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19-2547; Rev 0; 7/02
Smallest 1A, 1.4MHz Step-Down Regulators
General Description
The MAX1973/MAX1974 are constant-frequency 1.4MHz pulse-width-modulated (PWM) current-mode step-down regulators. The output voltage can be set as low as 0.75V using an external voltage-divider, or it can be set to preset outputs of 1V, 1.5V (MAX1974), 1.8V, or 2.5V (MAX1973) without requiring external resistors. The MAX1973 also includes a voltage-margining feature that offsets the output voltage up or down 4% to facilitate board-level production testing. A fixed 1.4MHz operating frequency ensures operation outside the DSL frequency band, provides fast transient response, and allows the use of small external components. Only 4.7F input and output ceramic capacitors are needed for 1A applications. Forced PWM operation ensures a constant switching frequency over all load conditions. Output voltage accuracy is 1% over load, line, and temperature operating ranges. The MAX1973 features voltage margining; the MAX1974 provides a POK output to indicate when the output has reached 90% of its nominal regulation voltage. Both devices are available in small 10-pin MAX packages. o Tiny Circuit Footprint of 0.19in2 o Ultra-Low Circuit Height of 1.8mm o 4.7F Ceramic Input and Output Capacitors o 2.6V to 5.5V Input Voltage o 1A Output Current o 1% Accurate o Built-In 4% Logic-Controlled Voltage Margining (MAX1973) o Preset 1V, 1.5V, 1.8V, 2.5V, or 0.75V to VIN Adjustable Output o Fixed-Frequency PWM Current-Mode Operation o 1.4MHz Switching Frequency, Operate Outside DSL Band o 100% Duty-Cycle Dropout Capability o Small External Components
Features
MAX1973/MAX1974
Applications
Network Equipment Cellular Base Stations DSL and Wireless Modems/Routers Optical Modules Central-Office DSL and Telecom DSP/ASIC Core and IO supplies
Selector Guide appears at end of data sheet.
TOP VIEW
FBSEL 1 COMP FB INPUT 2.6V TO 5.5V IN COMP FB FBSEL MAX1973 SS GND CTL1 CTL2 PGND VOLTAGE MARGINING ON/OFF FBSEL 1 COMP FB SS GND MAX1974 OUTPUT DOWN TO 0.75V 2 3 4 5 LX OUTPUT 1.25V TO VIN 1A SS GND 2 3 4 5
Ordering Information
PART MAX1973EUB MAX1974EUB TEMP RANGE -40C to +85C -40C to +85C PIN-PACKAGE 10 MAX 10 MAX
Pin Configurations
Typical Operating Circuit
10 CTL1 9 IN LX PGND CTL2
MAX1973
8 7 6
MAX
10 ON 9 IN LX PGND POK
MAX1974
8 7 6
MAX
________________________________________________________________ Maxim Integrated Products
1
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Smallest 1A, 1.4MHz Step-Down Regulators MAX1973/MAX1974
ABSOLUTE MAXIMUM RATINGS
IN, POK, CTL1, CTL2, FBSEL, ON to GND ..............-0.3V to +6V COMP, FB, SS to GND ................................-0.3V to (VIN + 0.3V) PGND to GND .......................................................-0.3V to +0.3V LX Current (Note 1) ...............................................-2.4A to +2.4A Continuous Power Dissipation (TA = +70C) 10-Pin MAX (derate 5.6mW/C above +70C) .......... 444mW Operating Temperature Range ...........................-40C to +85C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Note 1: LX has internal clamp diodes to IN and PGND. Applications that forward bias these diodes should take care not to exceed the IC package power dissipation limit.
