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 0.1µA. 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.

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

Ω to 100kΩ 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

ance. See the Typical Operating Characteristics.

Applications Information

Output Voltage Selection

The output voltage can be set to one of two preset val-

ues, 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

Ω to 22kΩ range, and then calcu-

late the value of R1 from the following equation:

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 mini-

Inductor Selection

A 2.2µH to 4.7µH inductor with a saturation current of at

least 1.25A is recommended for full-load (1mA) applica-

tions. 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.

derived from the following equation:

L

VV

V

V

LIR

I

f

INIT

OUT

IN

OUT

IN

OUT MAX

SW

=

−

()

××

×

()

RR

V

V

OUT

FB

12

1

=×

−













Smallest 1A, 1.4MHz Step-Down Regulators

______________________________________________________________________________________

11

CTL1

CTL2

FUNCTION

GND

GND

Shutdown

GND

IN

Positive voltage margining,

regulation voltage increased 4% from

normal operation

IN

GND

Negative voltage margining,

regulation voltage lowered 4% from

normal operation

IN

IN

Normal operation

Table 1. CTL_ Input Functions (MAX1973)

OUTPUT VOLTAGE

FBSEL

MAX1973

MAX1974

GND

1.8V

1V

IN

2.5V

1.5V

Not Connected

Adjustable down

to 1.25V

Adjustable down to

0.75V

Table 2. Preset Output Voltages