LTC4447

8

4447f

OPERATION

Rise/Fall Time

Since the power MOSFETs generally account for the major-

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

turn them on and off, thereby minimizing the transition

time and power loss. The LTC4447’s peak pull-up current

of 3.2A for both BG and TG produces a rapid turn-on

transition for the MOSFETs. This high current is capable

of driving a 3nF load with an 8ns risetime.

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

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

on the BG pin will rise above the threshold voltage 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

thereby introducing substantial power loss. A similar effect

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 LTC4447 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 LTC4447

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 LTC4447’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) in parallel with a low resistance P-channel MOSFET

(P1 and P2). This powerful combination rapidly pulls

BOOST, respectively). Both BG and TG have N-channel

MOSFET pull-down devices (N1 and N2) which pull BG

and TG down to their negative rails, GND and TS. An ad-

ditional NPN bipolar junction transistor (Q3) is present on

BG to increase its pull-down drive current capacity. The

rail-to-rail voltage swing of the BG and TG output pins

is important in driving external power MOSFETs, whose

BOOST

LTC4447

TG

TS

HIGH SIDE

POWER

MOSFET

BG

N2

P2

Q2

GND

4447 F02

LOW SIDE

POWER

MOSFET

LOAD

INDUCTOR

Q3

N1

P1

Q1

Figure 2. Capacitance Seen by BG and TG During Switching