Low-Cost, Wide Input Range, Step-Down

Controllers with Foldback Current Limit

8

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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-

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

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).

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

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

I

mV

R

MAX

LIMIT

DS ON

=

165

8546

()

()

I

mV

R

MAX

MAX

LIMIT

DS ON

=

320

8545

8548

()

(/

)