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19-1457; Rev 0; 5/99
L MANUA ION KIT ALUAT DATA SHEET EV WS FOLLO
2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches
General Description Features
o 1% Output Accuracy o 95% Efficiency o Internal PMOS and NMOS Switches 70m On-Resistance at VIN = +4.5V 100m On-Resistance at VIN = +3V o Output Voltage +3.3V or +2.5V Pin-Selectable +1.1V to VIN Adjustable o +3V to +5.5V Input Voltage Range o 360A (max) Operating Supply Current o < 1A Shutdown Supply Current o Programmable Constant-Off-Time Operation o 350kHz (max) Switching Frequency o Idle Mode Operation at Light Loads o Thermal Shutdown o Adjustable Soft-Start Inrush Current Limiting o 100% Duty Cycle During Low-Dropout Operation o Output Short-Circuit Protection o 16-Pin SSOP Package
MAX1644
The MAX1644 constant-off-time, PWM step-down DCDC converter is ideal for use in applications such as PC cards, CPU daughter cards, and desktop computer bus-termination boards. The device features internal synchronous rectification for high efficiency and reduced component count. It requires no external Schottky diode. The internal 0.10 PMOS power switch and 0.10 NMOS synchronous-rectifier switch easily deliver continuous load currents up to 2A. The MAX1644 produces a preset +3.3V or +2.5V output voltage or an adjustable output from +1.1V to VIN. It achieves efficiencies as high as 95%. The MAX1644 uses a unique current-mode, constantoff-time, PWM control scheme, which includes an Idle ModeTM to maintain high efficiency during light-load operation. The programmable constant-off-time architecture sets switching frequencies up to 350kHz, allowing the user to optimize performance trade-offs between efficiency, output switching noise, component size, and cost. The device also features an adjustable soft-start to limit surge currents during start-up, a 100% duty cycle mode for low-dropout operation, and a lowpower shutdown mode that disconnects the input from the output and reduces supply current below 1A. The MAX1644 is available in a 16-pin SSOP package.
Applications
+5V to +3.3V/+2.5V Conversion CPU I/O Supply +3.3V PC Card and CardBus Applications Notebook and Subnotebook Computers Desktop Bus-Termination Boards CPU Daughter Card Supply
Ordering Information
PART MAX1644EAE TEMP. RANGE -40C to +85C PIN-PACKAGE 16 SSOP
Pin Configuration
TOP VIEW
OUTPUT +1.1V TO VIN
Typical Operating Circuit
INPUT +3V TO +5.5V IN LX SHDN 1 IN 2 LX 3 IN 4 GND REF SS SS 5 COMP 6 TOFF 7 RTOFF FB 8 16 LX 15 PGND 14 LX
MAX1644 FB
VCC FBSEL SHDN COMP TOFF PGND
MAX1644
13 PGND 12 VCC 11 FBSEL 10 REF 9 GND
Idle Mode is a trademark of Maxim Integrated Products.
SSOP 1
________________________________________________________________ Maxim Integrated Products
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.
