LTC3868 Datasheet, PDF(22/38 Page) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

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LTC3868 Datasheet, PDF (22/38 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

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LTC3868

Applications Information

The following list summarizes the four possible connec-

tions for EXTVCC:

1. EXTVCC Left Open (or Grounded). This will cause INTVCC

to be powered from the internal 5.1V regulator result-

ing in an efficiency penalty of up to 10% at high input

voltages.

2. EXTVCC Connected Directly to VOUTâ. This is the normal

connection for a 5V to 14V regulator and provides the

highest efficiency.

3. EXTVCC Connected to an External Supply. If an external

supply is available in the 5V to 14V range, it may be

used to power EXTVCC. Ensure that EXTVCC < VIN.

4. EXTVCC Connected to an Output-Derived Boost Network.

For 3.3V and other low voltage regulators, efficiency

gains can still be realized by connecting EXTVCC to an

output-derived voltage that has been boosted to greater

than 4.7V. This can be done with the capacitive charge

pump shown in Figure 8. Ensure that EXTVCC < VIN.

CIN

VIN

MTOP

TG1

1/2 LTC3868

EXTVCC

MBOT

BG1

PGND

BAT85

VN2222LL

RSENSE

BAT85

VOUT

COUT

3868 F08

Figure 8. Capacitive Charge Pump for EXTVCC

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).

When adjusting the gate drive level, the final arbiter is the

total input current for the regulator. If a change is made

and the input current decreases, then the efficiency has

improved. If there is no change in input current, then there

is no change in efficiency.

Fault Conditions: Current Limit and Current Foldback

When the output current hits the current limit, the output

voltage begins to drop. If the output voltage falls below

70% of its nominal output level, then the maximum sense

voltage is progressively lowered from about one-half of

its maximum selected value. Under short-circuit condi-

tions with very low duty cycles, the LTC3868 will begin

cycle skipping in order to limit the short-circuit current.

In this situation the bottom MOSFET will be dissipating

most of the power but less than in normal operation. The

short-circuit ripple current is determined by the minimum

on-time, tON(MIN), of the LTC3868 (â95ns), the input volt-

age and inductor value:

ÎIL(SC)

tON(MIN)

ï£«

ï£¬

ï£

VIN

ï£¶

ï£·

ï£¸

The resulting average short-circuit current is:

ISC

50% â¢ ILIM(MAX)

RSENSE

âIL(SC)

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

Fault Conditions: Overvoltage Protection (Crowbar)

The overvoltage crowbar is designed to blow a system

input fuse when the output voltage of the regulator rises

much higher than nominal levels. The crowbar causes huge

currents to flow, that blow the fuse to protect against a

shorted top MOSFET if the short occurs while the control-

ler is operating.

3868fb

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