SC172 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC172
Semtech Corporation
'SC172' PDF : 27 Pages View PDF
SC172
Applications Information (continued)
FB
PWM
-
REF +
SQ
R
VOUT
VIN On-Shot Timing
Generator
RTON
Time = K x VOUT/VIN
Hi-Side
and
Lo-Side
Gate
Drivers
VIN
LPF VOUT
Q1
L
VLX
Q2 ESR
FB
COUT +
Figure 2 — On-Time Generation
The SC172 uses an external resistor to set the on-time
which indirectly sets the frequency. The on-time can
be programmed to provide operating frequency from
200kHz to 1MHz using a resistor between the TON pin
and ground. The resistor value is selected by the fol-
lowing equation.
R TON
= ( VOUT ) ⋅
1
VFB 100pF ⋅ fSW
VOUT Voltage Selection
The switcher output voltage is regulated by comparing
VOUT as seen through a resistor divider at the FB pin to
the internal 750mV reference voltage, see Figure 3.
VOUT
R1
R2
To FB pin
Enable and Power-save Inputs
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 power switches are tri-stated. When EN/
PSV is allowed to float, the pin voltage will float to 41%
of the voltage at VDD. The switching regulator turns on
with power-save disabled and all switching is in forced
continuous mode.
When EN/PSV is high (above 60% of the voltage at VDD),
the switching regulator turns on with ultrasonic power-
save enabled. The SC172 ultrasonic power-save opera-
tion maintains a minimum switching frequency of 25kHz,
for applications with stringent audio requirements.
Forced Continuous Mode Operation
The SC172 operates in Forced Continuous Mode (FCM)
by floating the EN/PSV pin (see Figure 4). In this mode
one of the power MOSFETs is always on, with no inten-
tional 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 switching of the
MOSFETs.
FB Ripple
Voltage (VFB)
FB threshold
(750mV)
Inductor
Current
DC Load Current
Figure 3 — Output Voltage Selection
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.
VOUT
=
0.75V
⋅ 1+
R1
R2
+
VRIPPLE
2
© 2010 Semtech Corporation
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 4 — Forced Continuous Mode Operation
12
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