2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches MAX1644
ABSOLUTE MAXIMUM RATINGS
VCC, IN to GND ........................................................-0.3V to +6V IN to VCC .............................................................................0.3V GND to PGND.....................................................................0.3V LX to PGND .................................................-0.3V to (VIN + 0.3V) All Other Pins to GND.................................-0.3V to (VCC + 0.3V) Continuous LX Output Current..............................................2.5A REF Short Circuit to GND Duration ............................Continuous ESD Protection .....................................................................2kV Continuous Power Dissipation (TA = +70C) SSOP (derate 16.7mW/C above +70C; part mounted on 1 in.2 of 1oz. copper) ............................1.2W Operating Temperature Range ...........................-40C to +85C Storage Temperature Range .............................-65C to +150C 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 = VCC = +3.3V, FBSEL = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Input Voltage SYMBOL VIN, VCC VIN = VCC = 4V to 5.5V, FBSEL = unconnected Preset Output Voltage VOUT ILOAD = 0 to 2A, VIN = VCC = 3V to 5.5V, VFB = VOUT FBSEL = VCC VIN = VCC = 3V to 5.5V, FBSEL = REF Adjustable Output Voltage Range AC Load Regulation Error DC Load Regulation Error Dropout Voltage Reference Voltage Reference Load Regulation PMOS Switch On-Resistance NMOS Switch On-Resistance Current-Limit Threshold Idle Mode Current Threshold Switching Frequency No-Load Supply Current Shutdown Supply Current PMOS Switch Off-Leakage Current Thermal Shutdown Threshold VDO VREF VREF RON, P RON, N ILIMIT IIM f IIN + ICC ICC(SHDN) IIN TSHDN (Note 1) VFB = 1.2V SHDN = GND SHDN = GND Hysteresis = 15C 150 240 <1 IREF = -1A to +10A ILX = 0.5A ILX = 0.5A VIN = 4.5V VIN = 3V VIN = 4.5V VIN = 3V 2.5 0.25 VIN = VCC = 3V to 5.5V, ILOAD = 0, FBSEL = GND or REF FBSEL = GND FBSEL = REF, VCC, or unconnected FBSEL = GND FBSEL = REF, VCC, or unconnected VIN = VCC = 3V, ILOAD = 1A, FBSEL = VCC 1.089 1.100 0.5 70 100 70 100 2.9 0.45 CONDITIONS MIN 3.0 3.300 2.500 1.089 VREF 1 2 0.2 0.4 200 1.111 1 150 200 150 200 3.3 0.65 350 360 3 15 3.333 2.525 1.100 TYP MAX 5.5 3.366 2.550 1.111 VIN V % % mV V mV m m A A kHz A A A C V UNITS V
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2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VCC = +3.3V, FBSEL = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Undervoltage Lockout Threshold FB Input Bias Current Off-Time Default Period Off-Time Start-Up Period On-Time Period SS Source Current SS Sink Current SHDN Input Current SHDN Input Low Threshold SHDN Input High Threshold FBSEL Input Current SYMBOL VUVLO IFB tOFF tOFF tON ISS ISS I SHDN VIL VIH FBSEL = GND FBSEL = REF FBSEL Logic Thresholds FBSEL = unconnected FBSEL = VCC 2.0 -5 0.9 0.7 * VCC - 0.2 VCC - 0.2 +5 0.2 1.3 0.7 * VCC + 0.2 CONDITIONS VIN falling, hysteresis = 40mV VFB = 1.2V RTOFF = 150k RTOFF = 30.1k RTOFF = 499k FB = GND 0.4 3.5 100 -0.5 MIN 2.5 0 1.13 0.20 TYP 2.6 80 1.33 0.33 4.3 4 * tOFF 5 6.5 0.5 0.8 MAX 2.7 200 1.53 5.6 s s A A A V V A UNITS V nA s
MAX1644
VSS = 1V V SHDN = 0 to VCC
V
ELECTRICAL CHARACTERISTICS
(VIN = VCC = +3.3V, FBSEL = GND, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 2) PARAMETER Input Voltage SYMBOL VIN VIN = VCC = 4V to 5.5V, ILOAD = 0 to 2A, FBSEL = unconnected VIN = 3V to 5.5V, FBSEL = VCC VFB = VOUT VIN = 3V to 5.5V, FBSEL = REF VIN = 3.0V to 5.5V, ILOAD = 0, FBSEL = GND or REF VREF RON, P RON, N ILIMIT IIM IIN + ICC IFB tOFF VFB = 1.2V VFB = 1.2V RTOFF = 150k ILX = 0.5A ILX = 0.5A VIN = 4.5V VIN = 3V VIN = 4.5V VIN = 3V 2.3 0.2 CONDITIONS MIN 3.0 3.276 2.48 1.08 VREF 1.08 TYP MAX 5.5 3.390 2.57 1.12 VIN 1.12 150 200 150 200 3.5 0.7 360 250 1.63 V UNITS V
