12

LT1336

Paralleling MOSFETs

is economically feasible with a single MOSFET, two or

more MOSFETs can be paralleled. The MOSFETs will

ratio as long as they are thermally connected (e.g., on a

common heat sink). The LT1336 top and bottom drivers

can each drive five power MOSFETs in parallel with only a

small loss in switching speeds (see Typical Performance

Characteristics). A low value resistor (10

Ω to 47Ω) in

series with each individual MOSFET gate may be required

to “decouple” each MOSFET from its neighbors to prevent

high frequency oscillations (consult manufacturer’s rec-

ommendations). If gate decoupling resistors are used, the

corresponding Gate Feedback pin can be connected to any

one of the gates as shown in Figure 3.

Driving multiple MOSFETs in parallel may restrict the

operating frequency to prevent overdissipation in the

LT1336 (see the following Gate Charge and Driver

Dissipation).

The actual increase in supply current is slightly higher due

to LT1336 switching losses and the fact that the gates are

being charged to more than 10V. Supply Current vs

Switching Frequency is given in the Typical Performance

Characteristics.

The LT1336 junction temperature can be estimated by

using the equations given in Note 1 of the Electrical

Characteristics. For example, the LT1336IS is limited to

less than 31mA from a 12V supply:

= 126

°C exceeds absolute maximum

In order to prevent the maximum junction temperature

from being exceeded, the LT1336 supply current must be

verified while driving the full complement of the chosen

MOSFET type at the maximum switching frequency.

Ugly Transient Issues

In PWM applications the drain current of the top MOSFET

is a square wave at the input frequency and duty cycle. To

prevent large voltage transients at the top drain, a low ESR

electrolytic capacitor must be used and returned to the

power ground. The capacitor is generally in the range of

25

µF to 5000µF and must be physically sized for the RMS

current flowing in the drain to prevent heating and prema-

ture failure. In addition, the LT1336 requires a separate

10

µF capacitor connected closely between Pins 2 and 6.

The LT1336 top source is internally protected against

transients below ground and above supply. However, the

Gate Drive pins cannot be forced below ground. In most

applications, negative transients coupled from the source

to the gate of the top MOSFET do not cause any problems.

Switching Regulator Applications

The LT1336 is ideal as a synchronous switch driver to

improve the efficiency of step-down (buck) switching

regulators. Most step-down regulators use a high current

Schottky diode to conduct the inductor current when the

switch is off. The fractions of the oscillator period that the

switch is on (switch conducting) and off (diode conduct-

ing) are given by:

Figure 3. Paralleling MOSFETs

GATEDR

GATEFB

LT1336

HV

*OPTIONAL 10

Ω

1336 F03

+

Gate Charge and Driver Dissipation

A useful indicator of the load presented to the driver by a

the additional charge required by the gate-to-drain swing.

When the supply current is measured in a switching

application, it will be larger than given by the DC electrical

characteristics because of the additional supply current

associated with sourcing the MOSFET gate charge:

II

dQ

dt

dQ

dt

SUPPLY

DC

G

TOP

G

BOTTOM

=+









+







