MAX15035 Datasheet, PDF (16/27 Pages) Maxim Integrated Products – 15A Step-Down Regulator with Internal Switches

15A Step-Down Regulator with Internal Switches
When the on-time expires, the controller re-enables the
low-side MOSFET until the controller detects that the
inductor current drops below the zero-crossing thresh-
old. Starting with a negative inductor current pulse
greatly reduces the peak output voltage when com-
pared to starting with a positive inductor current pulse.
The output voltage at the beginning of the ultrasonic
pulse determines the negative ultrasonic current
threshold, resulting in the following equation:
V_ISONIC = I_L × 0.006 = (V_REFIN − V_FB ) × 0.7
where V_FB > V_REFIN.
Forced-PWM Mode (SKIP = V_DD)
The low-noise, forced-PWM mode (SKIP = V_DD) dis-
ables the zero-crossing comparator, which controls the
low-side switch on-time. This forces the low-side gate-
drive waveform to constantly be the complement of the
high-side gate-drive waveform, so the inductor current
reverses at light loads while LX maintains a duty factor
of V_OUT/V_IN. The benefit of forced-PWM mode is to
keep the switching frequency fairly constant. However,
forced-PWM operation comes at a cost: the no-load 5V
bias current remains between 10mA to 50mA, depend-
ing on the switching frequency.
The MAX15035 automatically always uses forced-PWM
operation during shutdown, regardless of the SKIP
configuration.
Automatic Pulse-Skipping Mode (SKIP = GND or REF)
In skip mode (SKIP = GND or 3.3V), an inherent auto-
matic switchover to PFM takes place at light loads. This
switchover is affected by a comparator that truncates
the low-side switch on-time at the inductor current’s
zero crossing. The zero-crossing comparator threshold
is set by the differential across LX to PGND.
DC output-accuracy specifications refer to the threshold
of the error comparator. When the inductor is in continu-
ous conduction, the MAX15035 regulates the valley of
the output ripple, so the actual DC output voltage is
higher than the trip level by 50% of the output ripple
voltage. In discontinuous conduction (SKIP = GND and
I_OUT < I_LOAD(SKIP)), the output voltage has a DC regu-
lation level higher than the error-comparator threshold
by approximately 1.5% due to slope compensation.
When SKIP is pulled to GND, the MAX15035 remains in
pulse-skipping mode. Since the output is not able to
sink current, the timing for negative dynamic output-volt-
age transitions depends on the load current and output
capacitance. Letting the output voltage drift down is
typically recommended to reduce the potential for audi-
ble noise since this eliminates the input current surge
during negative output-voltage transitions. See Figures
4 and 5.
REFIN
OUTPUT
VOLTAGE
INTERNAL
PWM CONTROL
LX
PGOOD
OVP
ACTUAL V_OUT
INTERNAL TARGET
SKIP
NO PULSES: V_OUT > V_TARGET
BLANK HIGH-Z
SET TO REF + 300mV
DYNAMIC REFIN WINDOW
BLANK HIGH-Z
EA TARGET + 300mV
DYNAMIC TRANSITION WHEN SKIP# = GND
Figure 4. Dynamic Transition when SKIP = GND
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