LTC1435
U
W U U
APPLICATIONS INFORMATION
1.19V ≤ V
≤ 9V
OUT
R2
+
+
V
IN
1µF
C
IN
V
OSENSE
0.22µF
BAT85
BAT85
100pF
LTC1435
SGND
R1
V
IN
LTC1435 • F05
BAT85
TG
N-CH
N-CH
VN2222LL
R
EXTV
L1
SENSE
CC
V
OUT
Figure 5. Setting the LTC1435 Output Voltage
LTC1435
+
C
OUT
SW
BG
3.3V OR 5V
RUN/SS
RUN/SS
D1
PGND
C
SS
C
SS
LTC1435 • F04b
Figure 4b. Capacitive Charge Pump for EXTVCC
LTC1435 • F06
Figure 6. RUN/SS Pin Interfacing
Topside MOSFET Driver Supply (CB, DB)
Soft start reduces surge currents from VIN by gradually
increasing the internal current limit. Power supply se-
quencing can also be accomplished using this pin.
An external bootstrap capacitor CB connected to the Boost
pinsuppliesthegatedrivevoltageforthetopsideMOSFET.
CapacitorCB intheFunctionalDiagramischargedthrough
diode DB from INTVCC when the SW pin is low. When the
topside MOSFET is to be turned on, the driver places the
CB 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 VIN and the Boost pin
rises to VIN + INTVCC. The value of the boost capacitor CB
needs to be 100 times greater than the total input capaci-
tance of the topside MOSFET. In most applications 0.1µF
isadequate.ThereversebreakdownonDB mustbegreater
than VIN(MAX).
An internal 3µA current source charges up an external
capacitor CSS. 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/RSENSE (at
VRUN/SS = 1.3V) and ends at 150mV/RSENSE (VRUN/SS
>
2.7V). The output current thus ramps up slowly, charging
theoutputcapacitor.IfRUN/SShasbeenpulledalltheway
to ground there is a delay before starting of approximately
500ms/µF, followed by an additional 500ms/µF to reach
full current.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
tDELAY = 5(105)CSS Seconds
PullingtheRUN/SSpinbelow1.3VputstheLTC1435into
a low quiescent current shutdown (IQ < 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
CSS to ramp up slowly for the soft start function; this
diode and CSS can be deleted if soft start is not needed.
The RUN/SS pin has an internal 6V Zener clamp (See
Functional Diagram).
R2
R1
V
= 1.19V 1+
OUT
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
softstartfunctionandameanstoshutdowntheLTC1435.
12
Linear [ Linear ]
