Datasheet

10
LTC1624
pin through a small Schottky diode (like a Central
CMDSH-3) to V
OUT
as shown in Figure 10. However, for
3.3V and other lower voltage regulators, additional cir-
cuitry is required to derive boost supply power from the
output.
For low input voltage operation (V
IN
< 7V), a Schottky
diode can be connected from V
IN
to BOOST to increase the
external MOSFET gate drive voltage. Be careful not to
exceed the maximum voltage on BOOST to SW pins
of 7.8V.
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 resistive divider is connected to the output as
shown in Figure 2, allowing remote voltage sensing. When
using remote sensing, a local 100Ω resistor should be
connected from L1 to R2 to prevent V
OUT
from running
away if the sense lead is disconnected.
APPLICATIONS INFORMATION
WUU
U
I
TH
/RUN
C
C
R
C
1624 F03
D1
3.3V
OR 5V
I
TH
/RUN
C
C
R
C
(a) (b)
D1
C1
R1
I
TH
/RUN
C
C
R
C
(c)
Figure 3. I
TH
/RUN Pin Interfacing
Soft start can be implemented by ramping the voltage on
I
TH
/RUN during start-up as shown in Figure 3(c). As the
voltage on I
TH/RUN
ramps from 1.19V to 2.4V the internal
peak current limit is also ramped at a proportional linear
rate. The peak current limit begins at approximately
10mV/R
SENSE
(at V
ITH/RUN
= 1.4V) and ends at:
160mV/R
SENSE
(V
ITH/RUN
= 2.4V)
The output current thus ramps up slowly, charging the
output capacitor. The peak inductor current and maximum
output current are as follows:
I
L(PEAK)
= (V
ITH/RUN
– 1.3V)/(6.8R
SENSE
)
I
OUT(MAX)
= I
LPEAK
– ∆I
L
/2
with ∆I
L
= ripple current in the inductor.
During normal operation the voltage on the I
TH
/RUN pin
will vary from 1.19V to 2.4V depending on the load current.
Pulling the I
TH
/RUN pin below 0.8V puts the LTC1624 into
a low quiescent current shutdown (I
Q
< 30µA). This pin can
be driven directly from logic as shown in Figures 3(a)
and 3(b).
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
I
TH
/RUN Function
The I
TH
/RUN pin is a dual purpose pin that provides the
loop compensation and a means to shut down the LTC1624.
Soft start can also be implemented with this pin. Soft start
reduces surge currents from V
IN
by gradually increasing
the internal current limit. Power supply sequencing can
also be accomplished using this pin.
An internal 2.5µA current source charges up the external
capacitor C
C.
When the voltage on I
TH
/RUN reaches 0.8V
the LTC1624 begins operating. At this point the error
amplifier pulls up the I
TH
/RUN pin to its maximum of 2.4V
(assuming V
OUT
is starting low).
Figure 2. Setting the LTC1624 Output Voltage
LTC1624
V
FB
GND
100pF
R2
L1
R1
V
OUT
1624 F02