User guide

40V, High-Performance, Synchronous
Buck Controller
MAX15046
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When the valley current limit is reached during soft-start,
the MAX15046 regulates to the output impedance times
the limited inductor current and turns off after 4096 clock
cycles. When starting up into a large capacitive load (for
example), the inrush current will not exceed the current-
limit value. If the soft-start is not completed before 4096
clock cycles, the device turns off. The device remains
off for 8192 clock cycles before trying to soft-start again.
This implementation allows the soft-start time to be
automatically adapted to the time necessary to keep the
inductor current below the limit while charging the output
capacitor.
Power-Good Output (PGOOD)
The MAX15046 includes a power-good comparator to
monitor the output voltage and detect the power-good
threshold, fixed at 93% of the nominal FB voltage. The
open-drain PGOOD output requires an external pullup
resistor. PGOOD sinks up to 2mA of current while low.
PGOOD goes high (high-Z) when the regulator output
increases above 93% of the designed nominal regulated
voltage. PGOOD goes low when the regulator output volt-
age drops to below 90% of the nominal regulated voltage.
PGOOD asserts low during the hiccup timeout period.
Startup into a Prebiased Output
When the MAX15046 starts into a prebiased output, DH
and DL are off so that the converter does not sink current
from the output. DH and DL do not start switching until
the PWM comparator commands the first PWM pulse.
The first PWM pulse occurs when the ramping reference
voltage increases above the FB voltage.
When the output voltage is biased above the output set
point, the controller tries to pull the output down to the
set point once the internal soft-start is complete. This
pulldown is controlled by the sink current limit, which is
slowly increased to its normal value to minimize output
undershoot.
Current-Limit Circuit (LIM)
The current-limit circuit employs a ‘valley’ and sink
current-sensing algorithm that uses the on-resistance
of the low-side MOSFET as a current-sensing element,
to eliminate costly sense resistors. The current-limit
circuit is also temperature compensated to track the
on-resistance variation of the MOSFET overtemperature.
The current limit is adjustable with an external resistor at
LIM and accommodates MOSFETs with a wide range of
on-resistance characteristics (see the Setting the Valley
Current Limit section). The adjustment range is from 0.3V
to 3V for the valley current limit, corresponding to resistor
values of 6kI to 60kI. The valley current-limit threshold
across the low-side MOSFET is precisely 1/10th of the
voltage at LIM, while the sink current-limit threshold is
1/20th of the voltage at LIM.
Valley current limit acts when the inductor current flows
towards the load, and CSP is more negative than PGND
during the low-side MOSFET on-time. If the magnitude of
the current-sense signal exceeds the valley current-limit
threshold at the end of the low-side MOSFET on-time, the
MAX15046 does not initiate a new PWM cycle and lets
the inductor current decay in the next cycle. The control-
ler also ‘rolls back’ the internal reference voltage so that
the controller finds a regulation point determined by the
current-limit value and the resistance of the short. In this
manner, the controller acts as a constant current source.
This method greatly reduces inductor ripple current
during the short event, which reduces inductor sizing
restrictions and reduces the possibility for audible noise.
After 4096 clock cycles, the device goes into hiccup
mode. Once the short is removed, the internal reference
voltage soft-starts back up to the normal reference volt-
age and regulation continues.
Sink current limit is implemented by monitoring the volt-
age drop across the low-side MOSFET when CSP is
more positive than PGND. When the voltage drop across
the low-side MOSFET exceeds 1/20th of the voltage at
LIM at any time during the low-side MOSFET on-time, the
low-side MOSFET turns off and the inductor current flows
from the output through the body diode of the high-side
MOSFET. When the sink current limit activates, the DH/
DL switching sequence is no longer complementary and
both MOSFETs are turned off.
Carefully observe the PCB layout guidelines to ensure
that noise and DC errors do not corrupt the current-
sense signals at CSP and PGND. Mount the MAX15046
close to the low-side MOSFET with short, direct traces
making a Kelvin-sense connection so that trace resis-
tance does not add to R
DS(ON)
of the low-side MOSFET.
Hiccup Mode Overcurrent Protection
Hiccup mode overcurrent protection reduces power
dissipation during prolonged short-circuit or severe
overload conditions. An internal 3-bit counter counts up
on each switching cycle when the valley current-limit
threshold is reached. The counter counts down on each
switching cycle when the threshold is not reached, and
stops at zero (000). When the current-limit condition
persists and the counter reaches 111 (= 7 events), the