LTC4443/LTC4443-1

8

4443fa

OPERATION

Rise/Fall Time

Since the power MOSFET generally accounts for the major-

ity of power loss in a converter, it is important to quickly

turn it on and off, thereby minimizing the transition time

and power loss. The LTC4443’s peak pull-up current of

2.4A for both BG and TG (Q1 and Q2) produces a rapid

turn-on transition for the MOSFETs. This high current is

capable of driving a 3nF load with a 12ns rise time.

It is also important to turn the power MOSFETs off quickly

to minimize power loss due to transition time; however,

an additional benefit of a strong pull-down on the driver

outputs is the prevention of cross-conduction current. For

example, when BG turns the low side power MOSFET off

and TG turns the high side power MOSFET on, the volt-

frequency positive voltage transient will couple through

the BG pin. If the BG pin is not held down sufficiently, the

voltage on the BG pin will rise above the threshold volt-

age of the low side power MOSFET, momentarily turning

it back on. As a result, both the high side and low side

MOSFETs will be conducting, which will cause significant

cross-conduction current to flow through the MOSFETs

capacitances of the high side MOSFET.

voltage levels eliminates false triggering due to noise

during switch transitions; however, care should be taken

to keep noise from coupling into the IN pin, particularly

in high frequency, high voltage applications.

Undervoltage Lockout

The LTC4443 contains undervoltage lockout detectors that

BG and TG are pulled to GND and TS, respectively. This

operate reliably, normal operation will resume.

Adaptive Shoot-Through Protection

Internal adaptive shoot-through protection circuitry

monitors the voltages on the external MOSFETs to ensure

that they do not conduct simultaneously. The LTC4443

does not allow the bottom MOSFET to turn on until the

gate-source voltage on the top MOSFET is sufficiently

low, and vice-versa. This feature improves efficiency by

eliminating cross-conduction current from flowing from

switch transition.

Output Stage

A simplified version of the LTC4443’s output stage is

shown in Figure 2. The pull-up device on both the BG and

TG outputs is an NPN bipolar junction transistor (Q1 and

Q2). The BG and TG outputs are pulled up to within an

respectively).BothBGandTGhaveN-channelMOSFETpull-

down devices (N1 and N2) which pull BG and TG down to

their negative rails, GND and TS. An additional NPN bipolar

junction transistor (Q3) is present on BG to increase its

pull-down drive current capacity. The large voltage swing of

the BG and TG output pins is important in driving external

BOOST

LTC4443

TG

N1

Q1

TS

HIGH SIDE

POWER

MOSFET

BG

N2

Q2

Q3

GND

4443 F02

LOW SIDE

POWER

MOSFET

LOAD

INDUCTOR

Figure 2. Capacitance Seen by BG and TG During Switching