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19-1400; Rev 0; 11/98
ANUAL N KIT M LUATIO ATA SHEET EVA WS D FOLLO
Low-Noise, 5.5V-Input, PWM Step-Down Regulator
Features
o +2.7V to +5.5V Input Range o Adjustable Output from 1.25V to VIN o 600mA Guaranteed Output Current o 95% Efficiency o No Schottky Diode Required o 85A Quiescent Current o 100% Duty Cycle in Dropout o 750kHz Fixed-Frequency PWM Operation o Synchronizable Switching Frequency o Accurate Reference: 1.25V (1.2%) o Small 10-Pin MAX Package
General Description
The MAX1692 is a low-noise, pulse-width-modulated (PWM), DC-DC step-down converter. It powers logic and transmitters in small wireless systems such as cellular phones, communicating PDAs, and handy-terminals. The device features an internal synchronous rectifier for high efficiency; it requires no external Schottky diode. Excellent noise characteristics and fixed-frequency operation provide easy post-filtering. The MAX1692 is ideally suited for Li-Ion battery applications. It is also useful for +3V or +5V fixed input applications. The device operates in one of four modes. Forced PWM mode operates at a fixed frequency regardless of the load. Synchronizable PWM mode allows an external switching frequency to control and minimize harmonics. Idle ModeTM (PWM/PFM) extends battery life by switching to a PFM pulse-skipping mode during light loads. Shutdown mode places the device in standby, reducing quiescent supply current to under 0.1A. The MAX1692 can deliver over 600mA. The output voltage can be adjusted from 1.25V to VIN with the input range of +2.7V to +5.5V. Other features of the MAX1692 include high efficiency, low dropout voltage, and a 1.2%-accurate 1.25V reference. It is available in a space-saving 10-pin MAX package with a height of only 1.11mm.
MAX1692
Ordering Information
PART MAX1692EUB TEMP. RANGE -40C to +85C PIN-PACKAGE 10 MAX
Pin Configuration
TOP VIEW
IN 1 BP GND REF FB 2 3 4 5 10 PGND 9 LX SHDN SYNC/PWM LIM
Applications
Cellular Phones Cordless Phones CPU I/O Supplies Notebook Chipset Supplies
MAX1692
8 7 6
PDAs and Handy-Terminals Battery-Operated Devices (1 Li-Ion or 3 NiMH/NiCd)
MAX
Typical Operating Circuit
L VIN = 2.7V TO 5.5V C1 IN SHDN LIM R1 LX C2 VOUT = 1.25V TO VIN
MAX1692
BP C3 AGND REF
FB R2
SYNC/PWM PGND
C4
Idle Mode is a trademark of Maxim Integrated Products.
________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769.
Low-Noise, 5.5V-Input, PWM Step-Down Regulator MAX1692
ABSOLUTE MAXIMUM RATINGS
IN, BP, SHDN, SYNC/PWM, LIM to GND ................ -0.3V to +6V BP to IN .................................................................-0.3V to +0.3V PGND to GND ...................................................... -0.3V to +0.3V LX to PGND................................................. -0.3V to (VIN + 0.3V) FB, REF to GND ......................................... -0.3V to (VBP + 0.3V) Reference Current ............................................................. 1mA LX Peak Current (internally limited)...................................... 1.6A Continuous Power Dissipation (TA = +70C) 10-Pin MAX (derate 5.6mW/C above +70C) ............444mW Operating Temperature Range .......................... -40C to +85C Maximum Junction Temperature .................................... +150C 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