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 = VCTL_ = 3.3V, FB = OUT, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER IN IN Voltage Range IN Undervoltage Lockout Threshold Supply Current Supply Current in Dropout Shutdown Supply Current FB Output Voltage Range MAX1973 MAX1974 FBSEL not connected MAX1973 FB Regulation Voltage MAX1974 FBSEL = GND FBSEL = IN FBSEL not connected FBSEL = GND FBSEL = IN FB Regulation Voltage Positive Voltage Margining FB Regulation Voltage Negative Voltage Margining FB Input Resistance to GND in Preset Output Modes FB Input Bias Current SS (REFERENCE OUTPUT) SS Voltage MAX1974 MAX1973 0.75 1.25 V FBSEL not connected MAX1973, CTL1 = GND, CTL2 = IN 1.25 0.75 1.2375 1.7820 2.4750 0.7425 0.99 1.485 +3 1.25 1.8 2.5 0.75 1.00 1.500 +4 VIN VIN 1.2625 1.8180 2.5250 0.7575 1.01 1.515 +5 % V V Rising and falling, hysteresis is 25mV (typ) Switching with no load VOUT set for 3.6V VIN = 5.5V 2.6 2.2 2.35 4.0 3 0.1 5.5 2.5 7.5 5 10 V V mA mA A CONDITIONS MIN TYP MAX UNITS
MAX1973, CTL1 = IN, CTL2 = GND
-3
-4
-5
%
10 -0.1
30 0.01
70 +0.1
k A
2
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Smallest 1A, 1.4MHz Step-Down Regulators
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VCTL_ = 3.3V, FB = OUT, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER SS Source Current SS Sink Current SS to GND Resistance in Shutdown FBSEL Low Input Threshold High Input Threshold Input Bias Current COMP Transconductance from FB to COMP COMP to GND Resistance in Shutdown Clamp Voltage Low Clamp Voltage High LX On-Resistance High On-Resistance Low Current-Sense Transresistance Positive Current-Limit Threshold Negative Current-Limit Threshold LX Shutdown Leakage Current Switching Frequency CTL1, CTL2 (MAX1973), ON (MAX1974) Logic-Low Input Threshold Logic-High Input Threshold Logic Input Current POK (MAX1974 only) Output Low Voltage Output Valid Threshold for POK POK sinking 1mA Percentage of nominal regulation voltage Rising Falling 90 88 10 92.5 90 100 95 92 mV % -1 0.6 1.6 +1 V V A VLX = VIN = 5.5V LX = GND, VIN = 5.5V -20 1.2 1.4 1.6 VIN = 3.3V VIN = 3.3V 0.275 1.1 -1.2 0.23 0.16 0.335 1.6 -0.8 0.46 0.32 0.425 1.75 -0.4 20 V/A A A A MHz 40 5 0.6 1.35 60 40 0.9 1.75 80 100 1.2 2.15 S V V FBSEL = GND or IN, VIN = 5.5V -20 10 0.3 VIN 0.3 +20 V V A CONDITIONS MIN -25 10 5 TYP -20 20 40 MAX -15 35 100 UNITS A A
MAX1973/MAX1974
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Smallest 1A, 1.4MHz Step-Down Regulators MAX1973/MAX1974
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VCTL_ = 3.3V, FB = OUT, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER THERMAL SHUTDOWN Thermal-Shutdown Threshold Thermal-Shutdown Hysteresis +170 20 C C CONDITIONS MIN TYP MAX UNITS
ELECTRICAL CHARACTERISTICS
(VIN = VFB = VCTL_ = 3.3V, TA = -40C to +85C, unless otherwise noted.) (Note 2)
PARAMETER IN IN Voltage Range IN Undervoltage Lockout Threshold Supply Current Supply Current in Dropout Shutdown Supply Current FB Output Voltage Range MAX1973 MAX1974 FBSEL not connected MAX1973 FB Regulation Voltage MAX1974 FB Regulation Voltage Positive Voltage Margining FB Regulation Voltage Negative Voltage Margining FB Input Resistance to GND in Preset Output Modes FB Input Bias Current SS (REFERENCE OUTPUT) SS Source Current SS Sink Current -25 10 -15 35 A A FBSEL not connected FBSEL = GND FBSEL = IN FBSEL not connected FBSEL = GND FBSEL = IN MAX1973, CTL1 = GND, CTL2 = IN MAX1973, CTL1 = IN, CTL2 = GND 1.25 0.75 1.2375 1.7820 2.4750 0.7425 0.99 1.485 3 -3 10 -0.15 VIN VIN 1.2625 1.8180 2.5250 0.7575 1.01 1.515 5 -5 70 +0.15 % % k A V V Rising and falling, hysteresis is 25mV (typ) Switching with no load VOUT set for 3.6V VIN = 5.5V 2.6 2.2 5.5 2.5 7.5 5 10 V V mA mA A CONDITIONS MIN TYP MAX UNITS
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Smallest 1A, 1.4MHz Step-Down Regulators
ELECTRICAL CHARACTERISTICS
(VIN = VCTL_ = 3.3V, FB = OUT, TA = -40C to +85C, unless otherwise noted.) (Note 2)
PARAMETER SS to GND Resistance in Shutdown FBSEL Low Input Threshold High Input Threshold Input Bias Current COMP Transconductance from FB to COMP COMP to GND Resistance in Shutdown Clamp Voltage Low Clamp Voltage High LX On-Resistance High On-Resistance Low Current-Sense Transresistance Positive Current-Limit Threshold Negative Current-Limit Threshold LX Shutdown Leakage Current Switching Frequency CTL1, CTL2 (MAX1973), ON (MAX1974) Logic-Low Input Threshold Logic-High Input Threshold Logic Input Current POK (MAX1974 only) Output Low Voltage Output Valid Threshold for POK POK sinking 1mA Percentage of nominal regulation voltage Rising Falling 90 88 100 95 92 mV % -1 0.6 1.6 1 V V A VLX = VIN = 5.5V LX = GND, VIN = 5.5V -20 1.2 1.6 VIN = 3.3V VIN = 3.3V 0.275 1.10 -1.20 0.46 0.32 0.425 1.85 -0.35 20 V/A A A A MHz 40 5 0.6 1.3 80 100 1.2 2.2 S V V FBSEL = GND or IN, VIN = 5.5V -20 0.3 VIN 0.4 +20 V V A CONDITIONS MIN 5 TYP 40 MAX 100 UNITS
MAX1973/MAX1974
Note 2: Specifications to -40C are guaranteed by design and not production tested.