Preset Output Voltage
VOUT
Adjustable Output Voltage Reference Voltage PMOS Switch On-Resistance NMOS Switch On-Resistance Current-Limit Threshold Idle Mode Current Threshold No-Load Supply Current FB Input Bias Current Off-Time Default Period
V V m m A A A nA s
0 1.03
Note 1: Recommended operating frequency, not production tested. Note 2: Specifications from 0C to -40C are guaranteed by design, not production tested. _______________________________________________________________________________________ 3
2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches MAX1644
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
DC LOAD-REGULATION ERROR vs. OUTPUT CURRENT
MAX1644-02
EFFICIENCY vs. OUTPUT CURRENT
MAX1644-01
EFFICIENCY vs. OUTPUT CURRENT
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 VIN = 5V, VOUT = 1.5V, L = 6.0H, RTOFF = 270k VIN = 3.3V, VOUT = 1.5V, L = 4.7H, RTOFF = 200k 0 DC LOAD-REGULATION ERROR (%) -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 0.01 0.1 1 10 -1.0 0.0001
90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.001 0.01 0.1 1 VIN = 5V, VOUT = 3.3V, L = 6.0H, RTOFF = 120k
A
B
D E C
10
0 0.001
0.001
0.01
0.1
1
10
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
OUTPUT CURRENT (A) A: VIN = 3.3V, VOUT = 1.5V, L = 4.7H, RTOFF = 200k, FBSEL = GND B: VIN = 3.3V, VOUT = 1.5V, L = 4.7H, RTOFF = 200k, FBSEL = REF C: VIN = 5V, VOUT = 3.3V, L = 6.0H, RTOFF = 120k, FBSEL = OPEN D: VIN = 5V, VOUT = 1.5V, L = 6.0H, RTOFF = 270k, FBSEL = GND E: VIN = 5V, VOUT = 1.5V, L = 6.0H, RTOFF = 270k, FBSEL = REF
SWITCHING FREQUENCY vs. OUTPUT CURRENT
MAX1644-04
OFF-TIME vs. RTOFF
4.0 3.5
MAX1644-06
350 300 SWITCHING FREQUENCY (kHz) 250 200 150 100 50 0 0 0.5 1.0 1.5 VIN = 5V, VOUT = 3.3V, L = 6.0H, RTOFF = 120k VIN = 5V, VOUT = 1.5V, L = 6.0H, RTOFF = 270k
4.5
VIN = 3.3V, VOUT = 1.5V, L = 4.7H, RTOFF = 200k
3.0 tOFF (s) 2.5 2.0 1.5 1.0 0.5 0
2.0
0
100
200
300 RTOFF (k)
400
500
600
OUTPUT CURRENT (A)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
500 450 SUPPLY CURRENT ICC (A) 400 350 300 250 200 150 100 50 0 0 1 2 3 4 5 6 SUPPLY VOLTAGE UNDERVOLTAGE LOCKOUT SHDN = GND SHDN = VIN = VCC IOUT = 0
MAX1644-07
START-UP AND SHUTDOWN TRANSIENTS
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 SHUTDOWN SUPPLY CURRENT IIN + ICC (A) 0
MAX1644-09
0.10
VSHDN 5V/div IIN 1A/div VOUT 2V/div
0
0
VSS 1V/div
0
VIN = 5.0V, VOUT = 3.3V, IOUT = 2A 2ms/div
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MAX1644-03
100
2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
LOAD-TRANSIENT RESPONSE (FBSEL = REF)
MAX1644-10
MAX1644
LINE-TRANSIENT RESPONSE
4V VIN 3V VOUT = 1.5V, IOUT = 2A VOUT 50mV/div 2A IL 0
VIN = 3.3V, VOUT = 1.5V
VOUT 20mV/div
20s/div
20s/div
Pin Description
PIN 1 2, 4 3, 14, 16 5 6 7 8 9 10 11 12 13, 15 NAME SHDN IN LX SS COMP TOFF FB GND REF FBSEL VCC PGND FUNCTION Shutdown Control Input. Drive SHDN low to disable the reference, control circuitry, and internal MOSFETs. Drive high or connect to VCC for normal operation. Supply Voltage Input for the internal PMOS power switch Connection for the drains of the PMOS power switch and NMOS synchronous-rectifier switch. Connect the inductor from this node to output filter capacitor and load. Soft-Start. Connect a capacitor from SS to GND to limit inrush current during start-up. Integrator Compensation. Connect a capacitor from COMP to VCC for integrator compensation. See Integrator Amplifier section. Off-Time Select Input. Sets the PMOS power switch off-time during constant-off-time operation. Connect a resistor from TOFF to GND to adjust the PMOS switch off-time. Feedback Input for both preset-output and adjustable-output operating modes. Connect directly to output for fixed-voltage operation or to a resistor-divider for adjustable operating modes. Analog Ground Reference Output. Bypass REF to GND with a 1F capacitor. Feedback Select Input. Selects AC load-regulation error and output voltage. See Table 2 for programming instructions. Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass VCC with a 10 and 2.2F lowpass filter. See Figure 1. Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch.
