MAX16904
2.1MHz, High-Voltage,
600mA Mini-Buck Converter
9
Detailed Description
The MAX16904 is a small, current-mode buck converter
that features synchronous rectification and requires no
external compensation network. The device is designed
for 600mA output current, and can stay in dropout by
running at 97% duty cycle. It provides an accurate out-
put voltage within the +6.5V to +18V input range.
Voltage quality can be monitored by observing the
PGOOD signal. The device operates at 2.1MHz (typ)
frequency, which allows for small external components,
reduced output ripple, and guarantees no AM band
interference.
The device features an ultra-low 25µA (typ) quiescent
supply current in standby mode. Standby mode is
entered when load currents are below 5mA and when
SYNC is low. The device operates from a +3.5V to
+28V supply voltage and tolerates transients up to
+42V, making it ideal for automotive applications. The
device is available in factory-trimmed output voltages
from 1.8V to 10.7V in 100mV steps. Contact the factory
for availability of voltage options.
Enable (EN)
The device is activated by driving EN high. EN is com-
patible from a +3.3V logic level to automotive battery
levels. EN can be controlled by microcontrollers and
automotive KEY or CAN inhibit signals. The EN input
has no internal pullup/pulldown current to minimize
overall quiescent supply current. To realize a program-
mable undervoltage lockout level, use a resistor-
divider from SUP to EN to GND.
BIAS/UVLO
The device 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 device features an internal soft-start timer. The out-
put 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 pattern
repeats until the short circuit has been removed.
Oscillator/Synchronization and
Efficiency (SYNC)
The device 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 operate 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 device offers the best of both
worlds. The device can be switched during operation
between forced PWM mode and SKIP mode by switch-
ing 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 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 (valley) drops below the 1.2V (typ) feed-
back voltage.
Achieving High Efficiency at Light Loads
The device operates with very low quiescent current at
light loads to enhance efficiency and conserve battery
life. When the device enters SKIP mode the output cur-
rent is monitored to adjust the quiescent current.
When the output current is < 5mA, the device operates in
the lowest quiescent current mode also called the stand-
by mode. In this mode, the majority of the internal circuit-
ry (excluding that necessary to maintain regulation) in the
device, including the internal high-voltage LDO, is turned
off to save current. Under no load and with SKIP mode
enabled, the device draws only 25µA (typ) current. For
load currents > 5mA, the device enters normal SKIP
mode while still maintaining very high efficiency.
Controlled EMI with Forced-Fixed Frequency
In forced PWM mode, the device attempts to operate at
a constant switching frequency for all load currents. For
tightest frequency control, apply the operating frequen-
cy 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 induc-
tor, 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 device 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 device is forced to
lower its switching frequency by skipping pulses.
