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LTC3810

3810fb

Fault Monitoring/Protection

Constant on-time current mode architecture provides ac-

curate cycle-by-cycle current limit protection—a feature

that is very important for protecting the high voltage power

supply from output short circuits. The cycle-by-cycle cur-

rent monitor guarantees that the inductor current will never

exceed the value programmed on the VRNG pin.

Foldback current limiting provides further protection if the

output is shorted to ground. As VFB drops, the buffered

current threshold voltage ITHB is pulled down and clamped

to 1V. This reduces the inductor valley current level to

one-sixth of its maximum value as VFB approaches 0V.

Foldback current limiting is disabled at start-up.

Overvoltage and undervoltage comparators OV and UV

pull the PGOOD output low if the output feedback voltage

exits a ±10% window around the regulation point after the

internal 120μs power bad mask timer expires. Furthermore,

in an overvoltage condition, M1 is turned off and M2 is

turned on immediately and held on until the overvoltage

condition clears.

The LTC3810 provides two undervoltage lockout com-

parators—one for the INTVCC/DRVCC supply and one for

the input supply VIN. The INTVCC UV threshold is 6.2V to

guarantee that the MOSFETs have sufﬁcient gate drive volt-

age before turning on. The VIN UV threshold (UVIN pin) is

0.8V with 10% hysteresis which allows programming the

VIN threshold with the appropriate resistor divider con-

nected to VIN. If either comparator inputs are under the

UV threshold, the LTC3810 is shut down and the drivers

are turned off.

Strong Gate Drivers

The LTC3810 contains very low impedance drivers capable

of supplying amps of current to slew large MOSFET gates

quickly. This minimizes transition losses and allows paral-

leling MOSFETs for higher current applications. A 100V

ﬂoating high side driver drives the top side MOSFET and

a low side driver drives the bottom side MOSFET (see

Figure 3). The bottom side driver is supplied directly

from the DRVCC pin. The top MOSFET drivers are biased

from ﬂoating bootstrap capacitor, CB, which normally is

recharged during each off cycle through an external diode

from DRVCC when the top MOSFET turns off. In pulse

skip mode operation, where it is possible that the bottom

MOSFET will be off for an extended period of time, an

internal timeout guarantees that the bottom MOSFET is

turned on at least once every 25μs for one on-time period

to refresh the bootstrap capacitor.

The bottom driver has an additional feature that helps

minimize the possibility of external MOSFET shoot-through.

When the top MOSFET turns on, the switch node dV/dt

pulls up the bottom MOSFET’s internal gate through the

Miller capacitance, even when the bottom driver is holding

the gate terminal at ground. If the gate is pulled up high

enough, shoot-through between the top side and bottom

side MOSFETs can occur. To prevent this from occurring,

the bottom driver return is brought out as a separate pin

(BGRTN) so that a negative supply can be used to reduce

the effect of the Miller pull-up. For example, if a –2V sup-

ply is used on BGRTN, the switch node dV/dt could pull

the gate up 2V before the VGS of the bottom MOSFET has

more than 0V across it.

IC/Driver Supply Power

The LTC3810’s internal control circuitry and top and bottom

MOSFET drivers operate from a supply voltage (INTVCC,

DRVCC pins) in the range of 6.2V to 14V. The LTC3810

has two integrated linear regulator controllers to easily

generate this IC/driver supply from either the high voltage

input or from the output voltage. For best efﬁciency the

supply is derived from the input voltage during start-up

and then derived from the lower voltage output as soon

as the output is higher than 6.7V. Alternatively, the supply

can be derived from the input continuously if the output

Figure 3. Floating TG Driver Supply and Negative BG Return

OPERATION

BOOST

BGRTN

DRVCC

LTC3810

VIN

CIN

VOUT

COUT

3810 F03

0V TO –5V

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