SC2542 datasheet(10/27 Pages) SEMTECH

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SC2542

POWER MANAGEMENT

Applications Information (Cont.)

Figure 4 further explains the basic theory of SKIP mode

operation. When the load current falls from CCM to DCM,

the duty cycle of DRVH (output of PWM comparator) will

shrink. The programmed minimum pulse width of

SC2542 with the DRVH signal to get the TG (top side

MOSFET gate driver) pulse output. The TG output will

maintain at least the programmed minimum pulse width.

This TG pulse pumps up the output voltage causing the

Error Amplifier output to decrease as the output voltage

moves up. The PWM comparator may skip several cycles

before sending another pulse when the output voltage

drops down. The BG (bottom side MOSFET gate driver)

output will terminate whenever the inductor current

crosses zero.

0.00 %

20 .00 %

40 .00 %

60 .00 %

80 .00 %

100.00 %

1 00

10 00

1 00 00

Io (m A)

Skip Mod e

S y nc hr onous CCM

Figure 5. Skip mode improves light load efficiency.

Figure 6 (a) and (b) show typical waveforms of SKIP mode

at different light load conditions. The frequency can ac-

tually fall very low at very light loads. When the switching

frequency drops to the acoustic frequency range, very

minor acoustic frequency noise might be generated, but

the level of the noise is usually very low due to very small

flux excursion in the magnetics. The acoustic noise can

be totally avoided by using well constructed magnetics

or by knowing the minimum system loads to program the

SKIP mode operation accordingly.

Figure 3. Conventional Buck converter duty cycle vs.

load current.

Figure 4. The waveforms of SKIP mode operation.

SKIP mode therefore leads to a reduction in the average

switching frequency. MOSFET switching losses and driver

losses, both of which are proportional to frequency, are

significantly reduced at these light loads resulting in in-

creased efficiency. SKIP mode also reduces the circulat-

ing currents associated with SPWM mode. The test re-

sult (Figure 5) shows the efficiency improvement in SKIP

mode.

(a)

Duty

Iout

Min. duty

clamp

DCM

CCM

Nominal

duty

Duty

Iout

Min. duty

clamp

DCM

CCM

Nominal

duty

Ramp

Veo

DRVH

Min pulse

DRVL

Ramp

Veo

DRVH

Min pulse

DRVL

Inductor

Current

Inductor

Current

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