sc2542 Semtech Corporation, sc2542 Datasheet - Page 10

• Manufacturer Part Number: sc2542
• Description: Sc2542 High Performance Wide Input Rangedual Synchronous Buck Controller
• Manufacturer: Semtech Corporation

Figure 3. Conventional Buck converter duty cycle vs.
load current.
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.
© 2005 Semtech Corp.
POWER MANAGEMENT
Applications Information (Cont.)
Figure 4. The waveforms of SKIP mode operation.
Ramp
Ramp
Min pulse
Min pulse
DRVH
DRVH
DRVL
DRVL
Veo
Veo
Vo
Vo
TG
TG
Nominal
Nominal
duty
duty
Min. duty
Min. duty
clamp
clamp
I
I
L
L
Duty
Duty
DCM
DCM
D
D
<
<
Vo
Vo
Vi
Vi
D
D
CCM
CCM
Vo
Vo
Vi
Vi
Iout
Iout
10
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.
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.
1 00 .00 %
80 .00 %
60 .00 %
40 .00 %
20 .00 %
0 .00 %
10
1 00
Io (m A)
(a)
10 00
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SC2542
1 00 00
Inductor
Inductor
Current
Current
TG
TG
BG
BG
Vo
Vo
Skip Mod e
Sy n c h r on o us CCM