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U
OPERATIO
the main control loop is enabled with the I
pulled up by the error amp.
A built-in comparator guards against transient output
overshoots > 7.5% by turning off the top MOSFET and
keeping it off until the output decreases.
Low Current Operation
The LTC1624 is capable of Burst Mode operation, in which
the external MOSFET operates intermittently based on
load demand. The transition to low current operation
begins when a comparator detects that the I
age is below 1.5V. If the voltage across R
exceed approximately 20mV for one full cycle, the top and
internal bottom drives will be disabled on the following
cycles. This continues until the I
which causes drive to be returned to TG on the next cycle.
INTV
Power/Boost Supply (C
CC
Power for the top and internal bottom MOSFET drivers is
derived from V
. An internal regulator supplies the power.
IN
To power the top driver in step-down applications, an
internal high voltage diode recharges the bootstrap ca-
pacitor C
during each off cycle from the internal supply.
B
A small internal N-channel MOSFET pulls the switch node
(SW) to ground each cycle after the top MOSFET has
turned off, ensuring that the bootstrap capacitor is kept
fully charged.
When the top side MOSFET is to be turned on, the driver
places the C
voltage across the gate-source of the MOSFET.
B
This enhances the MOSFET and turns on the top-side
switch. The switch node voltage SW rises to V
BOOST pin rises to V
+ 5V.
IN
Significant efficiency gains can be realized by supplying
top-side driver operating voltage from the output, since
the V
current resulting from the driver and control
IN
currents will be scaled by a factor of (Duty Cycle)/(Effi-
ciency). For 5V regulators this simply means connecting
the BOOST pin though a small Schottky diode (such as a
CMDH-3) to V
.
OUT
For low input voltage operation (V
diode can be connected from V
external MOSFET gate drive voltage. Be careful not to
/RUN voltage
TH
/RUN volt-
TH
does not
SENSE
voltage exceeds 1.5V,
TH
, D
)
B
B
and the
IN
< 7V), a Schottky
IN
to BOOST to increase the
IN
DEMO MANUAL DC164
exceed the maximum voltage on the BOOST to SW pins of
7.8V.
I
/ RUN Function
TH
The I
/RUN pin is a dual purpose pin that provides the
TH
loop compensation and a means to shut down the LTC1624.
An internal 2.5µA current source charges the external
capacitor C
(Figure 4). When the voltage on I
C
reaches 0.8V, the LTC1624 begins operating. At this point
the error amplifier pulls up the I
mum of 2.4V (assuming V
can also be implemented with this pin, as shown in Figure
4c. Soft start reduces surge currents from V
increasing the internal current limit. Power supply
sequencing can also be accomplished using this pin.
During normal operation the voltage on the I
will vary from 1.19V to 2.4V, depending on the load
current. Pulling the I
/RUN pin below 0.8V puts the
TH
LTC1624 into a low quiescent current shutdown (I
30µA). This pin can be driven directly from logic, as shown
in Figures 4a and 4b.
The operating frequency is set internally to 200kHz.
In addition to shutdown, the dual function pin I
allows external compensation for optimum load-step
3.3V
OR 5V
I
/RUN
TH
D1
J1
C
C
R
C
(4a)
Figure 4. I
/RUN Pin Interfacing
TH
TH
/RUN pin to its maxi-
TH
is starting low). Soft start
OUT
by gradually
IN
/RUN pin
TH
TH
I
/RUN
TH
J1
C
C
R
C
(4b)
I
/RUN
TH
R1
D1
J1
C1
C
C
R
C
(4c)
164 • F04
/RUN
<
Q
/RUN
5
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