Datasheet

LT3763
19
3763fa
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applicaTions inForMaTion
the programmed level (1.206V at the FB pin), the voltage
error amplifier overrides CTRL1 to set the inductor cur-
rent and regulate V
OUT
. When the output voltage exceeds
125% of the regulated voltage level (1.515V at the FB pin),
the internal overvoltage flag is set, terminating switching.
The regulated output voltage must be greater than 1.5V
and is set by the equation:
V
OUT
= 1.206V 1+
R2
R1
⎛
⎝
⎜
⎞
⎠
⎟
Fault Detection
The LT3763 detects that the load has had an open-circuit
or short-circuit event indicated by pulling the FAULT pin to
ground. These conditions are detected by comparing the
voltage at the FB pin to two internal reference voltages.
A short-circuit is defined as V
FB
lower than 0.25V. In an
open-circuit condition, the regulated inductor current will
charge the output capacitor, the voltage at FB will begin
to increase, and the voltage error amplifier will begin to
reduce the inductor current. The open-circuit condition will
be indicated at FAULT when FB is higher than 1.16V and
the inductor current is less than ten percent (C/10) of the
maximum value set by the sense resistor R
S
. The output
voltage will be regulated as determined by the resistor
divider to the FB pin.
Low Current Detection
When the inductor current decreases to ten percent of the
maximum current, the C/10 comparator will also disable
the low side gate driver, so the converter will become
non-synchronous and automatically transition into dis
-
continuous conduction mode when the inductor current
is low enough relative to the ripple.
The
low
current condition is an essential part of battery
charging applications. The LT3763 works well in this ap
-
plication delivering a constant current to the battery as it
charges and
then automatically reducing the current to a
trickle charge as the battery voltage approaches its fully
charged value. In this application, the signal at FAULT
triggered by the low current detection comparator serves
as an indicator that the trickle charge phase of charging
the battery has begun.
Programming Switching Frequency
The LT3763 has an operational switching frequency range
between 200kHz and 1MHz. This frequency is programmed
with an external resistor from the RT pin to ground. Do not
leave this pin open under any condition. The RT pin is also
current-limited to 55µA. See Table 4 and Figure10 for resis
-
tor values and the corresponding switching frequencies.
Table 4. Switching Frequency
SWITCHING FREQUENCY (MHz) R
T
(kΩ)
1.00 40.2
0.75 53.6
0.50 82.5
0.30 143
0.20 221
Switching Frequency Synchronization
The nominal switching frequency of the LT3763 is deter-
mined by the resistor from the RT pin to ground and may
be set from 200kHz to 1MHz. The internal oscillator may
also be synchronized to an external clock through the SYNC
pin. The external clock applied to the SYNC pin must have
a logic low below 1.5V and a logic high above 2.175V. The
input frequency must be 20% higher than the frequency
that would otherwise be determined by the resistor at the
RT pin. Input signals outside of these specified parameters
will cause erratic switching behavior and subharmonic
oscillations. Synchronization is tested at 500kHz with
a 221k R
T
resistor. Operation under other conditions is
guaranteed by design. When synchronizing to an external
R
T
(kΩ)
0.0
FREQUENCY (MHz)
0.4
0.8
1.2
0.2
0.6
1.0
100 150 200
3763 F10
250
500
Figure 10. Frequency vs R
T
Resistance