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MAX1644-11
5
2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches MAX1644
_______________Detailed Description
The MAX1644 synchronous, current-mode, constant-offtime, PWM DC-DC converter steps down input voltages of +3V to +5.5V to a preset output voltage of either +3.3V or +2.5V, or to an adjustable output voltage from +1.1V to VIN. The device delivers up to 2A of continuous load current. Internal switches composed of a 0.1 PMOS power switch and a 0.1 NMOS synchronous-rectifier switch improve efficiency, reduce component count, and eliminate the need for an external Schottky diode. The MAX1644 optimizes performance by operating in constant-off-time mode under heavy loads and in Maxim's proprietary Idle Mode under light loads. A single resistor-programmable constant-off-time control sets switching frequencies up to 350kHz, allowing the user to optimize performance trade-offs in efficiency, switching noise, component size, and cost. Under lowdropout conditions, the device operates in a 100% duty-cycle mode, where the PMOS switch remains permanently on. Idle Mode enhances light-load efficiency by skipping cycles, thus reducing transition and gatecharge losses. When power is drawn from a regulated supply, constantoff-time PWM architecture essentially provides constantfrequency operation. This architecture has the inherent advantage of quick response to line and load transients. The MAX1644's current-mode, constant-off-time PWM architecture regulates the output voltage by changing the PMOS switch on-time relative to the constant offtime. Increasing the on-time increases the peak inductor current and the amount of energy transferred to the load per pulse. grammed off-time (tOFF). If the output falls dramatically out of regulation--approximately VFB / 4--the PMOS switch remains off for approximately four times tOFF. The NMOS synchronous rectifier turns on shortly after the PMOS switch turns off, and it remains on until shortly before the PMOS switch turns back on.
Idle Mode Under light loads, the device improves efficiency by switching to a pulse-skipping Idle Mode. Idle Mode operation occurs when the current through the PMOS switch is less than the Idle Mode threshold current. Idle Mode forces the PMOS to remain on until the current through the switch reaches 0.4A, thus minimizing the unnecessary switching that degrades efficiency under light loads. In Idle Mode, the device operates in discontinuous conduction. Current-sense circuitry monitors the current through the NMOS synchronous switch, turning it off before the current reverses. This prevents current from being pulled from the output filter through the inductor and NMOS switch to ground. As the device switches between operating modes, no major shift in circuit behavior occurs.
100% Duty-Cycle Operation
When the input voltage drops near the output voltage, the duty cycle increases until the PMOS MOSFET is on continuously. The dropout voltage in 100% duty cycle is the output current multiplied by the on-resistance of the internal PMOS switch and parasitic resistance in the inductor. The PMOS switch remains on continuously as long as the current limit is not reached.
