Step-Down Controllers with Prebias Startup,

Lossless Sensing, Synchronization, and OVP

______________________________________________________________________________________

17

Use very short, wide traces, about 10 to 20 squares

(50 mils to 100 mils wide if the MOSFET is 1in from the

device) for the gate drive. The dead time at the other

edge (DH turning off) also has an adaptive dead-time

circuit operating in a similar manner. For both edges,

there is an additional fixed dead time after the adaptive

dead time expires.

High-Side Gate-Drive Supply (BST)

A flying capacitor boost circuit (Figure 3) generates the

gate-drive voltage for the high-side n-channel MOSFET.

The capacitor between BST and LX is charged from VL

low-side MOSFET is on. When the low-side MOSFET is

switched off, the stored voltage of the capacitor is

stacked above LX to provide the necessary turn-on

then closes an internal switch between BST and DH to

turn the high-side MOSFET on.

Current-Sense Amplifier

The MAX8543/MAX8544 current-sense circuit amplifies

gain of the current-sense amplifier is determined by the

states of ILIM and ILIM1. This amplified current-sense

signal and the internal slope-compensation signal are

parator’s inverting input. The PWM comparator shuts

The differential current sense is also used to provide

peak inductor current limiting. This current limit is more

accurate than the valley current limit, which is measured

across the low-side MOSFET’s on-resistance.

Current-Limit Circuit

The MAX8543/MAX8544 use both valley foldback current

limiting and peak constant current limiting, simultaneously

(Figure 4). The valley foldback current limit is used to

reduce power dissipation of external components, mainly

inductor and power MOSFETs, and upstream power

source, when output is severely overloaded or short

circuited. Thus the circuit can withstand short-circuit

conditions indefinitely without causing overheating of any

component. The peak constant current limit sets the cur-

rent-limit point more accurately since it does not have to

suffer the wide variation of the low-side power MOSFET’s

on-resistance due to tolerance and temperature.

The valley current is sensed across the on-resistance of

limit trips when the sensed current exceeds the valley

current-limit threshold. The valley current limit recovers

when the sensed current drops below the valley current-

limit threshold (except when using the latch-off option

with the MAX8544).

Set the minimum valley current-limit threshold, when the

output voltage is at a nominal regulated value, higher

than the maximum peak current-limit setting. With this

method, the current-limit point accuracy is controlled by

the peak current limit and is not interfered with by the

wide variation of MOSFET on-resistance. See the Setting

the Current Limits section for how to set these limits.

The MAX8543 has a fixed valley current-limit threshold

and fixed foldback ratio. The MAX8544 can select

between an adjustable valley current-limit threshold

with adjustable foldback ratio and a fixed valley current

limit without foldback for latch-off. When latch-off is

used (MODE is connected to VL), set the current-limit

threshold by only one resistor from ILIM2 to GND and

make sure this threshold is higher than the maximum

output current required by at least a 20% margin. Cycle

EN or input power to reset the current-limit latch.

The peak current limit is used to sense the inductor

current, and is more accurate than the valley current limit

since it does not depend upon the on-resistance of the

low-side MOSFET. The peak current can be measured

across the resistance of the inductor for the highest

efficiency, or alternatively, a current-sense resistor can

be used for more accurate current sensing. The

MAX8543/MAX8544 have four selectable peak current-

limit thresholds that are selected using ILIM (MAX8543)

or ILIM1 (MAX8544). See Table 3 for the current-limit

settings.

For more information on the current limit, see the

Setting the Current Limits section.

MAX8543/

MAX8544

BST

IN

DH

LX

DL

N

N

Figure 3. The boost circuit provides voltage for the high-side

MOSFET gate drive.