this time, the SS voltage directly controls the peak inductor current, allowing 0A at VSS = 0.5V to the full current limit at VSS = 1.5V. The maximum load current is available
Applications Information
Boost DC-DC converters using the MAX1790/MAX8715 can be designed by performing simple calculations for a first iteration. All designs should be prototyped and tested prior to production. Table 1 provides a list of
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Low-Noise Step-Up DC-DC Converters MAX1790/MAX8715
Table 1. Component Selection
VIN (V) MAX1790 3.3 3.3 3.3 3.3 MAX8715 3.3 9 1.2M 6.8 (Sumida CLQ4D10-6R8) 3 x 3.3 ceramic (Taiyo Yuden LMK325BJ335MD) 82 750 10 150 12 12 5 5 640k 1.2M 640k 1.2M 10 (Sumida CDRH5D18-100NC) 5.4 (Sumida CDRH5D18-5R4NC) 5.4 (Sumida CDRH5D18-5R4NC) 2.7 (Sumida CDRH4D18-2R7) 33 tantalum (AVX TPSD336020R0200) 33 tantalum (AVX TPSD336020R0200) 47 tantalum (6TPA47M) 47 tantalum (6TPA47M) 120 180 62 91 1200 650 820 390 22 20 56 33 250 250 800 800 VOUT (V) fOSC (Hz) L (H) COUT (F) RCOMP (k) CCOMP (pF) CCOMP2 (pF) IOUT(MAX) (mA)
Table 2. Component Suppliers
SUPPLIER Inductors Coilcraft Coiltronics Sumida USA TOKO Capacitors AVX Kemet Sanyo Taiyo Yuden Diodes Central Semiconductor International Rectifier Motorola Nihon Zetex 516-435-1110 310-322-3331 602-303-5454 847-843-7500 516-543-7100 516-435-1824 310-322-3332 602-994-6430 847-843-2798 516-864-7630 803-946-0690 408-986-0424 619-661-6835 408-573-4150 803-626-3123 408-986-1442 619-661-1055 408-573-4159 847-639-6400 561-241-7876 847-956-0666 847-297-0070 847-639-1469 561-241-9339 847-956-0702 847-699-1194 PHONE FAX
as well as maximum and minimum input voltages. Begin by selecting an inductor value. Once L is known, choose the diode and capacitors.
Inductor Selection
The minimum inductance value, peak current rating, and series resistance are factors to consider when selecting the inductor. These factors influence the converter's efficiency, maximum output load capability, transientresponse time, and output voltage ripple. Physical size and cost are also important factors to be considered. The maximum output current, input voltage, output voltage, and switching frequency determine the inductor value. Very high inductance values minimize the current ripple and therefore reduce the peak current, which decreases core losses in the inductor and I2R losses in the entire power path. However, large inductor values also require more energy storage and more turns of wire, which increase physical size and can increase I2R losses in the inductor. Low inductance values decrease the physical size but increase the current ripple and peak current. Finding the best inductor involves choosing the best compromise between circuit efficiency, inductor size, and cost. The equations used here include a constant LIR, which is the ratio of the inductor peak-to-peak ripple current to the average DC inductor current at the full load current. The best trade-off between inductor size and circuit efficiency for step-up regulators generally has an LIR between 0.3 and 0.5. However, depending on the AC characteristics of the inductor core material and the
components for a range of standard applications. Table 2 lists component suppliers. External component value choice is primarily dictated by the output voltage and the maximum load current,
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Low-Noise Step-Up DC-DC Converters
ratio of inductor resistance to other power path resistances, the best LIR can shift up or down. If the inductor resistance is relatively high, more ripple can be accepted to reduce the number of turns required and increase the wire diameter. If the inductor resistance is relatively low, increasing inductance to lower the peak current can decrease losses throughout the power path. If extremely thin high-resistance inductors are used, as is common for LCD-panel applications, the best LIR can increase to between 0.5 and 1.0. Once a physical inductor is chosen, higher and lower values of the inductor should be evaluated for efficiency improvements in typical operating regions. Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current (IMAIN(MAX)), the expected efficiency (TYP) taken from an appropriate curve in the Typical Operating Characteristics, and an estimate of LIR based on the above discussion:
2 V VMAIN - VIN TYP L = IN VMAIN IMAIN(MAX) x fOSC LIR
MAX1790/MAX8715
9V - 3.3V 0.85 3.3V L= 6.8H 9V 0.15A x 1.2MHz 0.5 Using the circuit's minimum input voltage (3V) and estimating efficiency of 80% at that operating point: IIN(DC,MAX) = 0.15A x 9V 0.6A 3V x 0.8
2
The ripple current and the peak current are: IRIPPLE = 3V x (9V - 3V) 0.25A 6.8H x 9V x 1.2MHz 0.25A 0.725A 2
IPEAK = 0.6A +
Diode Selection
The output diode should be rated to handle the output voltage and the peak switch current. Make sure that the diode's peak current rating is at least IPK and that its breakdown voltage exceeds VOUT. Schottky diodes are recommended.