(VIN = +3.6V, SYNC/PWM = GND, VLIM = 3.6V, SHDN = IN, circuit of Figure 2; TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Input Voltage Range SYMBOL VIN FB = OUT, VIN = VLIM = 2.7V to 5.5V, IOUT = 0 Output Voltage VOUT FB = OUT, VIN = 2.7V to 5.5V, IOUT = 0 to 600mA, LIM = IN or IOUT = 0 to 250mA, LIM = GND (Note 1) VFB FB = OUT, VIN = VLIM = 5.5V, IOUT = 0 (duty cycle = 23%) (Note 2) Duty cycle = 100% to 23% IOUT = 0 to 600mA, LIM = IN or IOUT = 0 to 250mA, LIM = GND IFB PRDS(ON) NRDS(ON) VFB = 1.4V ILX = 180mA ILX = 180mA VIN = 3.6V VIN = 2.7V VIN = 3.6V VIN = 2.7V 0.35 0.75 -450 0 80 SYNC/PWM = GND, VFB = 1.4V, LX unconnected SHDN = LX = GND, includes LX leakage current VIN = 5.5V, VLX = 0 or 5.5V fOSC dutyMAX dutyMIN VREF IREF = 0 1.235 1.250 -20 650 500 100 22 1.265 -50 CONDITIONS MIN 2.7 1.223 1.249 TYP MAX 5.5 1.275 V 1.190 VREF 1.223 1.249 +1 -1.3 0.01 0.3 0.4 0.4 0.5 0.6 1.2 -850 50 120 85 0.1 0.1 750 50 0.65 0.8 0.85 1.55 -1600 100 160 140 10 20 830 1000 1.232 1.275 VIN 1.275 V V % % nA A mA mA A A A kHz kHz % % V UNITS V
Output Adjustment Range Feedback Voltage Line Regulation Load Regulation FB Input Current P-Channel On-Resistance N-Channel On-Resistance P-Channel Current-Limit Threshold N-Channel Current-Limit Threshold Pulse-Skipping Current-Limit Threshold Quiescent Current Shutdown Supply Current LX Leakage Current Oscillator Frequency SYNC Capture Range Maximum Duty Cycle Minimum Duty Cycle Reference Output Voltage 2
LIM = GND LIM = IN VFB = 1.4V SYNC/PWM = IN, FB = REF
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Low-Noise, 5.5V-Input, PWM Step-Down Regulator
ELECTRICAL CHARACTERISTICS (continued)
(VIN = +3.6V, SYNC/PWM = GND, VLIM = 3.6V, SHDN = IN, circuit of Figure 2; TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Reference Load Regulation Undervoltage Lockout Threshold Logic Input High Logic Input Low Logic Input Current SYNC/PWM Minimum Pulse Width UVLO VIH VIL SYMBOL 0 IREF 50A VIN rising, typical hysteresis is 85mV SHDN, SYNC/PWM, LIM SHDN, SYNC/PWM, LIM SHDN, SYNC/PWM, LIM High or low -1 500 0.1 2.3 2 0.4 1 CONDITIONS MIN TYP 3 2.4 MAX 15 2.5 UNITS mV V V V A ns
MAX1692
ELECTRICAL CHARACTERISTICS
(VIN = +3.6V, SYNC/PWM = GND, VLIM = 3.6V, SHDN = IN, circuit of Figure 2, TA = -40C to +85C, unless otherwise noted.) (Note 3) PARAMETER Input Voltage Range SYMBOL VIN FB = OUT, VIN = VLIM = 2.7V to 5.5V, IOUT = 0 Output Voltage VOUT FB = OUT, VIN = 2.7V to 5.5V, IOUT = 0 to 600mA, LIM = IN or IOUT = 0 to 250mA, LIM = GND (Note 1) VFB IFB FB = OUT, VIN = VLIM = 5.5V, IOUT = 0 (duty cycle = 23%) (Note 2) VFB =1.4V LIM = GND LIM = IN SYNC/PWM = IN, FB = REF SYNC/PWM = GND, LX = unconnected, VFB = 1.4V SHDN = LX = GND, includes LX leakage current fOSC VREF UVLO VIH VIL IREF = 0 VIN rising, typical hysteresis is 85mV SHDN, SYNC/PWM, LIM SHDN, SYNC/PWM, LIM SHDN, SYNC/PWM, LIM -1 630 1.230 2.3 2 0.4 1 CONDITIONS MIN 2.7 1.213 MAX 5.5 1.285 V 1.185 REF 1.213 -50 0.3 0.7 -15 1.285 VIN 1.285 50 0.9 1.6 110 140 10 840 1.268 2.5 V V nA A mA A A kHz V V V V A UNITS V
