MAX16903
2.1MHz, High-Voltage, 1A Mini-Buck Converter
The MAX16903 features an ultra-low 25μA (typ) quies-
cent supply current in standby mode. Standby mode is
entered when load currents are below 5mA and when
SYNC is low. The MAX16903 operates from a +3.5V to
+28V supply voltage and tolerates transients up to
+42V, making it ideal for automotive applications. The
MAX16903 is available in factory-trimmed output volt-
ages from 1.8V to 10.7V in 100mV steps. Please con-
tact factory for availability of voltage options.
Enable (EN)
The MAX16903 is activated by driving EN high. EN is
compatible from a +3.3V logic level to automotive bat-
tery levels. EN can be controlled by microcontrollers
and automotive KEY or CAN inhibit signals. The EN
input has no internal pullup/pulldown current to mini-
mize overall quiescent supply current. To realize a pro-
grammable undervoltage lockout level, use a resistor-
divider from SUP to EN to GND.
BIAS/UVLO
The MAX16903 features undervoltage lockout. When the
device is enabled, an internal bias generator turns on.
LX begins switching after V BIAS has exceeded the inter-
nal undervoltage lockout level V UVLO = 3V (typ).
Soft-Start
The MAX16903 features an internal soft-start timer. The
output voltage soft-start ramp time is 8ms (typ). If a
short circuit or undervoltage is encountered, after the
soft-start timer has expired, the device is disabled for
30ms (typ) and it reattempts soft-start again. This pat-
tern repeats until the short circuit has been removed.
Oscillator/Synchronization and
Efficiency (SYNC)
The MAX16903 has an on-chip oscillator that provides
a switching frequency of 2.1MHz (typ). Depending on
the condition of SYNC, two operation modes exist. If
SYNC is unconnected or at GND, the device must oper-
ate in highly efficient pulse-skipping mode if the load
current is below the SKIP mode current threshold. If
SYNC is at BIAS or has a frequency applied to it, the
device is in forced PWM mode. The MAX16903 offers
the best of both worlds. The device can be switched
during operation between forced PWM mode and SKIP
mode by switching SYNC.
SKIP Mode Operation
SKIP mode is entered when the SYNC pin is connected to
ground or is unconnected and the peak load current is
< 350mA (typ). In this mode, the high-side FET is turned
on until the current in the inductor is ramped up to 350mA
(typ) peak value and the internal feedback voltage is
above the regulation voltage (1.2V typ). At this point, both
8
the high-side and low-side FETs are turned off.
Depending on the choice of the output capacitor and the
load current the high-side FET turns on when OUTS (val-
ley) drops below the 1.2V (typ) feedback voltage.
Achieving High Efficiency at Light Loads
The MAX16903 operates with very low quiescent current
at light loads to enhance efficiency and conserve battery
life. When the MAX16903 enters SKIP mode the output
current is monitored to adjust the quiescent current.
When the output current is < 5mA, the MAX16903 oper-
ates in the lowest quiescent current mode also called the
standby mode. In this mode, the majority of the internal
circuitry (excluding that necessary to maintain regulation)
in the MAX16903, including the internal high-voltage
LDO, is turned off to save current. Under no load and
with SKIP mode enabled, the IC draws only 25μA (typ)
current. For load currents > 5mA, the IC enters normal
SKIP mode still maintaining very high efficiency.
Controlled EMI with Forced-Fixed Frequency
In forced PWM mode, the MAX16903 attempts to oper-
ate at a constant switching frequency for all load cur-
rents. For tightest frequency control, apply the
operating frequency to SYNC. The advantage of this
mode is a constant switching frequency, which
improves EMI performance; the disadvantage is that
considerable current can be thrown away. If the load
current during a switching cycle is less than the current
flowing through the inductor, the excess current is
diverted to GND. With no external load present, the
operating current is in the 10mA range.
Extended Input Voltage Range
In some cases, the MAX16903 is forced to deviate from
its operating frequency independent of the state of SYNC.
For input voltages above 18V, the required duty cycle to
regulate its output may be smaller than the minimum on-
time (80ns, typ). In this event, the MAX16903 is forced to
lower its switching frequency by skipping pulses.
If the input voltage is reduced and the MAX16903
approaches dropout the device tries to turn on the high-
side FET continuously. In order to maintain gate charge
on the high-side FET, the BST capacitor must be period-
ically recharged. To ensure proper charge on the BST
capacitor when in dropout, the high-side FET is turned
off every 6.5μs and the low-side FET is turned on for
about 150ns. This gives an effective duty cycle of > 97%
and a switching frequency of 150kHz when in dropout.
Spread-Spectrum Option
The MAX16903 has an optional spread-spectrum version.
If this option is selected, then the internal operating fre-
quency varies by +6% relative to the internally generated
Maxim Integrated