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Smallest 1A, 1.4MHz Step-Down Regulators MAX1973/MAX1974
Typical Operating Characteristics
(Circuits of Figure 2, 3, and 4; TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 0.1 LOAD CURRENT (A) 1 VIN = 3.3V VOUT = 1.8V VOUT = 1V VOUT = 2.5V
MAX1973/74 toc01
EFFICIENCY vs. LOAD CURRENT
MAX1973/74 toc02
DROPOUT VOLTAGE vs. LOAD CURRENT
MAX1973/74 toc03
100
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0
VOUT = 3.3V VOUT = 2.5V
0.35 0.30 DROPOUT VOLTAGE (V) 0.25 0.20 0.15 0.10 0.05 VOUT = 3.3V VOUT = 2.5V
VIN = 5V 0.01 0.1 LOAD CURRENT (A) 1
0 0 0.2 0.4 0.6 LOAD CURRENT (A) 0.8
NO-LOAD CURRENT vs. INPUT VOLTAGE
MAX1973/74 toc04
FB VOLTAGE vs. LOAD CURRENT
MAX1973/74 toc05
CHANGE IN OUTPUT VOLTAGE vs. LOAD CURRENT
0.5 CHANGE IN OUTPUT VOLTAGE (mV) 0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 1.0 0.01 VIN = 5V 0.1 1 10 100 1000 1V VOUT = 1.8V 2.5V
MAX1973/74 toc06
12 10 NO-LOAD CURRENT (mA) 8 6 4 2 VOUT = 2.5V 0 0 1 2 3 4 5 INPUT VOLTAGE (V)
1.252 1.251 1.250 FB VOLTAGE (V) 1.249 1.248 1.247 1.246 1.245 1.244 0 0.2 0.4 0.6 0.8 LOAD CURRENT (A) MAX1973 VIN = 5V R1 = 22k R2 = 13k
1.0
LOAD CURRENT (A)
SWITCHING FREQUENCY vs. SUPPLY VOLTAGE
TA = +85C SWITCHING FREQUENCY (MHz) 1.35 1.30 1.25 TA = -40C 1.20 1.15 1.10 2 3 4 SUPPLY VOLTAGE (V) 5 6
MAX1973/74 toc07
1.40
TA = +25C
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Smallest 1A, 1.4MHz Step-Down Regulators MAX1973/MAX1974
Typical Operating Characteristics (continued)
(Circuits of Figure 2, 3, and 4; TA = +25C, unless otherwise noted.)
