8

LTC4440

4440i

Rise/Fall Time

Since the power MOSFET generally accounts for the

majority of the power loss in a converter, it is important to

quickly turn it on or off, thereby minimizing the transition

time in its linear region. The LTC4440 can drive a 1nF load

with a 10ns rise time and 7ns fall time.

The LTC4440’s rise and fall times are determined by the

peak current capabilities of Q1 and N1. The predriver that

drives Q1 and N1 uses a nonoverlapping transition scheme

to minimize cross-conduction currents. N1 is fully turned

off before Q1 is turned on and vice versa.

Power Dissipation

To ensure proper operation and long-term reliability, the

LTC4440 must not operate beyond its maximum tempera-

ture rating. Package junction temperature can be calcu-

lated by:

where:

PD = Power Dissipation

Power dissipation consists of standby and switching

power losses:

where:

The LTC4440 consumes very little current during standby.

only (250

µA + 110µA)(12V) = 4.32mW.

AC switching losses are made up of the output capacitive

load losses and the transition state losses. The capacitive

load losses are primarily due to the large AC currents

needed to charge and discharge the load capacitance

during switching. Load losses for the output driver driving

The power MOSFET’s gate capacitance seen by the driver

A power MOSFET’s capacitive load power dissipation can

Transition state power losses are due to both AC currents

required to charge and discharge the driver’s internal

nodal capacitances and cross-conduction currents in the

internal gates.

Undervoltage Lockout (UVLO)

The LTC4440 contains both low side and high side under-

6.2V, the internal buffer is disabled and the output pin OUT

OUT is pulled down to TS. When both supplies are under-

voltage, OUT is pulled low to TS and the chip enters a low

current mode, drawing approximately 25

86

µA from BOOST.

APPLICATIO S I FOR ATIO