Choose an available inductor value from an appropriate inductor family. Calculate the maximum DC input current at the minimum input voltage VIN(MIN) using conservation of energy and the expected efficiency at that operating point (MIN) taken from an appropriate curve in the Typical Operating Characteristics: IIN(DC,MAX) = IMAIN(MAX) x VMAIN VIN(MIN) x MIN
Input and Output Capacitor Selection
Low-ESR capacitors are recommended for input bypassing and output filtering. Low-ESR tantalum capacitors are a good compromise between cost and performance. Ceramic capacitors are also a good choice. Avoid standard aluminum electrolytic capacitors. A simple equation to estimate input and outputcapacitor values for a given voltage ripple is as follows: 0.5 x L x IPK 2 C VRIPPLE x VOUT where VRIPPLE is the peak-to-peak ripple voltage on the capacitor.
Calculate the ripple current at that operating point and the peak current required for the inductor: IRIPPLE = VIN(MIN) x (VMAIN - VIN(MIN) ) L x VMAIN x fOSC
I IPEAK = IIN(DC,MAX) + RIPPLE 2 The inductor's saturation current rating and the MAX1790/MAX8715s' LX current limit (ILIM ) should exceed IPEAK and the inductor's DC current rating should exceed IIN(DC,MAX). For good efficiency, choose an inductor with less than 0.1 series resistance. Considering the application circuit in Figure 4, the maximum load current (IMAIN(MAX)) is 150mA with a 9V output and a typical input voltage of 3.3V. Choosing an LIR of 0.5 and estimating efficiency of 85% at this operating point:
Output Voltage
The MAX1790/MAX8715 operate with an adjustable output from VIN to 13V. Connect a resistor voltagedivider to FB (see the Typical Operating Circuit) from the output to GND. Select the resistor values as follows: V R1 = R2 OUT - 1 VFB where VFB, the boost-regulator feedback set point, is 1.24V. Since the input bias current into FB is typically 0,
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Low-Noise Step-Up DC-DC Converters MAX1790/MAX8715
R2 can have a value up to 100k without sacrificing accuracy. Connect the resistor-divider as close to the IC as possible.
VIN 2.6V TO 5.5V C1 10F 10V C2 10F LX
Loop Compensation
The voltage feedback loop needs proper compensation to prevent excessive output ripple and poor efficiency caused by instability. This is done by connecting a resistor (R COMP ) and capacitor (C COMP ) in series from COMP to GND, and another capacitor (CCOMP2) from COMP to GND. RCOMP is chosen to set the high-frequency integrator gain for fast transient response, while CCOMP is chosen to set the integrator zero to maintain loop stability. The second capacitor, CCOMP2, is chosen to cancel the zero introduced by output-capacitance ESR. For optimal performance, choose the components using the following equations: RCOMP (200 / A2) x VOUT2 x COUT / L (MAX1790) RCOMP (274 / A) x VIN x VOUT x COUT / (L x IOUT) (MAX8715) CCOMP (0.4 x 10 -3 A/) x L / VIN CCOMP (0.36 x 10 -3 A/) x L / VIN (MAX1790) (MAX8715)
0.027F L1A 5.3H IN
SHDN
D1
VOUT 3.3V
MAX1790
FREQ GND
L1B 5.3H
COUT 22F 20V
SS CC
FB
R2 605k CCOMP2 56pF RCOMP 22k CCOMP 330pF
R1 1M
CCOMP2 (0.005 A2/) x RESR x L / VOUT2 (MAX1790) CCOMP2 (0.0036 A/) x RESR x L x IOUT / (VIN x VOUT) (MAX8715) For the ceramic output capacitor, where ESR is small, CCOMP2 is optional. Table 1 shows experimentally verified external component values for several applications. The best gauge of correct loop compensation is by inspecting the transient response of the MAX1790/ MAX8715. Adjust RCOMP and CCOMP as necessary to obtain optimal transient performance.
