Datasheet

Detailed Description
The MAX15046 synchronous step-down controller oper-
ates from a 4.5V to 40V input-voltage range and gener-
ates an adjustable output voltage from 85% of the input-
voltage down to 0.6V while supporting loads up to 25A. As
long as the device supply voltage is within 5.0V to 5.5V,
the input power bus (V
IN
) can be as low as 3.3V.
The MAX15046 offers adjustable switching frequency
from 100kHz to 1MHz with an external resistor. The
adjustable switching frequency provides design flexibility
in selecting passive components. The MAX15046 adopts
an adaptive synchronous rectification to eliminate exter-
nal freewheeling Schottky diodes and improve efficiency.
The device utilizes the on-resistance of the external low-
side MOSFET as a current-sense element. The current-
limit threshold voltage is resistor-adjustable from 30mV
to 300mV and is temperature-compensated, so that the
effects of the MOSFET R
DS(ON)
variation over tempera-
ture are reduced. This current-sensing scheme protects
the external components from damage during output
overloaded conditions or output short-circuit faults without
requiring a current-sense resistor. Hiccup-mode current
limit reduces power dissipation during short-circuit condi-
tions. The MAX15046 includes a power-good output and
an enable input with precise turn-on/-off threshold to be
used for monitoring and for power sequencing.
The MAX15046 features internal digital soft-start that
allows prebias startup without discharging the output. The
digital soft-start function employs sink current limiting to
prevent the regulator from sinking excessive current when
the prebias voltage exceeds the programmed steady-state
regulation level. The digital soft-start feature prevents the
synchronous rectifier MOSFET and the body diode of the
high-side MOSFET from experiencing dangerous levels
of current while the regulator is sinking current from the
output. The MAX15046 shuts down at a +150°C junction
temperature to prevent damage to the device.
DC-DC PWM Controller
The MAX15046 step-down controller uses a PWM volt-
age-mode control scheme (see the Functional Diagram).
Control-loop compensation is external for providing maxi-
mum flexibility in choosing the operating frequency and
output LC filter components. An internal transconduc-
tance error amplifier produces an integrated error voltage
at COMP that helps to provide higher DC accuracy. The
voltage at COMP sets the duty cycle using a PWM com-
parator and a ramp generator. On the rising edge of an
internal clock, the high-side n-channel MOSFET turns on,
and remains on, until either the appropriate duty cycle or
the maximum duty cycle is reached. During the on-time
of the high-side MOSFET, the inductor current ramps
up. During the second-half of the switching cycle, the
high-side MOSFET turns off and the low-side n-channel
MOSFET turns on. The inductor releases the stored
energy as the inductor current ramps down, providing cur-
rent to the output. Under overload conditions, when the
inductor current exceeds the selected valley current-limit
threshold (see the Current-Limit Circuit (LIM) section), the
high-side MOSFET does not turn on at the subsequent
clock rising edge and the low-side MOSFET remains on
to let the inductor current ramp down.
Internal 5.25V Linear Regulator
An internal linear regulator (V
CC
) provides a 5.25V nomi-
nal supply to power the internal functions and to drive the
low-side MOSFET. Connect IN and V
CC
together when
using an external 5V ±10% power supply. The maximum
regulator input voltage (V
IN
) is 40V. Bypass IN to GND
with a 1µF ceramic capacitor. Bypass the output of the
linear regulator (V
CC
) with a 4.7µF ceramic capacitor
to GND. The V
CC
dropout voltage is typically 180mV.
When V
IN
is higher than 5.5V, V
CC
is typically 5.25V. The
MAX15046 also employs an undervoltage lockout circuit
that disables the internal linear regulator when V
CC
falls
below 3.6V (typical). The 400mV UVLO hysteresis pre-
vents chattering on power-up/power-down.
MOSFET Gate Drivers (DH, DL)
DH and DL are optimized for driving large-size n-channel
power MOSFETs. Under normal operating conditions and
after startup, the DL low-side drive waveform is always
the complement of the DH high-side drive waveform,
with controlled dead time to prevent crossconduction or
“shoot-through.” An adaptive dead-time circuit monitors
the DH and DL outputs and prevents the opposite-side
MOSFET from turning on until the MOSFET is fully off.
Thus, the circuit allows the high-side driver to turn on only
when the DL gate driver has turned off and prevents the
low side (DL) from turning on until the DH gate driver has
turned off.
The adaptive driver dead time allows operation without
shoot-through with a wide range of MOSFETs, minimiz-
ing delays and maintaining efficiency. There must be a
low-resistance, low-inductance path from DL and DH to
the MOSFET gates for the adaptive dead-time circuits
to function properly. The stray impedance in the gate
MAX15046 40V, High-Performance, Synchronous
Buck Controller
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