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LTC4440

4440f

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:

TJ = TA + PD (θJA)

where:

TJ = Junction Temperature

TA = Ambient Temperature

PD = Power Dissipation

θJA = Junction-to-Ambient Thermal Resistance

Power dissipation consists of standby and switching

power losses:

PD = PSTDBY + PAC

where:

PSTDBY = Standby Power Losses

PAC = AC Switching Losses

The LTC4440 consumes very little current during standby.

The DC power loss at VCC = 12V and VBOOST–TS = 12V is

only (250µA + 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

a pure capacitive load COUT would be:

Load Capacitive Power = (COUT)(f)(VBOOST–TS)2

The power MOSFET’s gate capacitance seen by the driver

output varies with its VGS voltage level during switching.

A power MOSFET’s capacitive load power dissipation can

be calculated using its gate charge, QG. The QG value

corresponding to the MOSFET’s VGS value (VCC in this

case) can be readily obtained from the manufacturer’s QG

vs VGS curves:

Load Capacitive Power (MOS) = (VBOOST–TS)(QG)(f)

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-

voltage lockout detectors that monitor VCC and the

bootstrapped supply VBOOST–TS. When VCC falls below

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

is pulled down to TS. When VBOOST – TS falls below 6.9V,

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 25µA from VCC and

86µA from BOOST.

Bypassing and Grounding

The LTC4440 requires proper bypassing on the VCC and

VBOOST–TS supplies due to its high speed switching (nano-

seconds) and large AC currents (Amperes). Careless

component placement and PCB trace routing may cause

excessive ringing and under/overshoot.

To obtain the optimum performance from the LTC4440:

A. Mount the bypass capacitors as close as possible

between the VCC and GND pins and the BOOST and TS

pins. The leads should be shortened as much as pos-

sible to reduce lead inductance.

B. Use a low inductance, low impedance ground plane to

reduce any ground drop and stray capacitance. Remem-

ber that the LTC4440 switches >2A peak currents and

any significant ground drop will degrade signal integrity.

C. Plan the power/ground routing carefully. Know where

the large load switching current is coming from and

going to. Maintain separate ground return paths for the

input pin and the output power stage.

D. Keep the copper trace between the driver output pin and

the load short and wide.

E. When using the MS8E package, be sure to solder the

exposed pad on the back side of the LTC4440 package

to the board. Correctly soldered to a 2500mm2 double-

sided 1oz copper board, the LTC4440 has a thermal

resistance of approximately 40°C/W. Failure to make

good thermal contact between the exposed back side

and the copper board will result in thermal resistances

far greater than 40°C/W.

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