LTC1624
U
W U U
APPLICATIONS INFORMATION
3.3V
OR 5V
pin through a small Schottky diode (like a Central
CMDSH-3) to VOUT 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.
I
/RUN
I
/RUN
TH
TH
D1
C
C
C
C
R
C
R
C
For low input voltage operation (VIN < 7V), a Schottky
diode can be connected from VIN 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.
(a)
(b)
I
/RUN
TH
R1
D1
C
C
Output Voltage Programming
C1
R
C
The output voltage is set by a resistive divider according
to the following formula:
(c)
Figure 3. ITH/RUN Pin Interfacing
1624 F03
R2
R1
V
= 1.19V 1+
OUT
Soft start can be implemented by ramping the voltage on
ITH/RUN during start-up as shown in Figure 3(c). As the
voltage on ITH/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/RSENSE (at VITH/RUN = 1.4V) and ends at:
The external resistive divider is connected to the output as
showninFigure2, allowingremotevoltagesensing. When
using remote sensing, a local 100Ω resistor should be
connected from L1 to R2 to prevent VOUT from running
away if the sense lead is disconnected.
V
OUT
L1
160mV/RSENSE (VITH/RUN = 2.4V)
R2
R1
V
FB
The output current thus ramps up slowly, charging the
outputcapacitor.Thepeakinductorcurrentandmaximum
output current are as follows:
LTC1624
GND
100pF
1624 F02
IL(PEAK) = (VITH/RUN – 1.3V)/(6.8RSENSE
)
Figure 2. Setting the LTC1624 Output Voltage
IOUT(MAX) = ILPEAK – ∆IL/2
ITH/RUN Function
with ∆IL = ripple current in the inductor.
The ITH/RUN pin is a dual purpose pin that provides the
loopcompensationandameanstoshutdowntheLTC1624.
Soft start can also be implemented with this pin. Soft start
reduces surge currents from VIN by gradually increasing
the internal current limit. Power supply sequencing can
also be accomplished using this pin.
During normal operation the voltage on the ITH/RUN pin
willvaryfrom1.19Vto2.4Vdependingontheloadcurrent.
Pulling the ITH/RUN pin below 0.8V puts the LTC1624 into
alowquiescentcurrentshutdown(IQ<30µA).Thispincan
be driven directly from logic as shown in Figures 3(a)
and 3(b).
An internal 2.5µA current source charges up the external
capacitor CC. When the voltage on ITH/RUN reaches 0.8V
the LTC1624 begins operating. At this point the error
amplifier pulls up the ITH/RUN pin to its maximum of 2.4V
(assuming VOUT is starting low).
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
10
Linear [ Linear ]
