MAX15026
Low-Cost, Small, 4.5V to 28V Wide Operating
Range, DC-DC Synchronous Buck Controller
9
Maxim Integrated
Detailed Description
The MAX15026 synchronous step-down controller oper-
ates from a 4.5V to 28V 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 MAX15026 offers adjustable switching frequency
from 200kHz to 2MHz with an external resistor. The
adjustable switching frequency provides design flexi-
bility in selecting passive components. The MAX15026
adopts an adaptive synchronous rectification to elimi-
nate an external freewheeling Schottky diode and
improve efficiency. The device utilizes the on-resis-
tance 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 temper-
ature-compensated, so that the effects of the MOSFET
R
DS(ON)
variation over temperature are reduced. This
current-sensing scheme protects the external compo-
nents from damage during output overloaded condi-
tions or output short-circuit faults without requiring a
current-sense resistor. Hiccup-mode current limit
reduces power dissipation during short-circuit condi-
tions. The MAX15026 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 MAX15026 features internal digital soft-start that
allows prebias startup without discharging the output.
The digital soft-start function employs sink current limit-
ing to prevent the regulator from sinking excessive cur-
rent 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
MAX15026 shuts down at a junction temperature of
+150°C to prevent damage to the device.
DC-DC PWM Controller
The MAX15026 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 current
ramps up. During the second-half of the switching cycle,
the high-side MOSFET turns off and the low-side n-chan-
nel MOSFET turns on. The inductor releases the stored
energy as the inductor current ramps down, providing
current to the output. Under overload conditions, when
the inductor current exceeds the selected valley current-
limit threshold (see the
Current-Limit Circuit (LIM)
sec-
tion), 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 28V. Bypass IN
to GND with a 1µF ceramic capacitor. Bypass the output
of the linear regulator (V
CC
) with a 4.7µF ceramic capac-
itor to GND. The V
CC
dropout voltage is typically 125mV.
When V
IN
is higher than 5.5V, V
CC
is typically 5.25V. The
MAX15026 also employs an undervoltage lockout circuit
that disables the internal linear regulator when V
CC
falls
below 3.6V (typ). The 400mV UVLO hysteresis prevents
chattering on power-up/power-down.
The internal V
CC
linear regulator can source up to
70mA to supply the IC, power the low-side gate driver,
recharge the external boost capacitor, and supply small
external loads. The current available for external loads
depends on the current consumed by the MOSFET
gate drivers.
For example, when switching at 600kHz, a MOSFET
with 18nC total gate charge (at V
GS
= 5V) requires
(18nC x 600kHz) = 11mA. The internal control functions
consume 5mA maximum. The current available for
external loads is:
(70 – (2 x 11) – 5)mA ≅ 43mA
MOSFET Gate Drivers (DH, DL)
DH and DL are optimized for driving large-size n-chan-
nel power MOSFETs. Under normal operating condi-
tions 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 cross-
conduction 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
other 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 turn-
ing on until the DH gate driver has turned off.
