Data Sheet

© 2011 Fairchild Semiconductor Corporation www.fairchildsemi.com
FDMF6705B • Rev. 1.0.3 14
FDMF6705B - Extra-Small, High-Performance, High-Frequency DrMOS Module
Skip Mode (SMOD)
The SMOD function allows higher converter efficiency
under light-load conditions. During SMOD, the low-side
FET gate signal is disabled (held LOW), preventing
discharging of the output capacitors as the filter inductor
current attempts reverse current flow – also known as
Diode Emulation Mode. When the SMOD# pin is pulled
HIGH, the synchronous buck converter works in
Synchronous Mode, gating on the low-side FET. When
the SMOD# pin is pulled LOW, the low-side FET is
gated off. The SMOD# pin is connected to the PWM
controller, which enables or disables SMOD
automatically when the controller detects light-load
condition from output current sensing. Normally this pin
is active LOW. See Figure 27 for timing delays.
Table 2. SMOD Logic
DISB# PWM SMOD# GH GL
0 X X 0 0
1 3-State X 0 0
1 0 0 0 0
1 1 0 1 0
1 0 1 0 1
1 1 1 1 0
Note:
4. The SMOD feature is intended to have low
propagation delay between the SMOD signal and
the low-side FET V
GS
response time to control
diode emulation on a cycle-by-cycle basis.
Figure 27. SMOD Timing Diagram
t
D_DEADON
PWM
V
SWH
GH
to
V
SWH
GL
t
PD_PHGLL
t
PD_PLGHL
t
D_DEADOFF
V
IH_PWM
V
IL_PWM
90%
10%
90%
1.0V
2.2V
t
PD_PHGHH
t
PD_SHGLH
Delay from SMOD# going
HIGH to LS V
GS
HIGH
HS turn -on with SMOD# LOW
SMOD
#
t
PD_SLGLL
Delay from SMOD# going
LOW to LS V
GS
LOW
DCM
CCM
CCM
10%
V
IH_PWM
10%
V
OUT
V
IH_SMOD
V
IL_SMOD
10%