Output Adjustment Range Feedback Voltage FB Input Current P-Channel Current-Limit Threshold N-Channel Current-Limit Threshold Quiescent Current Shutdown Supply Current Oscillator Frequency Reference Output Voltage Undervoltage Lockout Threshold Logic Input High Logic Input Low Logic Input Current
Note 1: Guaranteed by minimum and maximum duty-factor tests. Note 2: The following equation can be used to calculate FB accuracy for output voltages other than 1.232V: (see Feedback Voltage vs. Load Current) VFB = VFB (NOMINAL) - (Line Reg) (VOUT / VIN - 0.23) / 0.77 - (Load Reg)(IOUT + 0.5 * IRIPPLE) / IMAX where: Line Reg = the line regulation Load Reg = the load regulation IRIPPLE = (1- VOUT / VIN) * VOUT / (fOSC * L) where L is the inductor value IMAX = 250mA (LIM = GND) or 600mA (LIM = IN) Note 3: Specifications to -40C are guaranteed by design, not production tested. _______________________________________________________________________________________ 3
Low-Noise, 5.5V-Input, PWM Step-Down Regulator MAX1692
Typical Operating Characteristics
(SYNC/PWM = GND, circuit of Figure 2, L = Sumida CD43-100, TA = +25C, unless otherwise noted.)
DROPOUT VOLTAGE vs. LOAD CURRENT
MAX1692-01
EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V)
MAX1692-02
EFFICIENCY vs. LOAD CURRENT (VOUT = 2.5V)
95 90 EFFICIENCY (%)
MAX1692-03
600 500 DROPOUT VOLTAGE (mV) 400 300 200 100 0 0 150 300 450 600 750 VOUT = 2.5V
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 LIM = IN R1 = 505k R2 = 301k 1 10 100 VIN = 5.0V VIN = 3.6V
100
85 80 75 70 65 60 55 50 1 10 VIN = 5.0V VIN = 3.6V
VIN = 2.7V
VOUT = 3.3V
LIM = IN R1 = 309k R2 = 301k 100 1000
900
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
EFFICIENCY vs. LOAD CURRENT (VOUT = 1.8V)
MAX1692-04
FEEDBACK VOLTAGE vs. LOAD CURRENT
MAX1692-05
BATTERY INPUT CURRENT vs. INPUT VOLTAGE
95 BATTERY INPUT CURRENT (A) 90 85 80 75 70 65 60 TA = -40C VOUT = 1.8V SYNC/PWM = GND 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 TA = +25C
MAX1692-06
100 95 90 EFFICIENCY (%) VIN = 3.6V VIN = 5.0V VIN = 2.7V
1.25 1.245 1.24 FB VOLTAGE (V) 1.235 1.23 1.225 1.22 1.215 LIM = GND LIM = IN VIN = 5.0V R1 = 309k R2 = 301k SYNC/PWM = GND
100 TA = +85C
85 80 75 70 65 60 55 50 1 10 100 1000 LOAD CURRENT (mA) LIM = IN R1 = 138k R2 = 301k
1.21 1.205 1.2 0 100 200 300 400 500 600 700 800 900 1000 LOAD CURRENT (mA)
INPUT VOLTAGE (V)
BATTERY INPUT CURRENT vs. INPUT VOLTAGE
MAX1692-07
BATTERY INPUT CURRENT vs. INPUT VOLTAGE AND TEMPERATURE
MAX1692-09
OUTPUT VOLTAGE vs. LOAD CURRENT
VIN = 2.7V VOUT = 1.8V R1 = 138k R2 = 301k
MAX1692-10
5.0 4.5 BATTERY INPUT CURRENT (mA) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 SYNC/PWM = IN VOUT = 1.8V VOUT = 2.5V VOUT = 3.3V
2.5 BATTERY INPUT CURRENT (mA)
1.84
2.0
OUTPUT VOLTAGE (V)
TA = +85C
1.82
1.80
TA = +25C 1.5 TA = -40C VOUT = 1.8V SYNC/PWM = IN 1.0
1.78
1.76
1.74 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 0 100 200 300 400 500 600 700 800 900 LOAD CURRENT (mA) INPUT VOLTAGE (V)
5.5
INPUT VOLTAGE (V)
4
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Low-Noise, 5.5V-Input, PWM Step-Down Regulator
Typical Operating Characteristics (continued)
(SYNC/PWM = GND, TA = +25C, unless otherwise noted.)