L1 = CTX8-1P COUT = TPSD226025R0200
Figure 3. MAX1790 in a SEPIC Configuration
IOUT = maximum output current during power-up stage VIN = minimum input voltage The load must wait for the soft-start cycle to finish before drawing a significant amount of load current. The duration after which the load can begin to draw maximum load current is: tMAX = 6.77 x 105 CSS
Soft-Start Capacitor
The soft-start capacitor should be large enough that it does not reach final value before the output has reached regulation. Calculate CSS to be:
VOUT 2 - VIN x VOUT CSS > 21 x 10 -6 x COUT VIN x IINRUSH - IOUT x VOUT
Application Circuits 1-Cell to 3.3V SEPIC Power Supply
Figure 3 shows the MAX1790 in a single-ended primary inductance converter (SEPIC) topology. This topology is useful when the input voltage can be either higher or lower than the output voltage, such as when converting a single lithium-ion (Li+) cell to a 3.3V output. L1A and L1B are two windings on a single inductor. The coupling capacitor between these two windings must be a lowESR type to achieve maximum efficiency, and must also be able to handle high ripple currents. Ceramic capacitors are best for this application. The circuit in Figure 3 provides 400mA output current at 3.3V output when operating with an input voltage from +2.6V to +5.5V.
where: COUT = total output capacitance including any bypass capacitor on the output bus VOUT = maximum output voltage IINRUSH = peak inrush current allowed
12
______________________________________________________________________________________
Low-Noise Step-Up DC-DC Converters MAX1790/MAX8715
V2 +26V 5mA 1F
D2
0.1F 3.3F
D3
0.1F 1F
D4
V3 -9V 10mA
1F
D1
3.0V TO 3.6V C1 0.47F
L1 274k IN
MAX1790 MAX8715
C2 LX FB
C3
C4
V1 9V 150mA
FREQ
44.2k
SHDN
GND
COMP 150k (MAX1790) 82k (MAX8715) 470pF (MAX1790) 750pF (MAX8715) 18pF (MAX1790) 10pF (MAX8715)
SS
27nF C1, C2, C3, C4: TAIYO YUDEN LMK325BJ335MD (3.3F, 10V) D1: ZETEX ZHCS1000 (20V, 1A, SCHOTTKY) OR MOTOROLA MBRM120ET3 D2, D3, D4: ZETEX BAT54S (30V, 200mA, SCHOTTKY) L1: SUMIDA CLQ4D10-6R8 (6.8H, 0.8A) OR SUMITOMO CXLM120-6R8
Figure 4. Multiple-Output, Low-Profile (1.2mm max) TFT-LCD Power Supply
AMLCD Application Figure 4 shows a power supply for active matrix (TFTLCD) flat-panel displays. Output-voltage transient performance is a function of the load characteristic. Add or remove output capacitance (and recalculate compensation-network component values) as necessary to meet transient performance. Regulation performance for secondary outputs (V2 and V3) depends on the load characteristics of all three outputs.
Layout Procedure
Good PC board layout and routing are required in highfrequency switching power supplies to achieve good reg-
ulation, high efficiency, and stability. It is strongly recommended that the evaluation kit PC board layouts be followed as closely as possible. Place power components as close together as possible, keeping their traces short, direct, and wide. Avoid interconnecting the ground pins of the power components using vias through an internal ground plane. Instead, keep the power components close together and route them in a star ground configuration using component-side copper, then connect the star ground to internal ground using multiple vias.
Chip Information
TRANSISTOR COUNT: 1012
______________________________________________________________________________________
13
Low-Noise Step-Up DC-DC Converters MAX1790/MAX8715
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)
8LUMAXD.EPS
4X S
8
8
INCHES DIM A A1 A2 b MIN 0.002 0.030 MAX 0.043 0.006 0.037
MILLIMETERS MAX MIN 0.05 0.75 1.10 0.15 0.95
O0.500.1
E
H
0.60.1
c D e E H L
1
1
0.60.1
S
D
BOTTOM VIEW
0.014 0.010 0.007 0.005 0.120 0.116 0.0256 BSC 0.120 0.116 0.198 0.188 0.026 0.016 6 0 0.0207 BSC
0.25 0.36 0.13 0.18 2.95 3.05 0.65 BSC 2.95 3.05 5.03 4.78 0.41 0.66 0 6 0.5250 BSC
TOP VIEW
A2
A1
A
c e b L
SIDE VIEW
FRONT VIEW
PROPRIETARY INFORMATION TITLE:
PACKAGE OUTLINE, 8L uMAX/uSOP
APPROVAL DOCUMENT CONTROL NO. REV.
21-0036
1 1
J
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.

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