LTC4444

8

4444f

OPERATION

Output Stage

A simplified version of the LTC4444’s output stage is shown

in Figure 1. The pull-up devices on the BG and TG outputs

are NPN bipolar junction transistors (Q1 and Q2). The BG

BG and TG have N-channel MOSFET pull-down devices

(M1 and M2) which pull BG and TG down to their nega-

tive rails, GND and TS. The large voltage swing of the BG

and TG output pins is important in driving external power

Rise/Fall Time

The LTC4444’s rise and fall times are determined by the

peak current capabilities of Q1 and M1. The predriver that

drives Q1 and M1 uses a nonoverlapping transition scheme

to minimize cross-conduction currents. M1 is fully turned

off before Q1 is turned on and vice versa.

Since the power MOSFET generally accounts for the ma-

jority of the power loss in a converter, it is important to

quickly turn it on or off, thereby minimizing the transition

time in its linear region. 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 (synchronous) power MOSFET off and TG turns the

high side power MOSFET on, the voltage on the TS pin

of the low side power MOSFET to the BG pin. If there is

an insufficient pull-down on the BG pin, the voltage on

the BG pin can rise above the threshold voltage of the low

side power MOSFET, momentarily turning it back on. With

both the high side and low side MOSFETs conducting,

significant cross-conduction current will flow through the

The powerful output driver of the LTC4444 reduces the

switching losses of the power MOSFET, which increase

with transition time. The LTC4444’s high side driver is

6

BOOST

LTC4444

8

TS

TG

7

UP TO 100V

Q1

M1

3

9

GND

4

BG

Q2

M2

LOW SIDE

POWER

MOSFET

HIGH SIDE

POWER

MOSFET

LOAD

INDUCTOR

Figure 1. Capacitance Seen by BG and TG During Switching

capable of driving a 1nF load with 8ns rise and 5ns fall

12V while its low side driver is capable of driving a 1nF

load with 6ns rise and 3ns fall times using a supply volt-

Undervoltage Lockout (UVLO)

The LTC4444 contains an undervoltage lockout detector

the output pins BG and TG are pulled down to GND and

TS, respectively. This turns off both external MOSFETs.

will resume.

Adaptive Shoot-Through Protection

Internal adaptive shoot-through protection circuitry moni-

tors the voltages on the external MOSFETs to ensure that

they do not conduct simultaneously. This feature improves

efficiency by eliminating cross-conduction current from

to ground during a switch transition. The adaptive shoot-

through protection circuitry also monitors the level of the

TS pin. If the TS pin stays high, BG will be turned on 150ns

after TG is turned off.