LTC4449

9

4449fa

APPLICATIONS INFORMATION

OPERATION

The LTC4449’s powerful parallel combination of the

N-channel MOSFET (N2) and NPN (Q3) on the BG

pull-down generates a phenomenal 4ns fall time on BG

while driving a 3nF load. Similarly, the 0.8Ω pull-down

Power Dissipation

To ensure proper operation and long-term reliability,

the LTC4449 must not operate beyond its maximum

temperature rating. Package junction temperature can

be calculated by:

where:

Power dissipation consists of standby, switching and

capacitive load power losses:

where:

The LTC4449 consumes very little quiescent current. The

+ 600μA)(5V) = 6.65mW.

At a particular switching frequency, the internal power loss

increases due to both AC currents required to charge and

discharge internal nodal capacitances and cross-conduc-

tion currents in the internal logic gates. The sum of the

quiescent current and internal switching current with no

load are shown in the Typical Performance Characteristics

plot of Switching Supply Current vs Input Frequency.

The gate charge losses are primarily due to the large AC

currents required to charge and discharge the capacitance

of the external MOSFETs during switching. For identical

frequency fin, the load losses would be:

be approximated as:

Unlike a pure capacitive load, a power MOSFET’s gate

voltage level during switching. A MOSFET’s capacitive load

power dissipation can be calculated using its gate charge,

on TG and BG:

To avoid damaging junction temperatures due to power

dissipation, the LTC4449 includes a temperature monitor

that will pull BG and TG low if the junction temperature

exceeds 160°C. Normal operation will resume when the

junction temperature cools to less than 135°C.

MOSFET (N1) on TG results in a rapid 7ns fall time with

a 3nF load. These powerful pull-down devices minimize

the power loss associated with MOSFET turn-off time and

cross-conduction current.