LTC3415 datasheet(20/36 Pages) LINER

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LTC3613

3613fa

APPLICATIONS INFORMATION

For applications where the main input power is less than

5.3V, tie the VIN and INTVCC pins together and tie the

combined pins to the PVIN 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 INTVCC LDO and will prevent INTVCC from dropping

too low due to the dropout voltage.

VIN 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 INTVCC voltage

to ensure that an adequate gate-drive voltage is present.

The comparator enables UVLO and locks out the switch-

ing action until INTVCC rises above 4.2V. Once UVLO is

released, the comparator does not retrigger UVLO until

INTVCC falls below 3.65V. This hysteresis prevents oscil-

lations when there are disturbances on INTVCC.

Another way to detect an undervoltage condition is to

monitor the VINsupply.BecausetheRUNpinhasaprecision

turn-on voltage of 1.2V, one can use a resistor divider from

VIN to turn on the IC when VIN is high enough. The RUN pin

has bias currents that depend on the RUN voltage as well as

SVIN 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, CSS, 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:

tSOFTSTART =0.6V •

CSS

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.

INTVCC

PVIN

LTC3613

SVIN

CVCC

RVIN

3613 F05

VIN

CIN

Figure 5. Setup for VIN ≤ 5V

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