Compound Gate Drivers

The LM5025A contains two unique compound gate drivers,

which parallel both MOS and Bipolar devices to provide high

drive current throughout the entire switching event. The Bi-

polar device provides most of the drive current capability and

provides a relatively constant sink current which is ideal for

driving large power MOSFETs. As the switching event nears

conclusion and the Bipolar device saturates, the internal

MOS device continues to provide a low impedance to com-

pete the switching event.

During turn-off at the Miller plateau region, typically around

2V - 3V, is where gate driver current capability is needed

most. The resistive characteristics of all MOS gate drivers

are adequate for turn-on since the supply to output voltage

differential is fairly large at the Miller region. During turn-off

however, the voltage differential is small and the current

source characteristic of the Bipolar gate driver is beneficial to

provide fast drive capability.

20107413

PWM Comparator

The PWM comparator compares the ramp signal (RAMP) to

the loop error signal (COMP). This comparator is optimized

for speed in order to achieve minimum controllable duty

cycles. The internal 5k

Ω pull-up resistor, connected between

the internal 5V reference and COMP, can be used as the

pull-up for an optocoupler. The comparator polarity is such

that 0V on the COMP pin will produce a zero duty cycle on

both gate driver outputs.

Volt Second Clamp

The Volt x Second Clamp comparator compares the ramp

signal (RAMP) to a fixed 2.5V reference. By proper selection

of RFF and CFF, the maximum ON time of the main switch

can be set to the desired duration. The ON time set by Volt

x Second Clamp varies inversely with the line voltage be-

cause the RAMP capacitor is charged by a resistor con-

nected to Vin while the threshold of the clamp is a fixed

voltage (2.5V). An example will illustrate the use of the Volt x

Second Clamp comparator to achieve a 50% duty cycle limit,

at 200KHz, at a 48V line input: A 50% duty cycle at a 200KHz

requires a 2.5µs of ON time. At 48V input the Volt x Second

product is 120V x µs (48V x 2.5µs). To achieve this clamp

level:

R

48 x 2.5µ / 2.5 = 48µ

Select C

R

Ω

The recommended capacitor value range for CFF is 100pF

to 1000pF.

The C

every cycle by an internal discharge switch controlled by

either the internal clock or by theVxS Clamp comparator,

whichever event occurs first.

Current Limit

The LM5025A contains two modes of over-current protec-

tion. If the sense voltage at the CS1 input exceeds 0.5V the

present power cycle is terminated (cycle-by-cycle current

limit). If the sense voltage at the CS2 input exceeds 0.5V, the

controller will terminate the present cycle, discharge the

softstart capacitor and reduce the softstart current source to

1µA. The softstart (SS) capacitor is released after being fully

discharged and slowly charges with a 1µA current source.

When the voltage at the SS pin reaches approximately 1V,

20107412

FIGURE 1.

www.national.com

11