LTC1624 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1624' PDF : 28 Pages View PDF
LTC1624
APPLICATIONS INFORMATION
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.
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.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
VOUT
=
1.19V 1 +
R2
R1
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 VOUT from running
away if the sense lead is disconnected.
VFB
LTC1624
GND
L1
VOUT
R2
100pF R1
1624 F02
Figure 2. Setting the LTC1624 Output Voltage
ITH/RUN Function
The ITH/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 VIN 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 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).
3.3V
OR 5V
ITH /RUN
D1
CC
RC
ITH /RUN
CC
RC
(a)
(b)
ITH/RUN
R1
D1
C1
CC
RC
(c)
1624 F03
Figure 3. ITH/ RUN Pin Interfacing
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:
160mV/RSENSE (VITH/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:
IL(PEAK) = (VITH/RUN – 1.3V)/(6.8RSENSE)
IOUT(MAX) = ILPEAK – ∆IL / 2
with ∆IL = ripple current in the inductor.
During normal operation the voltage on the ITH/RUN pin
will vary from 1.19V to 2.4V depending on the load current.
Pulling the ITH/RUN pin below 0.8V puts the LTC1624 into
a low quiescent current shutdown (IQ < 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
10
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