Shutdown
Drive SHDN to a logic-level low to place the MAX1644 in low-power shutdown mode and reduce supply current to less than 1A. In shutdown, all circuitry and internal MOSFETs turn off, and the LX node becomes high impedance. Drive SHDN to a logic-level high or connect to VCC for normal operation.
Modes of Operation
The current through the PMOS switch determines the mode of operation: constant-off-time mode (for load currents greater than 0.2A) or Idle Mode (for load currents less than 0.2A). Current sense is achieved through a proprietary architecture that eliminates current-sensing I2R losses.
Summing Comparator
Three signals are added together at the input of the summing comparator (Figure 1): an output voltage error signal relative to the reference voltage, an integrated output voltage error correction signal, and the sensed PMOS switch current. The integrated error signal is provided by a transconductance amplifier with an external capacitor at COMP. This integrator provides high DC accuracy without the need for a high-gain amplifier. Connecting a capacitor at COMP modifies the overall loop response (see Integrator Amplifier section).
Constant-Off-Time Mode Constant-off-time operation occurs when the current through the PMOS switch is greater than the Idle Mode threshold current (0.4A, which corresponds to a load current of 0.2A). In this mode, the regulation comparator turns the PMOS switch on at the end of each offtime, keeping the device in continuous-conduction mode. The PMOS switch remains on until the output is in regulation or the current limit is reached. When the PMOS switch turns off, it remains off for the pro-
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2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches MAX1644
0.01F FBSEL SS FB FEEDBACK SELECTION COMP 470pF 10 VCC REF Gm REF VIN 3.0V TO 5.5V 10F
MAX1644
CURRENT SENSE SKIP PWM LOGIC AND DRIVERS SUMMING COMPARATOR
IN
VIN
2.2F SHDN REF 1F GND
LX
VOUT COUT
REF
TIMER
CURRENT SENSE PGND
TOFF RTOFF
NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.
Figure 1. Functional Diagram
Synchronous Rectification
In a step-down regulator without synchronous rectification, an external Schottky diode provides a path for current to flow when the inductor is discharging. Replacing the Schottky diode with a low-resistance NMOS synchronous switch reduces conduction losses and improves efficiency. The NMOS synchronous-rectifier switch turns on following a short delay after the PMOS power switch turns off, thus preventing cross conduction or "shoot through." In constant-off-time mode, the synchronous-rectifier switch turns off just prior to the PMOS power switch turning on. While both switches are off, inductor current flows through the internal body diode of the NMOS switch. The internal body diode's forward voltage is relatively high. Junction-to-ambient thermal resistance, JA, is highly dependent on the amount of copper area immediately surrounding the IC leads. The MAX1644 evaluation kit has 0.5 in.2 of copper area and a thermal resistance of 60C/W with no airflow. Airflow over the IC significantly reduces the junction-to-ambient thermal resistance. For
heatsinking purposes, evenly distribute the copper area connected at the IC among the high-current pins.
Power Dissipation
Power dissipation in the MAX1644 is dominated by conduction losses in the two internal power switches. Power dissipation due to supply current in the control section and average current used to charge and discharge the gate capacitance of the internal switches are less than 30mW at 300kHz. This number is reduced when the switching frequency decreases as the part enters Idle Mode. Combined conduction losses in the two power switches are approximated by: PD = IOUT2 * RON The junction-to-ambient thermal resistance required to dissipate this amount of power is calculated by: JA = (TJ,MAX - TA,MAX) / PD where: JA = junction-to-ambient thermal resistance TJ,MAX = maximum junction temperature TA,MAX = maximum ambient temperature
Thermal Resistance
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7
2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches MAX1644
__________________Design Procedure
For typical applications, use the recommended component values in Table 1. For other applications, take the following steps: 1) Select the desired PWM-mode switching frequency; 300kHz is a good starting point. 2) Select the constant-off-time as a function of input voltage, output voltage, and switching frequency. 3) Select RTOFF as a function of off-time. 4) Select the inductor as a function of output voltage, off-time, and peak-to-peak inductor current.
