Datasheet

LTC3613
20
3613fa
APPLICATIONS INFORMATION
For applications where the main input power is less than
5.3V, tie the V
IN
and INTV
CC
pins together and tie the
combined pins to the PV
IN
input with an optional 1 or
2.2 resistor as shown in Figure 5 to minimize the voltage
drop caused by the gate charge current. This will override
the INTV
CC
LDO and will prevent INTV
CC
from dropping
too low due to the dropout voltage.
V
IN
Undervoltage Lockout (UVLO)
The LTC3613 has two functions that help protect the con-
troller in case of input undervoltage conditions. A precision
UVLO comparator constantly monitors the INTV
CC
voltage
to ensure that an adequate gate-drive voltage is present.
The comparator enables UVLO and locks out the switch-
ing action until INTV
CC
rises above 4.2V. Once UVLO is
released, the comparator does not retrigger UVLO until
INTV
CC
falls below 3.65V. This hysteresis prevents oscil-
lations when there are disturbances on INTV
CC
.
Another way to detect an undervoltage condition is to
monitor the V
IN
supply. Because the RUN pin has a precision
turn-on voltage of 1.2V, one can use a resistor divider from
V
IN
to turn on the IC when V
IN
is high enough. The RUN pin
has bias currents that depend on the RUN voltage as well as
SV
IN
voltage. These bias currents should be taken into ac-
count when designing the voltage divider and UVLO circuit
to prevent faulty conditions. Generally for RUN < 3V a bias
current of 1.3A flows out of the RUN pin, and for RUN >
3V, correspondingly increasing current flows into the pin,
reaching about 35A for RUN = 6V.
Soft-Start and Tracking
The LTC3613 has the ability to either soft-start by itself
with a capacitor or track the output of an external supply.
Soft-start or tracking features are achieved not by limiting
the maximum output current of the switching regulator
but by controlling the regulator’s output voltage according
to the ramp rate on the TRACK/SS pin.
When configured to soft-start by itself, a capacitor should
be connected to the TRACK/SS pin. TRACK/SS is pulled
low until the RUN pin voltage exceeds 1.2V and UVLO is
released, at which point an internal current of 1A charges
the soft-start capacitor, C
SS
, connected to TRACK/SS.
Current foldback is disabled during this phase to ensure
smooth soft-start or tracking. The soft-start or tracking
range is defined to be the voltage range from 0V to 0.6V
on the TRACK/SS pin. The total soft-start time can be
calculated as:
t
SOFTSTART
=0.6V•
C
SS
1µA
When the LTC3613 is configured to track another supply,
a voltage divider can be used from the tracking supply to
the TRACK/SS pin to scale the ramp rate appropriately.
Two common implementations of tracking as shown in
Figure 6 are coincident and ratiometric. For coincident
tracking, make the divider ratio from the external supply
the same as the divider ratio for the differential feedback
voltage. Ratiometric tracking could be achieved by using
a different ratio than the differential feedback (Figure 7).
Note that the small soft-start capacitor charging current is
always flowing, producing a small offset error. To minimize
this error, select the tracking resistive divider values to be
small enough to make this offset error negligible.
INTV
CC
PV
IN
LTC3613
SV
IN
C
VCC
R
VIN
3613 F05
V
IN
C
IN
Figure 5. Setup for V
IN
≤ 5V