SC461 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC461' PDF : 31 Pages View PDF
SC461
Applications Information (continued)
to the switcher output due to its maximum voltage rating.
An additional resistor divider network is required to
connect from the switcher output to the VOUT pin as
shown in Figure 4.
LX
L
COUT
VOUT > 5V
RV1
To VOUT pin
RV2
Figure 4 — Resistor Divider For VOUT Exceeding 5V
trade-off being reduced efficiency at light loads due to
the high-frequency switching of the MOSFETs.
The PSV pin contains a 5μA current sink to prevent stray
leakage current from pulling the PSV pin up to the VDDA
supply when the PSV pin is floated to select Power-Save
operation. To select Forced Continuous Mode operation,
the maximum recommended resistance between the
VDDA supply and the PSV pin is 40kW.
FB Ripple
Voltage (VFB)
FB threshold
The resistors must be chosen so that the VOUT does not
exceed the VDDA supply. Note that the VOUT pin has an
internal 500kW resistor connected to AGND. To minimize
the effect of this resistor on the resistor divider ratio, the
maximum recommend value for resistor RV2 in Figure 4 is
10kW.
In addition to the resistor divider, the RTON resistor value
must be adjusted. The on-time is calculated according to
the voltage at the VOUT pin. In order to select the desired
on-time and operating frequency, the RTON resistor
should be adjusted to a higher value to compensate for
the reduced voltage at the VOUT pin. For output voltages
exceeding 5V, the required RTON value can be determined
by the following equation.
5721
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9287
9,1 u I6:
QV¸¸¹· u
S) u 9287
9,1
u
¨¨©§
5 9
5 9
¸¸¹·
For applications where VOUT exceeds 5V, FCM operation is
recommended.
Forced Continuous Mode Operation
The SC461 operates the switcher in Forced Continuous
Mode (FCM) by connecting the PSV pin to VDDA. The PSV
pin should never exceed the VDDA supply. See Figure 5
for FCM waveforms. In this mode one of the power
MOSFETs is always on, with no intentional dead time other
than to avoid cross-conduction. This results in more
uniform frequency across the full load range, with the
Inductor
Current
DC Load Current
On-time DH on-time is triggered when
(TON) VFB reaches the FB Threshold.
DH
DL
DL drives high when on-time is completed.
DL remains high until VFB falls to the FB threshold.
Figure 5 — Forced Continuous Mode Operation
Power-Save Mode Operation
The SC461 provides power-save operation at light loads
with no minimum operating frequency, selected by float-
ing the PSV pin (no connection). In this mode of opera-
tion, the zero cross comparator monitors inductor current
via the voltage across the low-side MOSFET during the
off-time. If the inductor current falls to zero for 8 consecu-
tive switching cycles, the controller enters power-save
operation. It will then turn off the low-side MOSFET on
each subsequent cycle, provided that the current falls to
zero. After the low-side MOSFET is off, both high-side and
low-sides MOSFETs remain off until VFB drops to the 600mV
threshold. While the MOSFETs are off the load is supplied
15
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