Datasheet

12
LTC1435
APPLICATIONS INFORMATION
WUU
U
EXTV
CC
V
IN

TG



BG
PGND
LTC1435
N-CH
N-CH
+
C
IN
V
IN
+
C
OUT
L1
R
SENSE
V
OUT
+
1µF
LTC1435 • F04b
VN2222LL
BAT85 BAT85
BAT85
0.22µF
SW
Figure 4b. Capacitive Charge Pump for EXTV
CC
Topside MOSFET Driver Supply (C
B
, D
B
)
An external bootstrap capacitor C
B
connected to the Boost
pin supplies the gate drive voltage for the topside MOSFET.
Capacitor C
B
in the Functional Diagram is charged through
diode D
B
from INTV
CC
when the SW pin is low. When the
topside MOSFET is to be turned on, the driver places the
C
B
voltage across the gate source of the MOSFET. This
enhances the MOSFET and turns on the topside switch.
The switch node voltage SW rises to V
IN
and the Boost pin
rises to V
IN
+ INTV
CC
. The value of the boost capacitor C
B
needs to be 100 times greater than the total input capaci-
tance of the topside MOSFET. In most applications 0.1µF
is adequate. The reverse breakdown on D
B
must be greater
than V
IN(MAX).
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
VV
R
R
OUT
=+






119 1
2
1
.
The external resistor divider is connected to the output as
shown in Figure 5 allowing remote voltage sensing.
Run/Soft Start Function
The RUN/SS pin is a dual purpose pin which provides the
soft start function and a means to shut down the LTC1435.
Figure 6. RUN/SS Pin Interfacing
LTC1435 • F06
C
SS
D1
3.3V OR 5V RUN/SS
C
SS
RUN/SS
Soft start reduces surge currents from V
IN
by gradually
increasing the internal current limit. Power supply se-
quencing can also be accomplished using this pin.
An internal 3µA current source charges up an external
capacitor C
SS.
When the voltage on RUN/SS reaches 1.3V
the LTC1435 begins operating. As the voltage on RUN/SS
continues to ramp from 1.3V to 2.4V, the internal current
limit is also ramped at a proportional linear rate. The
current limit begins at approximately 50mV/R
SENSE
(at
V
RUN/SS
= 1.3V) and ends at 150mV/R
SENSE
(V
RUN/SS
>
2.7V). The output current thus ramps up slowly, charging
the output capacitor. If RUN/SS has been pulled all the way
to ground there is a delay before starting of approximately
500ms/µF, followed by an additional 500ms/µF to reach
full current.
t
DELAY
= 5(10
5
)C
SS
Seconds
Pulling the RUN/SS pin below 1.3V puts the LTC1435 into
a low quiescent current shutdown (I
Q
< 25µA). This pin
can be driven directly from logic as shown in Figure 6.
Diode D1 in Figure 6 reduces the start delay but allows
C
SS
to ramp up slowly for the soft start function; this
diode and C
SS
can be deleted if soft start is not needed.
The RUN/SS pin has an internal 6V Zener clamp (See
Functional Diagram).
V
OSENSE
LTC1435
R1
R2
LTC1435 • F05
100pF
1.19V ≤ V
OUT
≤ 9V
SGND
Figure 5. Setting the LTC1435 Output Voltage