MAX1692
OSCILLATOR FREQUENCY vs. SUPPLY VOLTAGE
MAX1692-11
MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE
MAX1692-12
START-UP FROM SHUTDOWN
VSHDN 2V/div
MAX1692-14
800 TA = +85C OSCILLATOR FREQUENCY (kHz) 750 TA = +25C 700 TA = -40C 650 IOUT = 200mA 600 2.7 3.1 3.5 3.9 4.3 4.7 5.1
1.4 MAXIMUM OUTPUT CURRENT (A)
LIM = IN 1.1
VOUT 1V/div 0.8 LIM = GND 0.5 VOUT = 1.8V 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 2ms/div INPUT VOLTAGE (V) IIN 0.5A/div
5.5
SUPPLY VOLTAGE (V)
HEAVY LOAD SWITCHING WAVEFORMS
MAX1692-15
LOAD-TRANSIENT RESPONSE
MAX1692-17
LINE-TRANSIENT RESPONSE
VIN ACCOUPLED 2V/div VOUT ACCOUPLED 50mV/div
MAX1692-18
VLX 5V/div ILX 0.5A/div
VLX 5V/div
IOUT 2.5A/div VOUT ACCOUPLED 100mV/div VOUT AC-COUPLED 100mV/div 2ms/div VIN = 5V, VOUT = 3.3V, IOUT = 700mA 500s/div ILOAD = 30mA to 700mA
2ms/div VIN = 3V to 5V, IOUT = 300mA
RECOVERY FROM 100% DUTY CYCLE
MAX1692-19
SWITCHING HARMONICS AND NOISE
MAX1692-22
VIN 2V/div
VLX 5V/div VOUT ACCOUPLED 500mV/div
1mV/div
2ms/div VIN = 3.3V to 4.5V , VOUT = 3.3V, IOUT = 500mA
100kHz IOUT = 500mA
1MHz 1ms/div
10MHz
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5
Low-Noise, 5.5V-Input, PWM Step-Down Regulator MAX1692
Pin Description
PIN 1 2 3 4 5 6 7 NAME IN BP GND REF FB LIM SYNC/ PWM SHDN LX PGND FUNCTION Supply Voltage Input. Input range from +2.7V to +5.5V. Bypass with a 10F capacitor. Supply Bypass Pin. Internally connected to IN. Bypass with a 0.1F capacitor. Do not connect to an external power source other than IN. Ground 1.25V, 1.2% Reference Output. Capable of delivering 50A to external loads. Bypass with a 0.22F capacitor to GND. Feedback Input Current-Limit Select Input. Connect LIM to GND for 0.6A current limit or LIM to IN for 1.2A current limit. Oscillator Sync and Low-Noise, Mode-Control Input. SYNC/PWM = IN (Forced PWM Mode) SYNC/PWM = GND (PWM/PFM Mode) An external clock signal connected to this pin allows for LX switching synchronization. Active-Low, Shutdown-Control Input. Reduces quiescent current to 0.1A. In shutdown, output becomes high impedance. Inductor Connection to the Drains of the Internal Power MOSFETs Power Ground
8 9 10
CHIP SUPPLY PFM CURRENT COMPARATOR SHDN 10
BP
MAX1692
IN
REF REF GND 12mV 120mV LIM COMPARATOR 0.1X
1
P LX
SENSE FET PWM COMPARATOR CONTROL AND DRIVER LOGIC FB REF PWM ON SIGNAL FB ON ON REF PFM COMPARATOR REF OVERVOLTAGE COMPARATOR FB NEGLIM COMPARATOR 5mV IN PFM ADJ. IN PWM 40mV 1 PGND
RAMP GEN SYNC CELL PWM SLOPE COMPENSATION
SENSE FET 0.1X N
SYNC/ PWM
Figure 1. Simplified Functional Diagram
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Low-Noise, 5.5V-Input, PWM Step-Down Regulator
Detailed Description
The MAX1692 step-down, pulse-width-modulated (PWM), DC-DC converter has an adjustable output range from 1.25V to the input voltage. An internal synchronous rectifier improves efficiency and eliminates an external Schottky diode. Fixed-frequency operation enables easy post-filtering, thereby providing excellent noise characteristics. As a result, the MAX1692 is an ideal choice for many small wireless systems. The MAX1692 accepts inputs as low as +2.7V while still delivering 600mA. The MAX1692 can operate in four modes to optimize performance. A forced (PWM) mode switches at a fixed frequency, regardless of load, for easy post-filtering. A synchronizable PWM mode uses an external clock to minimize harmonics. A PWM/PFM mode extends battery life by operating in PWM mode under heavy loads and PFM mode under light loads for reduced power consumption. Shutdown mode reduces quiescent current to 0.1A.
