MAX16975
28V, 1.2A Automotive Step-Down Converter
with Low Operating Current
10Maxim Integrated
Detailed Description
The MAX16975 is a constant-frequency, current-mode
automotive buck converter with an integrated high-side
switch. The device operates with input voltages from
3.5V to 28V and tolerates input transients up to 42V.
During undervoltage events, such as cold-crank condi-
tions, the internal pass device maintains 94% duty cycle
for a short time.
An open-drain, active-low reset output helps to monitor
the output voltage. The device offers an adjustable reset
threshold that helps to keep microcontrollers alive down
to the lowest specified input voltage and a capacitor-
programmable reset timeout to ensure proper startup.
The switching frequency is resistor-programmable from
220kHz to 1.0MHz to allow optimization for efficiency,
noise, and board space. A clock input, FSYNC, allows
the device to synchronize to an external clock.
During light-load conditions, the device enters skip
mode that reduces the quiescent current down to 45FA
and increases light-load efficiency. The 5V fixed output
voltage eliminates the need for external resistors and
reduces the supply current by up to 50FA.
Linear Regulator Output (BIAS)
The device includes a 5V linear regulator, VBIAS, that
provides power to the internal circuitry. Connect a 1FF
ceramic capacitor from BIAS to GND. When the output
voltage is set between 3V and 5.5V, the internal linear
regulator only provides power until the output is in regula-
tion. The internal linear regulator turns off once the output
is in regulation and allows OUT to provide power to the
device. The internal regulator turns back on once the
external load on the output of the device is higher than
100mA. In addition, the linear regulator turns on anytime
the output voltage is outside the 3V to 5.5V range.
External Clock Input (FSYNC)
The device synchronizes to an external clock signal
applied at FSYNC. The signal at FSYNC must have a fre-
quency of 10% higher than the internal clock frequency
for proper synchronization.
Adjustable Reset Level
The device features a programmable reset threshold
using a resistive divider between OUT, RESETI, and
GND. Connect RESETI to GND for the internal threshold.
RES asserts low when the output voltage falls to 93% of
the programmed level. RES deasserts when the output
voltage goes above 95% of the set voltage.
Some microprocessors accept a wide input voltage range
(3.3V to 5V, for example) and can operate during dropout
of the device. Use a resistive divider at RESETI to adjust
the reset activation level (RES goes low) to lower levels.
The reference voltage at RESETI is 1.25V (typ).
The device also offers a capacitor-programmable reset
timeout period. Connect a capacitor from CRES to GND
to adjust the reset timeout period. When the output volt-
age goes out of regulation, RES asserts low and the
reset timing capacitor discharges with a 1mA pulldown
current. Once the output is back in regulation the reset
timing capacitor recharges with 10FA (typ) current. RES
stays low until the voltage at CRES reaches 1.13V (typ).
Dropout Operation
The device features an effective maximum duty cycle to
help refresh the BST capacitor when continuously oper-
ated in dropout. When the high-side switch is on for three
consecutive clock cycles, the device forces the high-side
switch off during the final 35% of the fourth clock cycle.
When the high-side switch is off, the LX node is pulled low
by the current flowing through the inductor. This increases
the voltage across the BST capacitor. To ensure that the
inductor has enough current to pull LX to ground, an
internal load sinks current from VOUT when the device is
close to dropout and external load is small. Once the input
voltage is increased above the dropout region, the device
continues to regulate at the set output voltage.
The device operates with no load and no external clock
at an effective maximum duty cycle of 94% in deep drop-
out. This effective maximum duty cycle is influenced by
the external load and by the optional external synchro-
nized clock.
System Enable (EN)
An enable-control input (EN) activates the device from
the low-power shutdown mode. EN is compatible with
inputs from the automotive battery level down to 3.3V.
The high-voltage compatibility allows EN to be con-
nected to SUP, KEY/KL30, or the INH inputs of a CAN
transceiver.
EN turns on the internal regulator. Once VBIAS is above
the internal lockout level, VUVL = 3.15V (typ), the control-
ler activates and the output voltage ramps up within 2048
cycles of the switching frequency.
A logic-low at EN shuts down the device. During shut-
down, the internal linear regulator and gate drivers turn
off. Shutdown mode reduces the quiescent current to
9FA (typ). Drive EN high to turn on the device.