MAX1974 STARTUP WAVEFORMS
MAX1973/74 toc08
MAX1973 HIGH-CURRENT SWITCHING WAVEFORMS
MAX1973/74 toc09
ON POK
10V/div 0 2V/div 0
VOUT VLX
20mV/div
5V/div 0
VOUT
1V/div 0 200mA/div IL
500mA/div
IIN 0 VIN = 5V, 100k PULLUP RESISTOR POK TO VOUT 500ns/div VIN = 5V, VOUT = 2.5V, IOUT = 800mA 0
MAX1973 LOW-CURRENT SWITCHING WAVEFORMS
MAX1973/74 toc10
MAX1973 LOAD TRANSIENT
MAX1973/74 toc11
VOUT
20mV/div VOUT 20mV/div
VLX
5V/div 0 500mA/div IL 500mA/div 0
IL 0 500ns/div VIN = 5V, VOUT = 2.5V, IOUT = 10mA 500ns/div VIN = 5V, VOUT = 2.5V, IOUT = 400mA TO 800mA
MAX1973 LOAD TRANSIENT
MAX1973/74 toc12
MAX1973 LINE TRANSIENT
MAX1973/74 toc13
VOUT
200mV/div
VOUT
50mV/div
IL
500mA/div 0
VIN
2V/div
0 20s/div VIN = 5V, VOUT = 2.5V, IOUT = 600mA TO 800mA 200s/div VIN = 3.3V TO 5V TO 3.3V, IOUT = 800mA
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Smallest 1A, 1.4MHz Step-Down Regulators MAX1973/MAX1974
Typical Operating Characteristics (continued)
(Circuits of Figure 2, 3, and 4; TA = +25C, unless otherwise noted.)
MAX1973 VOLTAGE MARGIN STEP CHANGE RESPONSE
MAX1973/74 toc14
MAX1974 POK AND INPUT VOLTAGE
MAX1973/74 toc15
VOUT
1V/div
VPOK 2V/div
VCTL1
VCTL2 IIN
0 10V/div 0 10V/div 0 500mA/div
0 VOUT VIN 1V/div
0 2V/div
0 1ms/div VIN = 5V, VOUT = 2.5V, IOUT = 800mA, -4% TO +4% TO -4%
0 20ms/div MAX1974 WITH 100k PULLUP RESISTOR FROM POK TO IN, ILOAD = 100mA
Pin Description
PIN NAME MAX1973 MAX1974 FUNCTION Feedback-Mode Selector. Connect FBSEL to GND to set the output voltage to 1.8V (MAX1973) or 1V (MAX1974). Connect FBSEL to IN to set the output voltage to 2.5V (MAX1973) or 1.5V (MAX1974). Leave FBSEL unconnected to set the output voltage using a resistor-divider at FB. Compensation. Connect a series RC network to GND. COMP is internally pulled to GND when the device is in shutdown or in undervoltage lockout (see the Compensation Components section). Feedback Input. Connect to the output if a preset voltage is used, or to a resistordivider from the output to GND for an adjustable output voltage. Soft-Start Pin and Reference Output. Bypass to GND with at least 0.01F. Connect 0.1F to GND for a soft-start ramp time of 6.25ms for the MAX1973, or 3.75ms for the MAX1974. SS is internally pulled to GND when the device is shut down or in undervoltage lockout. Ground Control Input 2. Controls enable/disable and voltage margining (see Table 1). Power-OK Output. Open-drain output goes low when output is below 90% of nominal output. POK is also low when the device is shut down or in undervoltage lockout. Power Ground Inductor Connection. Connect an inductor from LX to the output. Input Supply Voltage. Input voltage range is 2.6V to 5.5V. Connect a 4.7F capacitor from IN to PGND. Control Input 1. Controls Enable/Disable and voltage margining (see Table 1). Enable Input. Connect to IN or drive high for normal operation. Drive low to put device in shutdown.
1
FBSEL
FBSEL
2
COMP
COMP
3
FB
FB
4
SS
SS
5 6 7 8 9
GND CTL2 -- PGND LX IN CTL1
GND -- POK PGND LX IN -- ON
10
--
8
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Smallest 1A, 1.4MHz Step-Down Regulators
Detailed Description
The MAX1973/MAX1974 are 1.4MHz fixed-frequency PWM current-mode step-down DC/DC converters. A high 1.4MHz switching frequency allows use of small inductors and small capacitors for filtering and decoupling. An internal synchronous rectifier improves efficiency and eliminates the need for an external Schottky freewheeling diode. On-chip current sensing uses the on-resistance of the internal MOSFETs, eliminating current-sensing resistors and improving efficiency. The input voltage range is 2.6V to 5.5V. The output voltage is selectable to one of two presets, or adjustable by using a resistor-divider. The output voltage of the MAX1973 is preset to 1.8V or 2.5V by connecting FBSEL to GND or IN, respectively. The MAX1974 is preset to 1.0V or 1.5V by connecting FBSEL to GND or IN, respectively. In adjustable mode (see the Output Voltage Selection section), the output voltage is programmable down to 0.75V on the MAX1974, and down to 1.25V on the MAX1973. the load. Under overload conditions, when the inductor current exceeds the current limit, the high-side MOSFET is not turned on at the rising edge of the clock, and the low-side MOSFET remains on to let the inductor current ramp down.
