8/9 OUT_A/B

VCC

CNTRL

PGND

7

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LM5025D

www.ti.com

SNVSAB0 – JUNE 2015

Feature Description (continued)

7.3.4 Compound Gate Drivers

The LM5025D contains two unique compound gate drivers, which parallel both MOS and Bipolar devices to

provide high drive current throughout the entire switching event. The Bipolar 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 compete the switching event.

During turn-off at the Miller plateau region, typically around 2 V to 3 V, 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.

Figure 11. Compound Gate Drivers

7.3.5 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 5-k

Ω pull-up resistor,

connected between the internal 5-V reference and COMP, can be used as the pull-up for an optocoupler. The

comparator polarity is such that 0 V on the COMP pin produces a zero duty cycle on both gate driver outputs.

7.3.6 Volt Second Clamp

The Volt x Second Clamp comparator compares the ramp signal (RAMP) to a fixed 2.5-V 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 because the RAMP capacitor is charged

by either the internal clock or by the V x S Clamp comparator, whichever event occurs first.

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