Interleaved High-Efficiency, Dual Power-Supply

Controllers for Notebook Computers

DH_

LX_

MAX8716 DL_

MAX8717

MAX8757 PGND

NH

NL

CSH_

CSL_

a) OUTPUT SERIES RESISTOR SENSING

DH_

LX_

MAX8716 DL_

MAX8717

MAX8757 PGND

NH

NL

CSH_

CSL_

b) LOSSLESS INDUCTOR SENSING

INPUT (VIN)

CIN

L

DL

RSENSE

INPUT (VIN)

CIN

INDUCTOR

DL

REQ CEQ

RBIAS = REQ

COUT

COUT

Figure 8. Current-Sense Configurations

This configuration constantly monitors the inductor cur-

rent, allowing accurate current-limit protection.

Alternatively, high-power applications that do not

require highly accurate current-limit protection may

reduce the overall power dissipation by connecting a

series RC circuit across the inductor (Figure 8b) with an

equivalent time constant:

L

RL

= CEQ × REQ

where RL is the inductor’s series DC resistance. In this

configuration, the current-sense resistance equals the

inductor’s DC resistance (RSENSE = RL). Use the worst-

case inductance and RL values provided by the induc-

tor manufacturer, adding some margin for the

inductance drop over temperature and load.

Output Capacitor Selection

The output filter capacitor must have low enough equiv-

alent series resistance (ESR) to meet output ripple and

load-transient requirements, yet have high enough ESR

to satisfy stability requirements. The output capaci-

tance must be high enough to absorb the inductor

energy while transitioning from full-load to no-load con-

ditions without tripping the overvoltage fault protection.

When using high-capacitance, low-ESR capacitors (see

the Output-Capacitor Stability Considerations section),

the filter capacitor’s ESR dominates the output voltage

ripple. So the output capacitor’s size depends on the

maximum ESR required to meet the output-voltage-rip-

ple (VRIPPLE(P-P)) specifications:

VRIPPLE(P-P) = RESRILOAD(MAX)LIR

In idle mode, the inductor current becomes discontinu-

ous, with peak currents set by the idle-mode current-

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