LX VOUT
MAX1644
R2
FB R1 = 50k R2 = R1(VOUT / VREF - 1) VREF = 1.1V R1
Table 1. Recommended Component Values (IOUT = 2A, fPWM = 300kHz)
VIN (V) 5 5 5 5 3.3 3.3 3.3 VOUT (V) 3.3 2.5 1.8 1.5 2.5 1.8 1.5 L (H) 6.0 6.8 6.8 6.0 3.3 4.7 4.7 RTOFF (k) 120 180 240 270 82 180 200
Figure 2. Adjustable Output Voltage
leave unconnected (3.3V output voltage). Internal resistor-dividers divide down the output voltage, regulating the divided voltage to the internal reference voltage. For output voltages other than 2.5V or 3.3V, or for tighter AC load regulation, connect FBSEL to GND (1% regulation) or to REF (2% regulation), and connect FB to a resistor divider between the output voltage and ground (Figure 2). Regulation is maintained for adjustable output voltages when VFB equals VREF. Use 50k for R1. R2 is given by the equation: V R2 = R1 OUT - 1 VREF where VREF is typically 1.1V.
Table 2. Output Voltage and AC LoadRegulation Selection
PIN FBSEL VCC Unconnected REF GND FB Output Voltage Output Voltage Resistor Divider Resistor Divider OUTPUT VOLTAGE (V) 2.5 3.3 Adjustable Adjustable AC LOADREGULATION ERROR (%) 2 2 2 1
Programming the Switching Frequency and Off-Time
The MAX1644 features a programmable PWM mode switching frequency, which is set by the input and output voltage and the value of R TOFF, connected from TOFF to GND. RTOFF sets the PMOS power switch offtime in PWM mode. Use the following equation to select the off-time according to your desired switching frequency in PWM mode (IOUT > 0.2A): t OFF = where:
(VIN - VOUT - VPMOS ) fPWM ( VIN - VPMOS + VNMOS )
Setting the Output Voltage
The output of the MAX1644 is selectable between one of two preset output voltages: (2.5V or 3.3V) with a 2% AC load-regulation error, or an adjustable output voltage from the reference voltage (nominally 1.1V) up to VIN with a 1% or 2% AC load-regulation error. For a preset output voltage, connect FB to the output voltage, and connect FBSEL to VCC (2.5V output voltage) or
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tOFF = the programmed off-time VIN = the input voltage VOUT = the output voltage VNMOS = the voltage drop across the internal PMOS power switch VPMOS = the voltage drop across the internal NMOS synchronous-rectifier switch
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2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches
f PWM = switching frequency in PWM mode (IOUT > 0.2A) Select RTOFF according to the formula: RTOFF = (tOFF - 0.07s) (150k / 1.26s) Recommended values for RTOFF range from 39k to 470k for off-times of 0.4s to 4s. The output filter capacitor affects the output voltage ripple, output load-transient response, and feedback loop stability. For stable operation, the MAX1644 requires a minimum output ripple voltage of VRIPPLE 2% * VOUT (with 2% load regulation setting). The minimum ESR of the output capacitor should be: ESR > 2%
MAX1644
Inductor Selection
Three key inductor parameters must be specified: inductor value (L), peak current (IPEAK), and DC resistance (RDC). The following equation includes a constant, denoted as LIR, which is the ratio of peakto-peak inductor AC current (ripple current) to maximum DC load current. A higher value of LIR allows smaller inductance but results in higher losses and ripple. A good compromise between size and losses is found at approximately a 25% ripple-current to loadcurrent ratio (LIR = 0.25), which corresponds to a peak inductor current 1.125 times higher than the DC load current: L= VOUT t OFF IOUT LIR
L t OFF
Stable operation requires the correct output filter capacitor. When choosing the output capacitor, ensure that: COUT (tOFF / VOUT) * (64FV / s) With an AC load regulation setting of 1%, the COUT requirement doubles, and the minimum ESR of the output capacitor is halved.