MAX1973/MAX1974
100% Duty-Cycle Operation
The MAX1973/MAX1974 can operate at 100% duty cycle. In this state, the high-side P-channel MOSFET is turned on (not switching). The dropout voltage in 100% duty-cycle operation is the output current multiplied by the sum of the on-resistance of the P-channel MOSFET (RDS(ON)P) and the inductor resistance (RL). VDROPOUT = IOUT ( RDS(ON)P + RL )
Current Sense and Current Limit
The current-sense circuit amplifies the current-sense voltage generated by the high-side MOSFET's on-resistance and the inductor current (RDS(ON) INDUCTOR). This amplified current-sense signal and the internal slope compensation signal are summed together at the PWM comparator's inverting input. The PWM comparator turns off the internal high-side MOSFET when this sum exceeds the integrated feedback voltage. The internal high-side MOSFET has a current limit of 1.6A (typ). If the current flowing out of LX exceeds this maximum, the high-side MOSFET turns off and the synchronous rectifier MOSFET turns on. This lowers the duty cycle and causes the output voltage to droop until the current limit is no longer exceeded. There is also a synchronous rectifier current limit of -0.85A, to protect the device from current flowing into LX. If this negative current limit is exceeded, the synchronous rectifier turns off, and the inductor current continues to flow through the high-side MOSFET body diode back to the input until the beginning of the next cycle, or until the inductor current drops to zero.
PWM Control Scheme
The MAX1973/MAX1974 use a fixed-frequency PWM current-mode control scheme. The heart of the PWM current-mode controller is an open-loop comparator that compares the integrated voltage feedback signal against the sum of the amplified current-sense signal and the slope compensation ramp (see Figure 1). At each rising edge of the internal clock, the internal highside MOSFET turns on until the PWM comparator trips. During this on-time, current ramps up through the inductor, sourcing current to the output and storing energy in a magnetic field. The current-mode feedback system regulates the peak inductor current as a function of the output voltage error signal. Because the average inductor current is nearly the same as the peak inductor current (assuming that the inductor value is relatively high to minimize ripple current), the circuit acts as a switch-mode transconductance amplifier. It pushes the output LC filter pole, normally found in a voltage-mode PWM, to a higher frequency. To preserve inner loop stability and eliminate inductor staircasing, an internal slope-compensation ramp is summed into the main PWM comparator. During the second half of the switching cycle (off-time), the internal high-side MOSFET turns off and the internal low-side N-channel MOSFET turns on. The inductor releases the stored energy as its current ramps down while still providing current to the output. The output capacitor stores charge when the inductor current exceeds the load current and discharges when the inductor current is lower, smoothing the voltage across
Soft-Start
To reduce the supply inrush current, soft-start circuitry ramps up the output voltage during startup by charging the SS capacitor with a 20A current source. When SS reaches its nominal value, the output is in full regulation. The soft-start time (tSS) is determined from: V t SS = SS x CSS ISS where VSS is the soft-start (reference) voltage (1.25V for the MAX1973; 0.75V for the MAX1974), I SS is 20A, and CSS is the value of the capacitor connected to SS. Soft-start occurs when power is first applied and when the device exits shutdown. The part also goes through
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Smallest 1A, 1.4MHz Step-Down Regulators MAX1973/MAX1974
VOLTAGE CLAMP COMP
MAX1974 ONLY
POK
OUT PMOS CURRENT SENSE
IN
FB
FEEDBACK SELECT
CTL1 MAX1973 ONLY CTL2 PWM COMPARATOR ON MAX1974 ONLY SS REFERENCE SOFT-START PGND VOLTAGE MARGINING ERROR AMP LX PWM LOGIC BLOCK
NMOS CURRENT LIMIT
GND 1.4MHz OSCILLATOR REFERENCE READY BIAS IN UNDERVOLTAGE LOCK OUT CHIP ENABLE SLOPE COMP
MAX1973 MAX1974
Figure 1. Functional Diagram
soft-start when coming out of undervoltage lockout (UVLO) or thermal-overload protection.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation and protects the IC from damage in case of an overload or short-circuit condition. When the IC junction temperature (TJ) exceeds +170C, the device shuts down. The part turns on again after the junction temperature cools by 20C. This results in a pulsed output during continuous thermal-overload conditions.
