Datasheet

Automotive Power-Management IC with
Three Step-Down Converters and Linear Regulator
MAX16920
13
voltage following the normal soft-start sequence. If the
MAX16920 die reaches the thermal limit of +170˚C (typ)
(temperature rising), OUT3 is immediately shut off.
Power-Good Output (PG3)
DC-DC3 provides an open-drain power-good output
(PG3). PG3 is an active-high output that pulls high when
the output voltage (OUT3) is above 91.5% of its nominal
value. In standby mode, PG3 is high impedance.
Linear Regulator (OUTA)
The MAX16920 features a fixed 5V output linear
regulator (OUTA). OUTA provides up to 150mA load
current. Connect an external capacitor of at least 4.7μF
from OUTA to GND. OUTA is activated by driving ENA
high.
Power-Good Output (PGA)
OUTA provides an open-drain power-good output
(PGA). PGA is an active-high output that pulls high when
the output voltage (OUTA) is above 91.5% of its nominal
value. In standby mode, PGA is high impedance.
Overcurrent Protection
The OUTA output current is limited to 200mA (min). If the
output current exceeds the current limit, OUTA drops out
of regulation. Excess power dissipation in the device can
cause the device to turn off due to thermal shutdown.
Dropout
The dropout voltage for the linear regulator (OUTA)
is 500mV (max) at 100mA load. To avoid dropout,
make sure the input supply voltage is greater than the
output voltage plus the dropout voltage based on the
application output current requirements.
Switching Frequency
The MAX16920AATJ/V+ provides a fixed 400kHz switch-
ing frequency, whereas the MAX16920BATJ/V+ pro-
vides a fixed 2.2MHz switching frequency. Connect
SYNC to GND when using the internal switching fre-
quency. For external synchronization, see the DC-DC
Synchronization (SYNC) section.
Spread-Spectrum Option
The MAX16920AATJS/V+ and MAX16920BATJS/V+
have a built-in spread-spectrum oscillator. The inter-
nal operating frequency varies by +10% relative to the
internally generated operating frequency of 400kHz
(typ) for MAX16920AATJS/V+ and 2.025MHz (typ) for
MAX16920BATJS/V+. Spread spectrum improves the EMI
performance of the MAX16920 in some applications. By
varying the frequency 10% only in the positive direction,
the switching frequency of MAX16920BATJS/V+ never
drops below the AM radio band upper limit of 1.8MHz.
The internal spread spectrum does not interfere with the
external clock applied on the SYNC pin. It is active only
when generating the switching frequency internally.
DC-DC Synchronization (SYNC)
The SYNC input provides for synchronization of the
DC-DC converters. The MAX16920 defaults to the
internal switching frequency during startup or when
a SYNC input signal is not provided. For external
synchronization, provide a signal between 600kHz and
1000kHz for the MAX16920A and between 1800kHz and
2400kHz for the MAX16920B. The MAX16920A divides
the SYNC frequency by a factor of 2 before feeding to
the DC-DC converters, whereas the MAX16920B feeds
the SYNC frequency as it is to the DC-DC converters.
The duty cycle of the SYNC signal should be in the 30%
to 70% range. The SYNC input has an internal 200kΩ
pulldown resistor.
Controlled EMI with
Forced-Fixed Frequency
Under normal operating conditions, the MAX16920
attempts to operate at a constant switching frequency
for all load currents (DC-DC1 enters skip mode below
its skip-mode threshold). For tightest frequency control,
apply the operating frequency to SYNC. The advantage
of this is a more accurate switching frequency and more
predictable EMI characteristics.
Extremes of Input Voltage
In some cases, especially at high switching frequen-
cies, the MAX16920 is forced to deviate from its operat-
ing frequency independent of the state of SYNC. For
input voltages above 18V, the required duty cycle to
regulate the output can be lower than the minimum on-
time of the high-side switch (90ns typ). In this event, the
MAX16920 is forced to lower its switching frequency
by skipping pulses. In an analogous fashion when the
input voltage is reduced and the MAX16920 approaches
dropout, the device attempts to turn on the high-side
FET continuously. However, to maintain gate charge
on the high-side FET, the BST capacitor must be peri-
odically recharged. To ensure proper charge on the BST
capacitor when in dropout, the high-side FET is turned
off every 5.5μs for the MAX16920B and the low-side FET
is turned on for about 150ns. This gives an effective duty
cycle of > 97% and a switching frequency of 180kHz when
in dropout. (The MAX16920A refreshes the BST capacitor
every 7.5μs for 150ns and switches at 133kHz when in
dropout.)