SC403MLTRT Semtech, SC403MLTRT Datasheet - Page 16

• Manufacturer Part Number: SC403MLTRT
• Manufacturer: Semtech

Specifications of SC403MLTRT

• Lead Free Status / Rohs Status: Supplier Unconfirmed

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 V
internal 750mV 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 compara-
tor and a capacitor. One comparator input is connected to
V
the on-time begins, the internal capacitor charges from
zero volts through a current which is proportional to V
When the capacitor voltage reaches V
completed and the high-side MOSFET turns off.
This method automatically produces an on-time that is
proportional to V
Under steady-state conditions, the switching frequency
can be determined from the on-time by the following
equation.
The SC403 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.
The maximum R
ing equation.
FB
R
750mV
V
V
OUT
TON
OUT
IN
, the other input is connected to the capacitor. When
R
R
f
SW
FB Comparator
TON
TON
On-time = K x R
_
-
+
T
MAX
One-Shot
ON
t (
V
Figure 2 — On-Time Generation
Timer
ON
OUT
25
V
10
pF
TON
IN
10
V
OUT
IN
ns
value allowed is shown by the follow-
1
V
_
5 .
)
MIN
and inversely proportional to V
OUT
TON
V
A
Drives
Gate
IN
x (V
DH
DL
OUT
/V
V
IN
IN
V
)
Q2
Q1
LX
OUT
FB
C
ESR
OUT
L
is less than the
, the on-time is
+
V
OUT
FB
IN
IN
.
.
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:
TON limitations and V
For V
the input voltage.
The original R
relationship over the entire V
If V
range, the R
V
as follows.
Note that when V
is fixed and does not vary with V
condition, the switching frequency will vary inversely with
V
The switcher output voltage is regulated by comparing
V
internal 750mV reference voltage, see Figure 3.
V
IN
IN
OUT
•
•
OUT
R
> (V
rather than approximating a fixed frequency.
IN
V
TON
as seen through a resistor divider at the FB pin to the
V
DD
exceeds (V
VFB falls below the 750mV 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.
IN
Voltage Selection
OUT
DD
below 4.5V, the TON accuracy may be limited by
< (V
Figure 3 — Output Voltage Selection
- 1.6V) x 10, the R
(T
TON
ON
DD
TON
equation is not accurate. In all cases where
- 1.6V) x 10
10ns)
DD
IN
equation is accurate if V
> (V
25pF
- 1.6V) x 10, for all or part of the V
DD
(V
R
DD
- 1.6V) x 10 , the actual on-time
DD
TON
V
1
OUT
Supply Voltage
IN
equation must be modified,
range, as follows.
1.6V)
IN
. When operating in this
R
2
10
IN
To FB pin
satisfies the
SC403
16
IN