SC403BEVB 查看數據表（PDF） - Semtech Corporation
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SC403BEVB Datasheet PDF : 32 Pages
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SC403B
Applications Information (continued)
One-Shot Timer and Operating Frequency
One-shot timer operation is shown in Figure 2. The FB
Comparator output goes high when VFB is less than the
internal reference. This feeds into the gate drive and turns
on the high-side MOSFET, and starts the one-shot timer.
The one-shot timer uses an internal comparator and a
capacitor. One comparator input is connected to VOUT, the
other input is connected to the capacitor. When the on-
time begins, the internal capacitor charges from zero volts
through a current which is proportional to VIN. When the
capacitor voltage reaches VOUT, the on-time is completed
and the high-side MOSFET turns off.
FB Comparator
FB
-
+
Gate
Drives VIN
DH
Q1
VLX L
VOUT
VIN
One-Shot
Timer
ESR
DL
Q2
COUT
+
RTON On-time = K x RTON x (VOUT/VIN)
VOUT
FB
Figure 2 — On-Time Generation
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 SC403B uses an external resistor to set the on-time
which indirectly sets the frequency. The on-time can be
programmed to provide an operating frequency from
200kHz to 1MHz using a resistor between the TON pin and
ground. The resistor value is selected by the following
equation.
RTON
(tON 10ns) u VIN
25pF u VOUT
The maximum RTON value allowed is shown by the follow-
ing equation.
RTON _ MAX
VIN _ MIN
10 u1.5PA
Immediately after the on-time, the DL (drive signal for the
low side FET) output drives high to turn on the low-side
MOSFET. DL has a minimum high time of ~320ns, after
which DL continues to stay high until one of the following
occurs:
• VFB falls below the reference
• The zero cross detector senses that the voltage
on the LX node is below ground. Power save is
activated eight switching cycles after a zero
crossing is detected.
TON Limitations and VDD Supply Voltage
For VDD below 4.5V, the TON accuracy may be limited by
the input voltage.
The original RTON equation is accurate if VIN satisfies the
relationship over the entire VIN range, as follows.
VIN < (VDD - 1.6V) x 10
If VIN exceeds (VDD - 1.6V) x 10, for all or part of the VIN
range, the RTON equation is not accurate. In all cases where
VIN > (VDD - 1.6V) x 10, the RTON equation must be modified,
as follows.
RTON
(TON 10ns) u (VDD 1.6V) u10
25pF u VOUT
Note that when VIN > (VDD - 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 a 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 reference voltage, see Figure 3.
VOUT
To FB pin
R1
R2
Figure 3 — Output Voltage Selection
16
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