VIN +2.7V TO +5.5V IN C1 10F LIM LX C2 47F L1 10H VOUT = 1.8V @ 600mA
MAX1692
MAX1692
ON/OFF C4 0.22F SHDN REF BP C3 0.1F SYNC/ PWM GND PGND C5 47pF FB R2 300k R1 138k
PWM Control Scheme
The MAX1692 uses a slope-compensated, currentmode PWM controller capable of achieving 100% duty cycle. The device uses an oscillator-triggered, minimum on-time, current-mode control scheme. The minimum on-time is approximately 150ns unless in dropout. The maximum on-time is approximately 2/fOSC, allowing operation to 100% duty cycle. Current-mode feedback provides cycle-by-cycle current limiting for superior load- and line-response and protection of the internal MOSFET and rectifier. At each falling edge of the internal oscillator, the SYNC cell sends a PWM ON signal to the control and drive logic, turning on the internal P-channel MOSFET (main switch) (Figure 1). This allows current to ramp up through the inductor (Figure 2) to the load, and stores energy in a magnetic field. The switch remains on until either the current-limit (LIM) comparator is tripped or the PWM comparator signals that the output is in regulation. When the switch turns off during the second half of each cycle, the inductor's magnetic field collapses, releasing the stored energy and forcing current through the N-channel synchronous rectifier to the output-filter capacitor and load. The output-filter capacitor stores charge when the inductor current is high and releases it when the inductor current is low, thus smoothing the voltage across the load. During normal operation, the MAX1692 regulates output voltage by switching at a constant frequency and then modulating the power transferred to the load each cycle using the PWM comparator. A multi-input comparator sums three weighted differential signals: the
Figure 2. Standard Application Circuit
output voltage with respect to the reference, the main switch current sense, and the slope-compensation ramp. It modulates output power by adjusting the inductor-peak current during the first half of each cycle, based on the output-error voltage. The MAX1692's loop gain is relatively low to enable the use of a small, lowvalued output-filter capacitor. The resulting load regulation is 1.3% (typ) at 0 to 600mA.
100% Duty-Cycle Operation
The maximum on-time can exceed one internal oscillator cycle, which permits operation up to 100% duty cycle. As the input voltage drops, the duty cycle increases until the P-channel MOSFET is held on continuously. Dropout voltage in 100% duty cycle is the output current multiplied by the on-resistance of the internal switch and inductor, around 280mV (I OUT = 600mA). In PWM mode, subharmonic oscillation can occur near dropout but subharmonic voltage ripple is small, since the ripple current is low.
