SC461 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC461' PDF : 31 Pages View PDF
SC461
Applications Information (continued)
This method automatically produces an on-time that is
proportional to VOUT and inversely proportional to VIN.
Under steady-state conditions, the switching frequency
can be determined from the on-time by the following
equation.
fSW
VOUT
TON u VIN
The SC461 uses an external resistor to set the on-time which
indirectly sets the frequency. The on-time can be pro-
grammed to provide an operating frequency of up to 1MHz
using a resistor between the TON pin and ground. The resis-
tor value is selected by the following equation.
5721
721 QV u 9,1
S) u 9287
¨¨©§
9287
9,1 u I6:
QV ¸¸¹· u 9,1
S) u 9287
5721
721 QV u 9''$ 9 u
S) u 9287
Note that when VIN is greater than ((VDDA - 1.6V) x 10), the
actual on-time is fixed and does not vary with VIN. When
operating in this condition, the switching frequency will
vary inversely with VIN rather than approximating fixed
frequency.
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 600mV reference voltage (see Figure 3).
VOUT
To FB pin
R1
R2
The maximum recommended RTON value is shown by the
following equation.
5721B0$;
9,1B0,1
uÈ$
Immediately after the on-time, the DL output drives high
to energize the low-side MOSFET. DL has a minimum high
time of ~250ns, after which DL continues to stay high until
one of the following occurs:
• The FB input falls below the 600mV reference
• The Zero Cross Detector trips if power-save is
active
TON Limitations and VDDA Supply Voltage
For VDDA below 4.5V, the TON accuracy may be limited by
VIN. The previous RTON equation is accurate if VIN satisfies
the below relation over the entire VIN range:
VIN < (VDDA - 1.6V) x 10
If VIN exceeds ((VDDA - 1.6V) x 10) for all or part of the VIN
range, the previous RTON equation is not accurate. In all
cases where VIN > ((VDDA - 1.6V) x 10), the RTON equation
must be modified as follows.
Figure 3 — Output Voltage Selection
Note that this control method regulates the valley of the
output ripple voltage, not the DC value. The DC value of
VOUT is offset by the output ripple according to the follow-
ing equation.
VOUT
0.6
u
¨¨©§1
R1
R2
¸¸¹·
¨§
©
VRIPPLE
2
¸·
¹
In some applications a small capacitor CTOP is placed in
parallel with R1 to provide a larger ripple signal from VOUT
to the FB pin. In these applications, the output voltage
VOUT is calculated according to the following equation in
which w represents the switching frequency.
9287
u
¨¨©§
5
5
¸¸¹·
¨§
©
95,33/(
¸·
¹
u
5Ȧ&723
¨¨©§
5
5
u
5
5
Ȧ&723
¸¸¹·
Configuring VOUT Greater Than 5V
The switcher output voltage can be programmed higher
than 5V with careful attention to the VOUT and RTON pins.
In these applications the VOUT pin cannot connect directly
14
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