SC417/SC427
17
internal 500mV reference. This feeds into the gate drive
and turns on the high-side MOSFET, and also starts the
one-shot timer. The one-shot timer uses an internal com-
parator and a capacitor. One comparator input is con-
nected to V
OUT
, 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
V
IN
. When the capacitor voltage reaches V
OUT
, the on-time is
completed and the high-side MOSFET turns off.
Gate
Drives
FB Comparator
One-Shot
Timer
On-time = K x R
TON
x (V
OUT
/V
IN
)
V
OUT
V
IN
FB
500mV
Q1
Q2
L
C
OUT
V
IN
ESR
+
V
OUT
V
LX
FB
DH
DL
R
TON
+
-
Figure 2 On-Time Generation
This method automatically produces an on-time that is
proportional to V
OUT
and inversely proportional to V
IN
.
Under steady-state conditions, the switching frequency
can be determined from the on-time by the following
equation.
IN
ON
OUT
SW
V
T
V
f
The SC417/SC427 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 following
equation.
OUT
IN
ON
TON
V
pF
25
V
)
ns
10
T
(
R
The maximum R
TON
value allowed is shown by the follow-
ing equation.
A
15
V
R
MIN
_
IN
MAX
_
TON
V
OUT
Voltage Selection
The switcher output voltage is regulated by comparing
V
OUT
as seen through a resistor divider at the FB pin (see
Figure 3) to the internal 500mV reference voltage, see the
Detailed Application Circuit.
R
1
V
OUT
To FB pin
R
2
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 V
OUT
is offset by the output ripple according to the
following equation.
2
V
R
R
1
5
.
0
V
RIPPLE
2
1
OUT
When a large capacitor is placed in parallel with R1 (C
TOP
)
V
OUT
is shown by the following equation.
2
TOP
1
2
1
2
2
TOP
1
RIPPLE
2
1
OUT
C
R
R
R
R
1
)
C
R
(
1
2
V
R
R
1
5
.
0
V
The switcher output voltage can be programmed higher
than 5V. The VOUT pin is not allowed to connect directly
to the switcher output due to its the maximum voltage
rating. An additional resistor divider network is required
to connect from the switcher output to the VOUT pin.
When SC417/SC427 operates in self-biased mode, the
minimum difference between the voltages for the VOUT
and the VLDO pins should be ?00mV to avoid unwanted
switchover function due to resistor divider voltage drop.
For example, the voltage at the VOUT pin can be 4V if
VLDO is set for 5V. When the SC417/SC427 operates from
an external power source and the LDO is disabled, the
voltage at the VOUT pin can be as high as shown in
Recommended Operating Conditions. R
TON
is calculated
according to the voltage at the VOUT pin not the voltage
of the switcher output.
Enable and Power-save Input
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,
Applications Information (continued)
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