Synchronous Rectification
An N-channel, synchronous-rectifier improves efficiency during the second half of each cycle (off time). When the inductor current ramps below the threshold set by the NEGLIM comparator (Figure 1) or when the PWM reaches the end of the oscillator period, the synchronous rectifier turns off. This keeps excess current from flowing backward through the inductor, from the output-filter capacitor to GND, or through the switch and synchronous rectifier to GND. During PWM operation, the NEGLIM threshold adjusts to permit small
7
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Low-Noise, 5.5V-Input, PWM Step-Down Regulator MAX1692
amounts of reverse current to flow from the output during light loads. This allows regulation with a constantswitching frequency and eliminates minimum load requirements. The NEGLIM comparator threshold is 50mA if VFB < 1.25V, and decreases as VFB exceeds 1.25V to prevent the output from rising. The NEGLIM threshold in PFM mode is fixed at 50mA. (See Forced PWM and PWM/PFM Operation section.)
SYNC Input and Frequency Control
The MAX1692's internal oscillator is set for a fixedswitching frequency of 750kHz or can be synchronized to an external clock. Connect SYNC to IN for forcedPWM operation. Do not leave SYNC/PWM unconnected. Connecting SYNC/PWM to GND enables PWM/PFM operation to reduce supply current at light loads. SYNC/PWM is a negative-edge triggered input that allows synchronization to an external frequency ranging between 500kHz and 1000kHz. When SYNC/PWM is clocked by an external signal, the converter operates in forced PWM mode. If SYNC is low or high for more than 100s, the oscillator defaults to 750kHz.
Forced PWM and PWM/PFM Operation
Connect SYNC/PWM to IN for normal forced PWM operation. Forced PWM operation is desirable in sensitive RF and data-acquisition applications, to ensure that switching-noise harmonics do not interfere with sensitive IF and data-sampling frequencies. A minimum load is not required during forced PWM operation, since the synchronous rectifier passes reverse-inductor current as needed to allow constant-frequency operation with no load. Forced PWM operation uses higher supply current with no load (2mA typ). Connecting SYNC/PWM to GND enables PWM/PFM operation. This proprietary control scheme overrides PWM mode and places the MAX1692 in PFM mode at light loads to improve efficiency and reduce quiescent current to 85A. With PWM/PFM enabled, the MAX1692 initiates pulse-skipping PFM operation when the peak inductor current drops below 120mA. During PFM operation, the MAX1692 switches only as needed to service the load, reducing the switching frequency and associated losses in the internal switch, the synchronous rectifier, and the external inductor. During PFM mode, a switching cycle initiates when the PFM comparator senses that the output voltage has dropped too low. The P-channel MOSFET switch turns on and conducts current to the output-filter capacitor and load until the inductor current reaches the PFM peak current limit (120mA). Then the switch turns off and the magnetic field in the inductor collapses, forcing current through the synchronous rectifier to the output filter capacitor and load. Then the MAX1692 waits until the PFM comparator senses a low output voltage again. The PFM current comparator controls both entry into PWM mode and the peak switching current during PFM mode. Consequently, some jitter is normal during transition from PFM to PWM modes with loads around 100mA, and it has no adverse impact on regulation. Output ripple is higher during PFM operation. A larger output-filter capacitor can be used to minimize ripple.
Shutdown Mode
Connecting SHDN to GND places the MAX1692 in shutdown mode. In shutdown, the reference, control circuitry, internal switching MOSFET, and the synchronous rectifier turn off and the output falls to 0V. Connect SHDN to IN for normal operation.
