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 decou-

pling. An internal synchronous rectifier improves effi-

ciency and eliminates the need for an external Schottky

freewheeling diode. On-chip current sensing uses the

on-resistance of the internal MOSFETs, eliminating cur-

rent-sensing resistors and improving efficiency.

The input voltage range is 2.6V to 5.5V. The output volt-

age 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, respec-

Selection section), the output voltage is programmable

down to 0.75V on the MAX1974, and down to 1.25V on

the MAX1973.

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 high-

side 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 transconduc-

tance amplifier. It pushes the output LC filter pole, nor-

mally 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

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.

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

Current Sense and Current Limit

The current-sense circuit amplifies the current-sense

voltage generated by the high-side MOSFET’s on-resis-

This amplified current-sense signal and the internal

slope compensation signal are summed together at the

PWM comparator’s inverting input. The PWM compara-

tor 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 syn-

chronous 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.

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 20µA current source. When SS

reaches its nominal value, the output is in full regula-

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

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