Undervoltage Lockout (UVLO)
If VIN drops below 2.35V (typ), the MAX1973/MAX1974 assume that the supply voltage is too low to provide a valid output voltage, and the UVLO circuit inhibits switching. Once VIN rises above 2.4V, UVLO is disabled and the soft-start sequence begins.
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Smallest 1A, 1.4MHz Step-Down Regulators
Voltage Margining and Shutdown
A voltage-margining feature is provided on the MAX1973 to shift the output voltage up or down by 4%. Voltage margining is useful for the automatic testing of systems at high and low supply conditions to find potential failures. See Table 1 for the MAX1973 voltage margining and shutdown truth table. A shutdown feature is included on both the MAX1973 and the MAX1974. Shutdown turns off the IC and reduces the supply current about 0.1A. For the MAX1974, drive ON high for normal operation, or low for shutdown. For the MAX1973, drive both CTL1 and CTL2 high for normal operation, or drive both low for shutdown. For a simple enable/shutdown function with no voltage margining on the MAX1973, connect CTL1 to CTL2 and drive as one input.
MAX1973/MAX1974
Table 1. CTL_ Input Functions (MAX1973)
CTL1 GND GND CTL2 GND IN Shutdown Positive voltage margining, regulation voltage increased 4% from normal operation Negative voltage margining, regulation voltage lowered 4% from normal operation Normal operation FUNCTION
IN IN
GND IN
Table 2. Preset Output Voltages
FBSEL GND IN Not Connected OUTPUT VOLTAGE MAX1973 1.8V 2.5V Adjustable down to 1.25V MAX1974 1V 1.5V Adjustable down to 0.75V
Power-OK Output (POK)
A power-OK output (POK) is provided on the MAX1974. This is an open-drain output indicating when the output voltage is in regulation. If the output voltage falls below 90% of its nominal value, POK goes low. POK remains low until the output voltage rises to 92.5% of its nominal value. At that point, POK goes high impedance. To use POK as a logic output, connect a 10k to 100k pullup resistor from POK to the power supply of the logic receiving the POK signal. POK continues to function in shutdown or UVLO. Note that a minimum voltage of 1V at IN is required to ensure that POK provides a valid output. When VIN drops to zero, POK is high impedance. See the Typical Operating Characteristics.
For the MAX1973, VFB = 1.25V, allowing its output to be set down to 1.25V. For the MAX1974, VFB = 0.75V, allowing its output to be set down to 0.75V The MAX1973/MAX1974 PWM circuitry is capable of a stable minimum duty cycle of 17%. This limits the minimum output voltage that can be generated to 0.17 VIN. Instability may result for VIN/VOUT ratios below 0.17.
Applications Information
Output Voltage Selection
The output voltage can be set to one of two preset values, or can be set by an external resistor-divider. For preset output voltages, connect FB to the output as shown in Figures 2 and 3. Connect FBSEL to GND or IN to select the desired preset output voltage (see Table 2). To set the output voltage to a value other than the preset values, FBSEL is not connected, and FB is connected to a voltage-divider as shown in Figures 4 and 5. Select a value for R2 in the 1k to 22k range, and then calculate the value of R1 from the following equation: V R1 = R2 x OUT - 1 VFB
Inductor Selection
A 2.2H to 4.7H inductor with a saturation current of at least 1.25A is recommended for full-load (1mA) applications. For lower load currents, the inductor current rating can be reduced. For most applications, use an inductor with a current rating 1.25 times the maximum required output current. For best efficiency, the inductor's DC resistance should be as small as possible. See Table 3 for recommended inductors and manufacturers. For most designs, the inductor value (L INIT) can be derived from the following equation: VOUT (VIN - VOUT )
LINIT =
VIN x LIR x IOUT(MAX) x fSW
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Smallest 1A, 1.4MHz Step-Down Regulators MAX1973/MAX1974
VIN = 2.6V TO 5.5V IN 4.7F 36k COMP 470pF FBSEL SS 0.1F GND PGND MAX1973 CTL1 CTL2 FB LX
3.3H
1.8V 4.7F
VIN = 2.6V TO 5.5V 4.7F CC RC
3.3H IN COMP FBSEL SS MAX1973 LX FB CTL1 R2 CTL2 PGND
1.25V TO VIN 4.7F
R1
0.1F GND
Figure 2. MAX1973 with 1.8V Preset Output
Figure 4. MAX1973 with Adjustable Output Voltage Set by R1 and R2
VIN = 2.6V TO 5.5V 4.7F 43k
3.3H IN COMP LX FB ON FBSEL MAX1974 100k SS POK POK GND PGND
1.5V 4.7F
VIN = 2.6V TO 5.5V 4.7F CC RC
3.3H IN COMP FBSEL SS MAX1974 LX FB IN ON R2 POK PGND R1
0.75V TO VIN 4.7F
330pF IN
IN
IN 100k POK
0.1F
0.1F GND
Figure 3. MAX1974 with Preset 1.5V Output
Figure 5. MAX1974 with Adjustable Output Voltage Set by R1 and R2
where fSW is the switching frequency (1.4106 Hz), and LIR is the inductor ripple current as a percentage of the maximum load current. Keep LIR between 20% and 40% for best compromise of cost, size, and performance. The peak inductor current is approximately: LIR IL(PEAK) = 1 + x IOUT(MAX) 2
capacitor must meet the ripple current requirement (IRMS) imposed by the switching currents defined by the following equation: I IRMS = OUT VOUT (VIN - VOUT ) VIN Choose a capacitor that exhibits less than 10C temperature rise at the maximum operating RMS current for optimum long-term reliability.