Current-Sense Comparators
The MAX1692 uses several internal current-sense comparators. In PWM operation, the PWM comparator sets the cycle-by-cycle current limit (Figure 1) and provides improved load and line response, allowing tighter specification of the inductor-saturation current limit to reduce inductor cost. A second 120mA current-sense comparator used across the P-channel switch controls entry into PFM mode. A third current-sense comparator monitors current through the internal N-channel MOSFET to set the NEGLIM threshold and determine when to turn off the synchronous rectifier. A fourth comparator (LIM) used at the P-channel MOSFET switch detects overcurrent. This protects the system, external components, and internal MOSFETs under overload conditions.
Applications Information
Output Voltage Selection
Select an output voltage between 1.25V and V IN by connecting FB to a resistor-divider between the output and GND (Figure 2). Select feedback resistor R2 in the 5k to 500k range. R1 is then given by: R1 = R2 [(VOUT / VFB) - 1] where V FB = 1.232V (See Note 2 of the Electrical Characteristics). Add a small ceramic capacitor (C5) around 47pF to 100pF in parallel with R1 to compensate for stray capacitance at the FB pin and output capacitor equivalent series resistance (ESR).
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Low-Noise, 5.5V-Input, PWM Step-Down Regulator
Capacitor Selection
Choose input- and output-filter capacitors to service inductor currents with acceptable voltage ripple. The input-filter capacitor also reduces peak currents and noise at the voltage source. In addition, connect a lowESR bulk capacitor (>10F suggested) to the input. Select this bulk capacitor to meet the input ripple requirements and voltage rating, rather than capacitor size. Use the following equation to calculate the maximum RMS input current: IRMS = IOUT[VOUT (VIN - VOUT)]1/2 * VIN When selecting an output capacitor, consider the output-ripple voltage and approximate it as the product of the ripple current and the ESR of the output capacitor. VRIPPLE = [VOUT (VIN - VOUT)] / [2 * fOSC(L) (VIN)] * ESRC2 ESR values are met: C2 > 2VREF(1 + VOUT/VIN(MIN)) / (VOUT * RSENSE * fOSC) RESR < (RSENSE)(VOUT) / (VREF) where C2 is the output filter capacitor, VREF is the internal reference voltage of 1.25V, VIN(min) is the minimum input voltage (2.7V), RSENSE is the internal sense resistance of 0.1, and fOSC is the internal oscillator frequency (typically 750kHz). These equations provide the minimum requirements. The value of C2 may need to be increased for operation at duty-cycle extremes. Tables 1 and 2 provide recommended inductor and capacitor sizes at various external sync frequencies. Table 3 lists suppliers for the various components used with the MAX1692.
MAX1692
Standard Application Circuits
Figures 2 and 3 are standard application circuits optimized for power and board space respectively. The circuit of Figure 2 is the most general of the two, and generates 1.8V at 600mA. The circuit of Figure 3 is optimized for smallest overall size. Cellular phones are using low voltage for baseband logic and have critical area and height restrictions. This circuit operates from a single Li-ion battery (2.9V to 4.5V) and delivers up to 200mA at 1.8V. It uses small ceramic capacitors at the input and output and a tiny chip inductor such as the NLC322522T series from TDK. With the MAX1692 in a 10-pin MAX package, the entire circuit can fit in only 60mm2 and have less than 2.4mm height.
