LTC3810-5 datasheet(21/36 Pages) LINER

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LTC3810-5

38105fc

VIN is typically much higher than 14V a separate supply

for the IC power (INTVCC) and driver power (DRVCC) must

be used. The LTC3810-5 has integrated bias supply con-

trol circuitry that allows the IC/driver supply to be easily

generated from VIN and/or VOUT with minimal external

components. There are four ways to do this as shown in

the simpliﬁed schematics of Figure 3 and explained in the

following sections.

Using the Linear Regulator for INTVCC/DRVCC Supply

In Mode 1, a small external SOT23 MOSFET, controlled

by the NDRV pin, is used to generate a 5.5V start-up

supply from VIN. The small SOT23 package can be used

because the NMOS is on continuously only during the brief

start-up period. As soon as the output voltage reaches

4.7V, the LTC3810-5 turns off the external NMOS and the

LTC3810-5 regulates the 5.5V supply from the EXTVCC

pin (connected to VOUT or a VOUT derived boost network)

through an internal low dropout regulator. For this mode to

work properly, EXTVCC must be in the range 4.7V < EXTVCC

< 15V. If VOUT < 4.7V, a charge pump or extra winding can

be used to raise EXTVCC to the proper voltage, or alter-

natively, Mode 2 should be used as explained later in this

section. If VOUT is shorted or otherwise goes below the

minimum 4.5V threshold, the MOSFET connected to VIN

is turned back on to maintain the 5.5V supply. However if

the output cannot be brought up within a timeout period,

the drivers are turned off to prevent the SOT23 MOSFET

from overheating. Soft-start cycles are then attempted at

low duty cycle intervals to try to bring the output back up

(see Figure 10). This fault timeout operation is enabled

by choosing the choosing RNDRV such that the resistor

current INDRV is greater than 270μA by using the follow-

ing formulas:

RNDRV ≤

PMOSFET(MAX) /ICC − VTH

270μA

where

ICC =(f) QG(TOP) +QG(BOTTOM)

()+3mA

and VTH is the threshold voltage of the MOSFET.

The value of RNDRV also affects the VIN(MIN) as follows:

VIN(MIN) = VINTVCC(MIN) + (40μA) RNDRV +VT

(1)

where VINTVCC(MIN) is normally 4.5V for driving logic-level

MOSFETs. If minimum VIN is not low enough, consider

reducing RNDRV and/or using a Darlington NPN instead

of an NMOS to reduce VT to ~1.4V.

When using RNDRV equal to the computed value, the

LTC3810-5 will enable the low duty cycle soft-start re-

tries only when the desired maximum power dissipation,

PMOSFET(MAX), in the MOSFET is exceeded and leave the

drivers on continuously otherwise. The shutoff/restart

times are a function of the TRACK/SS capacitor value.

The external NMOS for the linear regulator should be a

standard 3V threshold type (i.e., not a logic-level threshold).

The rate of charge of INTVCC from 0V to 5.5V is controlled

by the LTC3810-5 to be approximately 75μs regardless of

the size of the capacitor connected to the INTVCC pin. The

charging current for this capacitor is approximately:

IC =

5.5V

μs

CINTVCC

Figure 10. Fault Timeout Operation

SS/TRACK

VOUT

TG/BG

FAULT TIMEOUT

ENABLED

EXTVCC UV THRESHOLD

DRIVER OFF THRESHOLD

DRIVER POWER

FROM VIN

SHORT-CIRCUIT EVENT

START-UP INTO SHORT-CIRCUIT

START-UP

DRIVER POWER

FROM VOUT

DRIVER POWER

FROM VIN

ISS/TRACK = 1.4μA (SOURCE)

ISS/TRACK = 0.1μA (SINK)

38105 F10

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