Input Capacitor
A 4.7F ceramic input capacitor is recommended for most applications because of its low equivalent series resistance (ESR), equivalent series inductance (ESL), and cost. To ensure stability over a wide temperature range, an X5R or X7R dielectric is recommended. The input capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on the input caused by the circuit's switching. The input
12
Output Capacitor
A 4.7F ceramic output capacitor is recommended for most applications because of its low ESR, ESL, and lower cost. To ensure stability over a wide temperature range, an X5R or X7R dielectric is recommended. Key selection parameters for a ceramic output capacitor are capacitance, ESR, and voltage rating. These affect the overall stability, output ripple voltage, and transient
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Smallest 1A, 1.4MHz Step-Down Regulators MAX1973/MAX1974
Table 3. Recommended Inductors
MANUFACTURER Coilcraft Sumida Toko PART LPO1704-32M CDRD3D16-R3 A682AY-3R3M INDUCTANCE (H) 3.3 3.3 3.3 ESR (m) 160 85 134 SATURATION CURRENT (A) 1.3 1.1 0.97 DIMENSIONS L W H (mm) 5.5 6.6 1 4 4 1.8 4.4 4.4 3.1
response of the DC-DC converter. With ceramic capacitors, the voltage ripple from ESL is negligible. Output ripple is generated by variations in the charge stored in the output capacitance, and the voltage drop across the capacitor ESR. VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR) The output voltage ripple due to the output capacitance is: IP-P VRIPPLE(C) = 8 x COUT x fSW The output voltage ripple due to capacitor ESR is: VRIPPLE(ESR) = IP-P x ESR IP-P is the peak-to-peak inductor current: V -V V IP-P = IN OUT x OUT fSW x L VIN These equations are suitable for initial capacitor selection, but final values should be set by testing a prototype or evaluation circuit. As a rule, a smaller ripple current results in less output voltage ripple. Because the inductor ripple current is inversely proportional to inductor value, output voltage ripple decreases with larger inductance. Load transient response depends on the selected output capacitor. During a load transient, the output voltage instantly changes by ESR ILOAD. Before the controller can respond, the output deviates further, depending on the inductor and output capacitor values. After a short time (see the Typical Operating Characteristics), the controller responds by regulating the output voltage back to its nominal state. The controller response time depends on the closed-loop bandwidth. With a higher bandwidth the response time is faster. However, to maintain stable operation, the bandwidth should not be set above fSW/10.
