MAX8743(2004) 데이터 시트보기 (PDF) - Maxim Integrated

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MAX8743
(Rev.:2004)

Dual, High-Efficiency, Step-Down Controller with High Impedance in Shutdown

Maxim Integrated 

Dual, High-Efficiency, Step-Down

Controller with High Impedance in Shutdown

∆i = VBATT - VOUT

∆t

L

IPEAK

ILOAD = IPEAK / 2

IPEAK

ILOAD

ILIMIT

0 ON-TIME

TIME

Figure 4. Pulse-Skipping/Discontinuous Crossover Point

circuit to work properly. Otherwise, the sense circuitry

in the MAX8743 interprets the MOSFET gate as “off”

while there is actually still charge left on the gate. Use

very short, wide traces measuring 10 to 20 squares (50

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

MAX8743).

The dead time at the other edge (DH turning off) is

determined by a fixed 35ns (typ) internal delay.

The internal pulldown transistor that drives DL low is

robust, with a 0.5Ω typical on-resistance. This helps

prevent DL from being pulled up during the fast rise

time of the inductor node, due to capacitive coupling

from the drain to the gate of the low-side synchronous-

rectifier MOSFET. However, for high-current applica-

tions, some combinations of high- and low-side FETs

might be encountered that will cause excessive gate-

drain coupling, which can lead to efficiency-killing,

EMI-producing shoot-through currents. This is often

remedied by adding a resistor in series with BST, which

increases the turn-on time of the high-side FET without

degrading the turn-off time (Figure 6).

0

TIME

Figure 5. ‘‘Valley’’ Current-Limit Threshold Point

MAX8743

+5V

VIN

BST

5Ω

DH

LX

Figure 6. Reducing the Switching-Node Rise Time

POR, UVLO, and Soft-Start

Power-on reset (POR) occurs when VCC rises above

approximately 2V, resetting the fault latch and prepar-

ing the PWM for operation. Below 4.05V (min), the VCC

undervoltage-lockout (UVLO) circuitry inhibits switching

by keeping DH and DL low.

Soft-start allows a gradual increase of the internal cur-

rent-limit level during startup to reduce the input surge

currents. When ON1 or ON2 goes high, the respective

digital soft-start timer begins to ramp up the maximum

allowed current limit in five steps. During the first step,

the controller limits the current limit to only 20% of the full

current limit. The current limit is increased by 20% every

425µs. 100% current limit is available after 1.7ms ±50%.

A continuously adjustable analog soft-start function can

be realized by adding a capacitor in parallel with the

ILIM external resistor-divider network. This soft-start

method requires a minimum interval between power-

down and power-up to discharge the capacitor.

16 ______________________________________________________________________________________

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