LTC3613 Datasheet, PDF (20/36 Pages) Linear Technology – 24V, 15A Monolithic Step Down Regulator

LTC3613
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 1Ω or
2.2Ω 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.
LTC3613
PV_IN
INTV_CC
V_IN
C_IN
SV_IN
R_VIN
C_VCC
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Figure 5. Setup for V_IN ≤ 5V
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 1.3μA flows out of the RUN pin, and for RUN >
3V, correspondingly increasing current flows into the pin,
reaching about 35μA 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 1μA 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.
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