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 10kΩ 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
output. When V
IN
drops to zero, POK is high imped-
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 1kΩ to 22kΩ range, and then calcu-
late the value of R1 from the following equation:
For the MAX1973, V
FB
= 1.25V, allowing its output to be
set down to 1.25V. For the MAX1974, V
FB
= 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 mini-
mum output voltage that can be generated to 0.17
✕
V
IN
. Instability may result for V
IN
/V
OUT
ratios below 0.17.
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.
For most designs, the inductor value (L
INIT
) can be
derived from the following equation:
