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LTC3835

3835fc

APPLICATIONS INFORMATION

4. EXTVCCConnectedtoanOutput-DerivedBoostNetwork.

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 6.

Fault Conditions: Current Limit and Current Foldback

The LTC3835 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 LTC3835 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 cur-

rent is determined by the minimum on-time tON(MIN) of the

LTC3835 (≈180ns), the input voltage and inductor value:

ΔIL(SC) = tON(MIN) (VIN/L)

The resulting short-circuit current is:

SENSE

LSC

–

()

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 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 overvoltage 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.

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors CB connected to the BOOST

pins supply the gate drive voltages for the topside MOSFET.

Capacitor CB in the Functional Diagram is charged

though external diode DB from INTVCC when the SW pin

is low. When the topside MOSFET 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. 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.

Figure 6. Capacitive Charge Pump for EXTVCC

EXTVCC

VIN

TG1

BG1

PGND

LTC3835

RSENSE

VOUT

VN2222LL

COUT

3835 F06

N-CH

CIN

1μF

VIN

BAT85

0.22μF

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