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LT1683

1683f

MOSFET Selection

There is a wide variety of MOSFETs to choose from for this

part. The part will work with either normal threshold (3V to

4V) or logic level threshold devices (1V to 2V).

Select a voltage rating to insure under worst-case condi-

tions that the MOSFET will not break down. Next choose an

RON sufficiently low to meet both the power dissipation

capabilities of the MOSFET package as well as overall

efficiency needs of the converter.

The LT1683 can handle a large range of gate charges.

However at very large charge stability may be affected.

The power dissipation in the MOSFET depends on several

factors. The primary element is I2R heating when the

device is on. In addition, power is dissipated when the

device is slewing. An estimate for power dissipation is:

+ ∆

−+

∆

















































•

••

•

where I is the average current,

∆I is the ripple current in the

switch, ISR is the current slew rate, VSR is the voltage slew

rate, f is the oscillator frequency, DC is the duty cycle and

RON is the MOSFET on-resistance.

Setting GCL Voltage

Setting the voltage on the GCL pin depends on what type

of MOSFET is used and the desired gate drive undervolt-

age lockout voltage.

First determine the maximum gate drive that you require.

Typically you will want it to be at least 2V greater than the

maximum threshold. Higher voltages will lower the on

resistance and increase efficiency. Be certain to check the

maximum allowed gate voltage. Often this is 20V but for

some logic threshold MOSFETs it is only 8V to 10V.

VGCL needs to be set approximately 0.2V above the desired

max gate threshold. In addition VIN needs to be at least

1.6V above the gate voltage.

The GCL pin can be tied to VIN which will result in a

maximum gate voltage of VIN – 1.6V.

This pin also controls undervoltage lockout of the gate

drives. The undervoltage lockout will prevent the MOSFETs

from switching until there is sufficient drive present.

If GCL is tied to a voltage source or Zener less than 6.8V,

the gate drivers will not turn on until VIN exceeds the GCL

voltage by 0.8V. For VGCL above 6.5V, the gate drives are

insured to be off for VIN < 7.3V and they will be turned on

by VGCL + 0.8V.

If GCL is tied to VIN, the gate drivers are always on

(undervoltage lockout is disabled).

Approximately 50

µA of current can be sourced from this

pin if VIN > VGCL + 0.8V. This could be used to bias a Zener.

The GCL pin has an internal 19V Zener to ground that will

provide a failsafe for maximum gate voltage.

As an example say we are using a Siliconix Si4480DY

which has RDS(ON) rated at 6V. To get 6V, VGCL needs to be

set to 6.2V and VIN needs to be at least 7.6V.

APPLICATIO S I FOR ATIO

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