capacitor between COMP and GND to form a pole-zero pair. The external inductor, output capacitor, compensation resistor, and compensation capacitor determine the loop bandwidth and stability. The inductor and output capacitor are chosen based on performance, size, and cost. Additionally, the compensation resistor and capacitor are selected to optimize the control loop. Table 4 and Table 5 list typical component values. The rest of this section is a more detailed discussion on calculating compensation components. The controller uses a current-mode control scheme that regulates the output voltage by forcing the required current through the external inductor. The voltage across the internal high-side MOSFET's on-resistance is used to sense inductor current. Current-mode control eliminates the double pole caused by the inductor and output capacitor found in other control schemes. Simple Type 1 compensation with a single resistor (RC) and capacitor (CC) is all that is needed to provide a stable and high-bandwidth loop. Use the formula below to calculate the value of C C, then use the nearest standard value: CC = 1 1 VFB x x gm x 0.5 x IOUT(MAX) RCS 2 x fC
where VFB is 1.25V for the MAX1973 and 0.75V for the MAX1974, the current-sense transresistance (RCS) is 0.26 (typ), and the transconductance from FB to COMP (g m ) is 50S (typ). For best stability and response performance, the closed-loop unity-gain frequency (fC) should be approximately 140kHz (onetenth the switching frequency). Use the following equation to calculate RC: RC = COUT CC x VOUT 0.5 x IOUT(MAX)
Compensation Components
An internal transconductance error amplifier compensates the control loop. Connect a series resistor and
Below is a numerical example of calculating compensation values for a circuit using the MAX1973 with 2.5V output and maximum output current of 1A: MAX1973
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13
Smallest 1A, 1.4MHz Step-Down Regulators MAX1973/MAX1974
VOUT = 2.5V IOUT(MAX) = 1A COUT = 4.7F VFB = 1.25V RCS = 0.26 gm = 50S fC = 140kHz CC = = VFB 0.5 x IOUT(MAX) 1 x 1 RCS x gm x 1 1 2 x fC
PC Board Layout
A properly designed PC board layout is important in any switching regulator. The switching power stage requires particular attention. Follow these guidelines for good PC board layout: 1) Place decoupling capacitors as close to IC pins as possible. Keep the power ground plane (connected to PGND) and signal ground plane (connected to GND) separate. Connect the two ground planes with a single connection from PGND to GND. 2) Input and output capacitors are connected to the power ground plane; all other capacitors are connected to the signal ground plane. 3) Keep the high-current paths as short and wide as possible. 4) If possible, connect IN, LX, and PGND separately to a large land area to help cool the IC to further improve efficiency and long-term reliability. 5) Ensure all feedback connections are short and direct. Place feedback resistors (if used) as close to the IC as possible. 6) Route high-speed switching nodes (LX) away from sensitive analog areas (FB, COMP, SS).
x x 50 x 10-6 x = 547pF 0.5 x 1 0.26 2 x 140000
1.25
Select the nearest standard value: CC = 560pF RC = COUT CC
-12
x x
VOUT 0.5 x IOUT(MAX) 2.5 0.5 x 1
=
4.7 x 10-6 560 x 10
= 41.9k
Select the nearest standard value: RC = 43k
Table 4. Recommended Components for the MAX1973
VOUT (V) 2.5 1.8 CIN (F) 4.7 4.7 COUT (F) 4.7 4.7 CC (pF) 560 560 RC (k) 43 30
Table 5. Recommended Components for the MAX1974
VOUT (V) 1.5 1.0 CIN (F) 4.7 4.7 COUT (F) 4.7 4.7 CC (pF) 330 330 RC (k) 43 27
Selector Guide
PART MAX1973EUB MAX1974EUB FEATURES Voltage Margining Power-OK Output OUTPUT PRESET 1.8V or 2.5V 1V or 1.5V
Chip Information
TRANSISTOR COUNT: 1998 PROCESS: BiCMOS
14
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Smallest 1A, 1.4MHz Step-Down Regulators
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.)
10LUMAX.EPS
MAX1973/MAX1974
e
10
4X S
10
INCHES MAX DIM MIN 0.043 A 0.006 0.002 A1 A2 0.030 0.037 0.116 0.120 D1 0.114 0.118 D2 0.116 E1 0.120 E2 0.114 0.118 H 0.187 0.199 L 0.0157 0.0275 L1 0.037 REF b 0.007 0.0106 e 0.0197 BSC c 0.0035 0.0078 0.0196 REF S 0 6
MILLIMETERS MAX MIN 1.10 0.05 0.15 0.75 0.95 3.05 2.95 3.00 2.89 3.05 2.95 2.89 3.00 4.75 5.05 0.40 0.70 0.940 REF 0.177 0.270 0.500 BSC 0.090 0.200 0.498 REF 0 6
H y 0.500.1 0.60.1
1
1
0.60.1
TOP VIEW
BOTTOM VIEW
D2 GAGE PLANE A2 A b D1 A1
E2 c E1 L1
L
FRONT VIEW
SIDE VIEW
PROPRIETARY INFORMATION TITLE:
PACKAGE OUTLINE, 10L uMAX/uSOP
APPROVAL DOCUMENT CONTROL NO. REV.
21-0061
I
1 1
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 15 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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