LTC3826-1 Datasheet, PDF(19/32 Page) Linear Technology – 30μA IQ, Dual, 2-Phase Synchronous Step-Down Controller

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LTC3826-1 Datasheet, PDF (19/32 Pages) Linear Technology – 30μA IQ, Dual, 2-Phase Synchronous Step-Down Controller

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LTC3826-1

APPLICATIONS INFORMATION

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

For 3.3V and other low voltage regulators, efï¬ciency

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.

VIN

CIN

VIN

LTC3826-1

EXTVCC

TG1

N-CH

BG1

N-CH

PGND

1Î¼F

BAT85

0.22Î¼F

BAT85

VN2222LL

BAT85

RSENSE

VOUT

COUT

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Figure 8. Capacitive Charge Pump for EXTVCC

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 MOSFET and

turns on the topside switch. 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 ï¬nal arbiter is the

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

and the input current decreases, then the efï¬ciency has

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

is no change in efï¬ciency.

Fault Conditions: Current Limit and Current Foldback

The LTC3826-1 includes current foldback to help limit

load current when the output is shorted to ground. If the

output falls below 70% of its nominal output level, then

the maximum sense voltage is progressively lowered from

100mV to 30mV. Under short-circuit conditions with very

low duty cycles, the LTC3826-1 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 LTC3826-1 (â230ns), the input voltage and inductor

value:

ÎIL(SC) = tON(MIN) (VIN/L)

The resulting short-circuit current is:

ISC

30mV

RSENSE

ÎIL(SC)

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 ï¬ow, that blow the fuse to protect against a

shorted top MOSFET if the short occurs while the controller

is operating.

A comparator monitors the output for overvoltage

conditions. The comparator (OV) detects overvoltage faults

greater than 10% above the nominal output voltage. When

this condition is sensed, the top MOSFET is turned off and

the bottom MOSFET is turned on until the overvoltage

condition is cleared. The bottom MOSFET remains on

continuously for as long as the OV condition persists; if

VOUT returns to a safe level, normal operation automatically

resumes. A shorted top MOSFET will result in a high current

condition which will open the system fuse. The switching

regulator will regulate properly with a leaky top MOSFET

by altering the duty cycle to accommodate the leakage.

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