LTC4442/LTC4442-1

9

4442fb

OPERATION

8

BOOST

LTC4442

TG

N1

Q1

1

TS

HIGH SIDE

POWER

MOSFET

UP TO 38V

2

7

BG

N2

Q2

Q3

3

GND

4442 F02

LOW SIDE

POWER

MOSFET

4

LOAD

INDUCTOR

Figure 2. Capacitance Seen by BG and TG During Switching

eliminating cross-conduction current from flowing from

switch transition.

Output Stage

A simplified version of the LTC4442’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

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 LTC4442’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.

The LTC4442’s powerful parallel combination of the

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

generates a phenomenal 5ns fall time on BG while driving

a 3nF load. Similarly, the 1Ω pull-down MOSFET (N1) on

TG results in a rapid 8ns fall time with a 3nF load. These

powerful pull-down devices minimize the power loss as-

sociated with MOSFET turn-off time and cross-conduction

current.