Integrator Amplifier
An internal transconductance amplifier fine tunes the output DC accuracy. A capacitor, CCOMP, from COMP to VCC compensates the transconductance amplifier. For stability, choose: CCOMP 470pF A large capacitor value maintains a constant average output voltage but slows the loop response to changes in output voltage. A small capacitor value speeds up the loop response to changes in output voltage but decreases stability. Choose the capacitor values that result in optimal performance.
where: IOUT = maximum DC load current LIR = ratio of peak-to-peak AC inductor current to DC load current, typically 0.25 The peak inductor current at full load is 1.125 * IOUT if the above equation is used; otherwise, the peak current is calculated by: IPEAK = IOUT + VOUT 2

t OFF L
Setting the AC Loop Gain
The MAX1644 allows selection of a 1% or 2% AC loadregulation error when the adjustable output voltage mode is selected (Table 2). A 2% setting is automatically selected in preset output voltage mode (FBSEL connected to VCC or unconnected). A 2% load-regulation error setting reduces output filter capacitor requirements, allowing the use of smaller and less expensive capacitors. Selecting a 1% load-regulation error reduces transient load errors, but requires larger capacitors.
Choose an inductor with a saturation current at least as high as the peak inductor current. To minimize loss, choose an inductor with a low DC resistance.
Capacitor Selection
The input filter capacitor reduces peak currents and noise at the voltage source. Use a low-ESR and lowESL capacitor located no further than 5mm from IN. Select the input capacitor according to the RMS input ripple-current requirements and voltage rating: IRIPPLE = ILOAD VOUT VIN - VOUT VIN
(
)
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9
2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches MAX1644
Soft-Start
Soft-start allows a gradual increase of the internal current limit to reduce input surge currents at start-up and at exit from shutdown. A charging capacitor, CSS , placed from SS to GND sets the rate at which the internal current limit is changed. Upon power-up, when the device comes out of undervoltage lockout (2.6V typ) or after the SHDN pin is pulled high, a 5A constant-current source charges the soft-start capacitor and the voltage on SS increases. When the voltage on SS is less than approximately 0.7V, the current limit is set to zero. As the voltage increases from 0.7V to approximately 1.8V, the current limit is adjusted from 0 to 2.9A. The voltage across the soft-start capacitor changes with time according to the equation: VSS = 5A t CSS
SHDN 0 1.8V VSS (V) 0 2.9A ILIMIT (A) 0 t 0.7V
Figure 3. Soft-Start Current Limit over Time
The soft-start current limit varies with the voltage on the soft-start pin, SS, according to the equation: ILIMIT = (VSS - 0.7V) * 2.7A/V, for VSS > 0.7V The constant-current source stops charging once the voltage across the soft-start capacitor reaches 1.8V (Figure 3).
2) Connect the input filter capacitor less than 5mm away from IN. The connecting copper trace carries large currents and must be at least 2mm wide, preferably 5mm. 3) Place the LX node components as close together and as near to the device as possible. This reduces resistive and switching losses as well as noise. 4) A ground plane is essential for optimum performance. In most applications, the circuit is located on a multilayer board, and full use of the four or more layers is recommended. Use the top and bottom layers for interconnections and the inner layers for an uninterrupted ground plane.
Circuit Layout and Grounding
Good layout is necessary to achieve the MAX1644's intended output power level, high efficiency, and low noise. Good layout includes the use of a ground plane, appropriate component placement, and correct routing of traces using appropriate trace widths. The following points are in order of decreasing importance: 1) Minimize switched-current and high-current ground loops. Connect the input capacitor's ground, the output capacitor's ground, and PGND together.
___________________Chip Information
TRANSISTOR COUNT: 1758
10
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2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches
Package Information
SSOP.EPS
MAX1644
______________________________________________________________________________________
11
2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches MAX1644
NOTES
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