LTC1435I View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1435I' PDF : 20 Pages View PDF
LTC1435
APPLICATIONS INFORMATION
+
VIN
TG
EXTVCC
LTC1435
SW
VIN
CIN
BAT85
+
1µF
0.22µF
BAT85
N-CH
VN2222LL
BAT85
L1
RSENSE
VOUT
+
COUT
BG
PGND
N-CH
LTC1435 • F04b
VOSENSE
LTC1435
SGND
1.19V ≤ VOUT ≤ 9V
R2
100pF R1
LTC1435 • F05
Figure 5. Setting the LTC1435 Output Voltage
3.3V OR 5V
RUN/SS
D1
CSS
RUN/SS
CSS
Figure 4b. Capacitive Charge Pump for EXTVCC
Topside MOSFET Driver Supply (CB, DB)
An external bootstrap capacitor CB connected to the Boost
pin supplies the gate drive voltage for the topside MOSFET.
Capacitor CB in the Functional Diagram is charged through
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
is adequate. The reverse breakdown on DB must be greater
than VIN(MAX).
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
VOUT = 1.19V 1+ RR21
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.
LTC1435 • F06
Figure 6. RUN/SS Pin Interfacing
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 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
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.
tDELAY = 5(10 5)CSS Seconds
Pulling the RUN/SS pin below 1.3V puts the LTC1435 into
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).
12
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