User guide

40V, High-Performance, Synchronous
Buck Controller
MAX15046
10 _____________________________________________________________________________________
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 flex-
ibility in selecting passive components. The MAX15046
adopts an adaptive synchronous rectification to elimi-
nate external 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 temperature 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 conditions. 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 lev-
els of current while the regulator is sinking current from the
output. The MAX15046 shuts down at a +150NC 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 max-
imum flexibility in choosing the operating frequency and
output LC filter components. An internal transconduc-
tance error amplifier produces an integrated error volt-
age at COMP that helps to provide higher DC accuracy.
The voltage at COMP sets the duty cycle using a PWM
comparator 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 cur-
rent 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, pro-
viding current 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 Q10% power supply. The maximum
regulator input voltage (V
IN
) is 40V. Bypass IN to GND
with a 1FF ceramic capacitor. Bypass the output of the
linear regulator (V
CC
) with a 4.7FF ceramic capacitor to
GND. The V
CC
dropout voltage is typically 180mV. When
V
IN
is higher than 5.5V, V
VCC
is typically 5.25V. The
MAX15046 also employs an undervoltage lockout circuit
that disables the internal linear regulator when V
VCC
falls below 3.6V (typical). The 400mV UVLO hysteresis
prevents 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 preventing
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