MAX8545/MAX8546/MAX8548
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
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Detailed Description
The MAX8545/MAX8546/MAX8548 are BiCMOS switch-
mode power-supply controllers designed to implement
simple, buck-topology regulators in cost-sensitive
applications. The main power-switching circuit consists
of two n-channel MOSFETs, an inductor, and input/out-
put filter capacitors. An all n-channel synchronous-rec-
tified design provides high efficiency at reduced cost.
These devices have an internal 5V linear regulator that
steps down the input voltage to supply the IC and the
gate drivers. The low-side-switch gate driver is directly
powered from the 5V regulator (VL), while the high-
side-switch gate driver is indirectly powered from VL
plus an external diode-capacitor boost circuit.
Current-Limit and
Short-Circuit Protection
The MAX8545/MAX8546/MAX8548 employ a valley cur-
rent-sensing algorithm that uses the R
DS(ON)
of the low-
side n-channel MOSFET to sense the current. This
eliminates the need for an external sense resistor usually
placed in series with the output. The voltage measured
across the low-side MOSFET’s R
DS(ON)
is compared to
a fixed -320mV reference for the MAX8545/MAX8548
and a fixed -165mV reference for the MAX8546. The cur-
rent limit is given by the equations below:
Aside from current limiting, these devices feature fold-
back short-circuit protection. This feature is designed
to reduce the current limit by 80% as the output voltage
drops to 0V.
MOSFET Gate Drivers
The DH and DL drivers are optimized for driving n-chan-
nel MOSFETs with low gate charge. An adaptive dead-
time circuit monitors the DL output and prevents the
high-side MOSFET from turning on until the low-side
MOSFET is fully off. There must be a low-resistance,
low-inductance connection from the DL driver to the
MOSFET gate for the adaptive dead-time circuit to work
properly. Otherwise, the sense circuitry in the MAX8545/
MAX8546/MAX8548 may detect the MOSFET gate as off
while there is actually charge left on the gate. Use very
short, wide traces measuring no less than 50mils to 100
mils wide if the MOSFET is 1in away from the MAX8545/
MAX8546/MAX8548. The same type of adaptive dead-
time circuit monitors the DH off edge. The same recom-
mendations apply for the gate connection of the
high-side MOSFET.
The internal pulldown transistor that drives DL low is
robust, with a 1.1Ω (typ) on-resistance. This helps pre-
vent DL from being pulled up due to capacitive cou-
pling from the drain to the gate of the low-side
synchronous-rectifier MOSFET during the fast rise time
of the LX node.
Soft-Start
The MAX8545/MAX8546/MAX8548 feature an internally
set soft-start function that limits inrush current. It accom-
plishes this by ramping the internal reference input to the
controller’s transconductance error amplifier from 0 to
the 0.8V reference voltage. The ramp time is 1024 oscil-
lator cycles for the MAX8548 and 2048 oscillator cycles
for the MAX8545/MAX8546. At the nominal 100kHz and
300kHz switching rate, the soft-start ramp is approxi-
mately 10.2ms and 6.8ms, respectively.
High-Side Gate-Drive Supply (BST)
A flying-capacitor boost circuit generates gate-drive volt-
age for the high-side n-channel MOSFET. The flying
capacitor is connected between the BST and LX nodes.
On startup, the synchronous rectifier (low-side MOSFET)
forces LX to ground and charges the boost capacitor to
VL. On the second half-cycle, the MAX8545/MAX8546/
MAX8548 turn on the high-side MOSFET by closing an
internal switch between BST and DH. This provides the
necessary gate-to-source voltage to drive the high-side
MOSFET gate above its source at the input voltage.
Internal 5V Linear Regulator
All MAX8545/MAX8546/MAX8548 functions are internally
powered from an on-chip, low-dropout 5V regulator (VL).
These devices have a maximum input voltage (V
IN
) of
28V. Connect V
CC
to VL through a 10Ω resistor and
bypass V
CC
to GND with a 0.1µF ceramic capacitor. The
V
IN
-to-VL dropout voltage is typically 140mV, so when V
IN
is less than 5.5V, VL is typically V
IN
- 140mV.
The internal linear regulator can source a minimum of
25mA and a maximum of approximately 40mA to supply
power to the IC low-side and high-side MOSFET drivers.
Duty-Cycle Limitations for
Low V
OUT
/V
IN
Ratios
The MAX8545/MAX8546/MAX8548s’ output voltage is
adjustable down to 0.8V. However, the minimum duty
cycle can limit the ability to supply low-voltage outputs
