MAX15035 Datasheet, PDF (15/27 Pages) Maxim Integrated Products – 15A Step-Down Regulator with Internal Switches

15A Step-Down Regulator with Internal Switches
Power-on reset (POR) occurs when V_CC rises above
approximately 3V, resetting the fault latch and prepar-
ing the controller for operation. The V_CC UVLO circuitry
inhibits switching until V_CC rises above 4.25V. The con-
troller powers up the reference once the system
enables the controller, V_CC exceeds 4.25V, and EN is
driven high. With the reference in regulation, the con-
troller ramps the output voltage to the target REFIN volt-
age with a 1.2mV/µs slew rate:
t_START
=
V_FB
1.2mV
µs
=
V_FB
1.2V ms
The soft-start circuitry does not use a variable current
limit, so full output current is available immediately.
PGOOD becomes high impedance approximately
200µs after the target REFIN voltage has been reached.
The MAX15035 automatically uses pulse-skipping mode
during soft-start and uses forced-PWM mode during
soft-shutdown, regardless of the SKIP configuration.
For automatic startup, the input voltage should be pre-
sent before V_CC. If the controller attempts to bring the
output into regulation without the input voltage present,
the fault latch trips. The controller remains shut down
until the fault latch is cleared by toggling EN or cycling
the V_CC power supply below 0.5V.
If the V_CC voltage drops below 4.25V, the controller
assumes that there is not enough supply voltage to
make valid decisions. To protect the output from over-
voltage faults, the controller shuts down immediately
and forces a high impedance on LX.
Shutdown
When the system pulls EN low, the MAX15035 enters
low-power shutdown mode. PGOOD is pulled low
immediately, and the output voltage ramps down with a
1.2mV/µs slew rate:
t_SHDN
=
V_FB
1.2mV
µs
=
V_FB
1.2V ms
Slowly discharging the output capacitors by slewing
the output over a long period of time (typically 0.5ms to
2ms) keeps the average negative inductor current low
(damped response), thereby preventing the negative
output-voltage excursion that occurs when the con-
troller discharges the output quickly by permanently
turning on the low-side MOSFET (underdamped
response). This eliminates the need for the Schottky
diode normally connected between the output and
ground to clamp the negative output-voltage excursion.
After the controller reaches the zero target, the
MAX15035 shuts down completely—the drivers are dis-
abled (high impedance on LX)—the reference turns off,
and the supply currents drop to about 0.1µA (typ).
When a fault condition—output UVP or thermal shut-
down—activates the shutdown sequence, the protection
circuitry sets the fault latch to prevent the controller from
restarting. To clear the fault latch and reactivate the
controller, toggle EN or cycle V_CC power below 0.5V.
The MAX15035 automatically uses pulse-skipping mode
during soft-start and uses forced-PWM mode during
soft-shutdown, regardless of the SKIP configuration.
Modes of Operation
Ultrasonic Mode (SKIP = Open = 3.3V)
Leaving SKIP unconnected activates a unique pulse-
skipping mode with a minimum switching frequency of
18kHz. This ultrasonic pulse-skipping mode eliminates
audio-frequency modulation that would otherwise be
present when a lightly loaded controller automatically
skips pulses. In ultrasonic mode, the controller automati-
cally transitions to fixed-frequency PWM operation when
the load reaches the same critical conduction point
(I_LOAD(SKIP)) that occurs when normally pulse skipping.
An ultrasonic pulse occurs when the controller detects
that no switching has occurred within the last 33µs.
Once triggered, the ultrasonic controller turns on the
low-side MOSFET to induce a negative inductor current
(Figure 3). After the inductor current reaches the nega-
tive ultrasonic current threshold, the controller turns off
the low-side MOSFET and triggers a constant on-time.
33µs (typ)
INDUCTOR
CURRENT
0
I_SONIC
ZERO-CROSSING
DETECTION
ON-TIME (t_ON)
Figure 3. Ultrasonic Waveform
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