SC403BEVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC403BEVB Datasheet PDF : 32 Pages
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SC403B
Applications Information (continued)
Note that this control method regulates the valley of the
output ripple voltage, not the DC value. The DC output
voltage VOUT is offset by the output ripple according to the
following equation.
FB Ripple
Voltage (VFB)
FB threshold
VOUT
VFB
u ¨¨©§1
R1
R2
¸¸¹·
¨§
©
VRIPPLE
2
¸·
¹
When a large capacitor is placed in parallel with R1 (CTOP)
VOUT is shown by the following equation.
VOUT
VFB
u
¨¨©§1
R1
R2
¸¸¹·
¨§
©
VRIPPLE
2
¸·
¹
u
1 (R1ZCTOP )2
1
¨¨©§
R2
R2
u
R1
R1
ZCTOP
¸¸¹·2
Inductor
Current
DH
DC Load Current
On-time DH on-time is triggered when
(TON) VFB reaches the FB Threshold.
Enable and Power Save Input
The EN/PSV input is used to enable or disable the switch-
ing regulator. When EN/PSV is low (grounded), the switch-
ing regulator is off and in its lowest power state. When off,
the output of the switching regulator soft-discharges the
output into a 15Ω internal resistor via the VOUT pin. When
EN/PSV is allowed to float, the pin voltage will float to 33%
of the voltage at VDD. The switching regulator turns on
with PSAVE (power save) disabled and all switching is in
forced continuous mode.
When EN/PSV is high (above 45% of the voltage at VDD),
the switching regulator turns on with power-save
enabled.
Forced Continuous Mode Operation
The SC403B operates the switcher in FCM (Forced
Continuous Mode) by floating the EN/PSV pin (see Figure
4). In this mode one of the power MOSFETs is always on,
with no intentional dead time other than to avoid cross-
conduction. This feature results in uniform frequency
across the full load range with the trade-off being poor
efficiency at light loads due to the high-frequency switch-
ing of the MOSFETs. DH is the gate signal driving the
upper MOSFET. DL is the lower gate signal driving the
lower MOSFET.
DL
DL drives high when on-time is completed.
DL remains high until VFB falls to the FB threshold.
Figure 4 — Forced Continuous Mode Operation
Power-save Operation
The SC403B provides power-save operation at light loads
with no minimum operating frequency. With power-save
enabled, the internal zero crossing comparator monitors
the inductor current via the voltage across the low-side
MOSFET during the off-time. If the inductor current falls to
zero for 8 consecutive switching cycles, the controller
enters power-save operation. It will turn off the low-side
MOSFET on each subsequent cycle provided that the
current crosses zero. At this time both MOSFETs remain off
until VFB drops to the 600mV threshold. Because the
MOSFETs are off, the load is supplied by the output
capacitor.
17
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