LTC3890-3 Datasheet, PDF(23/40 Page) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

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LTC3890-3 Datasheet, PDF (23/40 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

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LTC3890-3

Applications Information

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors, CB, connected to the BOOST

pins supply the gate drive voltages for the topside MOSFETs.

Capacitor CB in the Functional Diagram is charged though

external diode DB from INTVCC when the SW pin is low.

When one of the topside MOSFETs is to be turned on, the

driver places the CB voltage across the gate-source of the

desired MOSFET. This enhances the top MOSFET switch

and turns it on. The switch node voltage, SW, rises to VIN

and the BOOST pin follows. With the topside MOSFET

on, the boost voltage is above the input supply: VBOOST =

VIN + VINTVCC. The value of the boost capacitor, CB, needs

to be 100 times that of the total input capacitance of the

topside MOSFET(s). The reverse breakdown of the external

Schottky diode must be greater than VIN(MAX).

The external diode DB can be a Schottky diode or silicon

diode, but in either case it should have low-leakage and

fast recovery. Pay close attention to the reverse leakage

current specification for this diode, especially at high

temperatures where it generally increases substantially.

For applications with output voltages greater than ~5V

that are switching infrequently, a leaky diode DB can fully

discharge the bootstrap capacitor CB, creating a current

path from the output voltage to the BOOST pin to INTVCC.

Not only does this increase the quiescent current of the

converter, but it can cause INTVCC to rise to dangerous

levels if the leakage exceeds the current consumption on

INTVCC.

Particularly, this is a concern in Burst Mode operation at

no load or very light loads, where the part is switching

very infrequently and the current draw on INTVCC is very

low (typically about 35ÂµA). Generally, pulse-skipping and

forced continuous modes are less sensitive to leakage,

since the more frequent switching keeps the bootstrap

capacitor CB charged, preventing a current path from the

output voltage to INTVCC.

However, in cases where the converter has been operat-

ing (in any mode) and then is shut down, if the leakage

of diode DB fully discharges the bootstrap capacitor CB

before the output voltage discharges to below ~5V, then

the leakage current path can be created from the output

voltage to INTVCC. In shutdown, the INTVCC pin is able to

sink about 30ÂµA. To accommodate diode leakage greater

than this amount in shutdown, INTVCC can be loaded

with an external resistor or clamped with a Zener diode.

Alternatively, the PGOOD resistor can be used to sink the

current (assuming the resistor pulls up to INTVCC) since

PGOOD is pulled low when the converter is shut down.

Nonetheless, using a low-leakage diode is the best choice

to maintain low quiescent current under all conditions.

Fault Conditions: Current Limit and Current Foldback

The LTC3890-3 peak current mode control architecture

limits the inductor current when the output is shorted to

ground. Under short-circuit conditions with very low duty

cycles, the LTC3890-3 will begin cycle skipping in order

to limit the short-circuit current. In this situation the bot-

tom MOSFET will be dissipating most of the power. The

short-circuit ripple current is determined by the minimum

on-time, tON(MIN), of the LTC3890-3 (â90ns), the input

voltage and inductor value:

âIL(SC)

tON(MIN)

ï£«

ï£ï£¬

VIN

ï£¶

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The resulting average short-circuit current is:

ISC

ILIM(MAX )

âIL(SC)

For more information www.linear.com/3890-3

38903f

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