MAX16922_10 Datasheet, PDF (11/17 Pages) Maxim Integrated Products – 2.2MHz, Dual, Step-Down DC-DC Converters, Dual LDOs, and RESET

2.2MHz, Dual, Step-Down DC-DC
Converters, Dual LDOs, and RESET
Detailed Description
The MAX16922 PMIC is designed for medium power
level automotive applications requiring multiple sup-
plies in a small footprint. As shown in the Typical
Applications Circuit, the MAX16922 integrates one
high-voltage power supply and three low-voltage cas-
caded power supplies. OUT1 and OUT2 are step-down
DC-DC converters, and OUT3 and OUT4 are linear reg-
ulators. The device also includes a reset output
(RESET) and a high-voltage compatible enable input
(EN). The operating input voltage range is from 3.5V to
28V and tolerant of transient voltages up to 45V.
OUT1 Step-Down DC-DC Regulator
Step-Down Regulator Architecture
OUT1 is a high-input voltage, high-efficiency 2.2MHz
PWM current-mode step-down DC-DC converter that
delivers up to 1.2A. OUT1 has an internal high-side n-
channel switch and uses a low forward-drop free-
wheeling diode for rectification. Under normal
operating conditions, OUT1 is fixed frequency to pre-
vent unwanted AM radio interference. However, under
light loads and high-input voltage, the step-down regu-
lator skips cycles to maintain regulation. The output
voltage is factory selectable from 3.0V to 5.5V in 50mV
increments.
Soft-Start
When initially powered up or enabled with EN, the
OUT1 step-down regulator soft-starts by gradually
ramping up the output voltage for approximately 2.2ms.
This reduces inrush current during startup. During soft-
start the full output current is available. Before a soft-
start sequence begins, the outputs of both DC-DC
regulators discharge below 1.25V through an internal
resistor. See the startup waveforms in the Typical
Operating Characteristics section.
Current Limit
The MAX16922 limits the peak inductor current
sourced by the n-channel MOSFET. When the peak
current limit is reached, the internal n-channel MOSFET
turns off for the remainder of the cycle. If the current
limit is exceeded for 16 consecutive cycles and the
output voltage is less than 1.25V, the n-channel MOS-
FET is turned off for 256 clock cycles to allow the
inductor current to discharge and then initiate a soft-
start sequence for all four outputs.
Dropout
The high-voltage, step-down converter (OUT1) of the
MAX16922 is designed to operate near 100% duty-
cycle. When the input voltage is close to the output
voltage, the device tries to maintain the high-side
switch on with 100% duty cycle. However, to maintain
proper gate charge, the high-side switch must be
turned off periodically so the LX pin can go to ground
and charge the BST capacitor. As the input voltage
approaches the output voltage, the effective duty cycle
of the n-channel MOSFET approaches 94%. Every 4th
cycle is limited to a maximum duty cycle of 75%
(recharge period is approximately 112ns) while the
remaining cycles can go to 100% duty cycle. As a
result, when the MAX16922 is in dropout, the switching
frequency is reduced by a factor of 4.
During dropout conditions under light load, the load
current may not be sufficient to enable the LX pin to
reach ground during the recharge period. To ensure
the LX pin is pulled to ground and proper BST capaci-
tor recharge occurs, an internal load is applied to
OUTS1 when PV1 falls below approximately 6.5V. This
load is approximately 70Ω and is connected between
OUTS1 and GND3 through an internal switch.
OUT2 Step-Down DC-DC Regulator
Step-Down Regulator Architecture
OUT2 is a low-input voltage, high-efficiency 2.2MHz
PWM current-mode step-down DC-DC converter that
outputs up to 600mA. OUT2 has an internal high-side
p-channel switch, and low-side n-channel switch for
synchronous rectification. The DC-DC regulator sup-
ports auto-PWM operation so that under light loads the
device automatically enters high-efficiency skip mode.
The auto-PWM mode can be disabled by connecting
the PWM input to OUTS1. The output voltage is factory
selectable from 1.0V to 3.9V in 50mV increments.
Soft-Start
OUT2 enters soft-start when OUT1 finishes its soft-start
sequence to prevent high startup current from exceed-
ing the maximum capability of OUT1. The step-down
regulator executes a soft-start by gradually ramping up
the output voltage for approximately 1.5ms. This
reduces inrush current during startup. During soft-start,
the full output current is available. The soft-start
sequence on OUT2 begins after the soft-start sequence
is completed on OUT1. See the startup waveforms in
the Typical Operating Characteristics section.
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