where ESRC2 is the equivalent-series resistance of the output capacitor. The MAX1692's loop gain is relatively low, enabling the use of small, low-value output filter capacitors. Higher values provide improved output ripple and transient response. Lower oscillator frequencies require a largervalue output capacitor. When PWM/PFM is used, verify capacitor selection with light loads during PFM operation, since output ripple is higher under these conditions. Low-ESR capacitors are recommended. Capacitor ESR is a major contributor to output ripple (usually more than 60%). Ordinary aluminum-electrolytic capacitors have high ESR and should be avoided. Low-ESR aluminum-electrolytic capacitors are acceptable and relatively inexpensive. Low-ESR tantalum capacitors are better and provide a compact solution for space-constrained surface-mount designs. Do not exceed the ripple-current ratings of tantalum capacitors. Ceramic capacitors have the lowest ESR overall, and OS-CONTM capacitors have the lowest ESR of the high-value electrolytic types. It is generally not necessary to use ceramic or OS-CON capacitors for the MAX1692; consider them only in very compact, high-reliability, or wide-temperature applications where the expense is justified. When using verylow-ESR capacitors, such as ceramic or OS-CON, check for stability while examining load-transient response. The output capacitor is determined by ensuring that the minimum capacitance value and maximum
VIN +2.9V TO +4.5V IN C5 4.7F BP LX
L1 10H
VOUT = 1.8V @ 200mA C2 10F 10F
MAX1692
ON/OFF C4 0.1F SHDN C5 47pF FB R2 301k GND PGND R1 138k
REF LIM SYNC/ PWM
OS-CON is a trademark of Sanyo Corp.
Figure 3. Miniaturized 200mA Output Circuit Fits in 60mm2
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Low-Noise, 5.5V-Input, PWM Step-Down Regulator MAX1692
Bypass Considerations
Bypass IN and OUT to PGND with 10F and 47F, respectively. Bypass BP and REF to GND with 0.1F and 0.22F, respectively. Locate the bypass capacitors as close as possible to their respective pins to minimize noise coupling. For optimum performance, place input and output capacitors as close to the device as feasible (see Capacitor Selection section). of which can result in instability or regulation errors. Connect the inductor, input filter capacitor, and output filter capacitor as close together as possible, and keep their traces short, direct, and wide. Connect their ground pins at a single common node in a star-ground configuration. The external voltage-feedback network should be very close to the FB pin, within 0.2in (5mm). Keep noisy traces, such as from the LX pin, away from the voltage-feedback network; also keep them separate, using grounded copper. Connect GND and PGND at the highest quality ground. The MAX1692 evaluation kit manual illustrates an example PC board layout and routing scheme.
PC Board Layout and Routing
High switching frequencies and large peak currents make PC board layout a very important part of design. Good design minimizes excessive EMI on the feedback paths and voltage gradients in the ground plane, both
Table 1. Suggested Inductors
OUTPUT VOLTAGE RANGE (V) INDUCTOR L VALUE (H) SUGGESTED INDUCTORS Sumida CD43-100 Coilcraft D01608C-103 Sumida CD54-100 TDK NLC322522-100T Sumida CD43-220 Sumida CD54-220 Sumida CD43-330 Sumida CD54-330
Table 3. Component Suppliers
COMPANY AVX Coilcraft Coiltronics Kemet Nihon Sanyo Sprague Sumida PHONE 843-946-0238 847-639-6400 561-241-7876 408-986-0424 USA 805- 867-2555 Japan 81-3-3494-7411 USA 619-661-6835 Japan 81-7-2070-6306 603-224-1961 USA 847-956-0666 Japan 81-3-3607-5111 408-573-4150 847-390-4373 FAX 843-626-3123 847-639-1469 561-241-9339 408-986-1442 805- 867-2698 81-3-3494-7414 619-661-1055 81-7-2070-1174 603- 224-1430 847- 956-0702 81-3-3607-5144 408-573-4159 847-390-4428
1.25 to 2.5
10
2.5 to 4.0 4.0 to 5.5
22 33
Table 2. Suggested Capacitors
MANUFACTURER PART NUMBER AVX TPSD476M016R0150 Sanyo 6TPA47M Sprague 594D686X9010C2T Taiyo Yuden JMK325BJ106MN TYPE Tantalum Poscap Tantalum Ceramic ESR (m) 150 100 95 50
Taiyo Yuden TDK
10
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Low-Noise, 5.5V-Input, PWM Step-Down Regulator
Chip Information
TRANSISTOR COUNT: 1462
MAX1692
Package Information
10LUMAXB.EPS
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11
Low-Noise, 5.5V-Input, PWM Step-Down Regulator MAX1692
NOTES
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.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1998 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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