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LT1160

Half-/Full-Bridge N-Channel Power MOSFET Drivers

Linear Technology 

LT1160/LT1162

APPLICATIONS INFORMATION

Power MOSFET Selection

Since the LT1160 (or 1/2 LT1162) inherently protects the

top and bottom MOSFETs from simultaneous conduction,

there are no size or matching constraints. Therefore selec-

tion can be made based on the operating voltage and

RDS(ON) requirements. The MOSFET BVDSS should be

greater than the HV and should be increased to approxi-

mately (2)(HV) in harsh environments with frequent fault

conditions. For the LT1160 maximum operating HV supply

of 60V, the MOSFET BVDSS should be from 60V to 100V.

The MOSFET RDS(ON) is specified at TJ = 25°C and is

generally chosen based on the operating efficiency re-

quired as long as the maximum MOSFET junction tem-

perature is not exceeded. The dissipation while each

MOSFET is on is given by:

P = D(IDS)2(1+∂)RDS(ON)

Where D is the duty cycle and ∂ is the increase in RDS(ON)

at the anticipated MOSFET junction temperature. From this

equation the required RDS(ON) can be derived:

( ) ( ) RDS(ON) =

P

2

D IDS 1+ ∂

For example, if the MOSFET loss is to be limited to 2W

when operating at 5A and a 90% duty cycle, the required

RDS(ON) would be 0.089Ω/(1 + ∂). (1 + ∂) is given for each

MOSFET in the form of a normalized RDS(ON) vs tempera-

ture curve, but ∂ = 0.007/°C can be used as an approxima-

tion for low voltage MOSFETs. Thus, if TA = 85°C and the

available heat sinking has a thermal resistance of 20°C/W,

the MOSFET junction temperature will be 125°C and

∂ = 0.007(125 – 25) = 0.7. This means that the required

RDS(ON) of the MOSFET will be 0.089Ω/1.7 = 0.0523Ω,

which can be satisfied by an IRFZ34 manufactured by

International Rectifier.

Transition losses result from the power dissipated in each

MOSFET during the time it is transitioning from off to on,

or from on to off. These losses are proportional to (f)(HV)2

and vary from insignificant to being a limiting factor on

operating frequency in some high voltage applications.

Paralleling MOSFETs

When the above calculations result in a lower RDS(ON) than

is economically feasible with a single MOSFET, two or

more MOSFETs can be paralleled. The MOSFETs will

inherently share the currents according to their RDS(ON)

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

common heat sink). The LT1160 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 (10Ω 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 1.

Driving multiple MOSFETs in parallel may restrict the

operating frequency to prevent overdissipation in the

LT1160 (see the following Gate Charge and Driver Dissi-

pation).

GATE DR

LT1160

RG*

RG*

GATE FB

*OPTIONAL 10Ω

1160 F01

Figure 1. Paralleling MOSFETs

Gate Charge and Driver Dissipation

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

power MOSFET is the total gate charge QG, which includes

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

QG is usually specified for VGS = 10V and VDS = 0.8VDS(MAX).

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:

ISUPPLY

=

IDC

+

⎛

⎝⎜

dQG

dt

⎞

⎠⎟

TOP

+

⎛

⎝⎜

dQG

dt

⎞

⎠⎟

BOTTOM

11602fb

9

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