LTC4446

7

4446f

OPERATION

Overview

The LTC4446 receives ground-referenced, low voltage digi-

tal input signals to drive two N-channel power MOSFETs in

a synchronous buck power supply configuration. The gate

depending on the state of the input. Similarly, the gate of

the high side MOSFET is driven to either BOOST or TS by

a supply bootstrapped off of the switching node (TS).

Input Stage

The LTC4446 employs CMOS compatible input thresholds

that allow a low voltage digital signal to drive standard

power MOSFETs. The LTC4446 contains an internal

voltage regulator that biases both input buffers for high

side and low side inputs, allowing the input thresholds

false triggering due to noise during switching transitions.

However, care should be taken to keep both input pins

(TINP and BINP) from any noise pickup, especially in high

frequency, high voltage applications. The LTC4446 input

buffers have high input impedance and draw negligible

input current, simplifying the drive circuitry required for

the inputs.

Output Stage

A simplified version of the LTC4446’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 LTC4446’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

Figure 1. Capacitance Seen by BG and TG During Switching

6

BOOST

LTC4446

8

TS

TG

7

UP TO 100V

Q1

M1

4446 F01

3

9

GND

4

BG

Q2

M2

LOW SIDE

POWER

MOSFET

HIGH SIDE

POWER

MOSFET

LOAD

INDUCTOR