LM5045 datasheet(31/46 Pages) TI

31 / 46 page

LO1

PGND

VCC

BST1

HO1

HS1

VIN

LM5045

www.ti.com

SNVS699H – FEBRUARY 2011 – REVISED JANUARY 2015

8.2.2.6 Augmenting the Gate Drive Strength

The LM5045 includes powerful 2-A integrated gate drivers. However, in certain high-power applications (> 500

W), it might be necessary to augment the strength of the internal gate driver to achieve higher efficiency and

better thermal performance. In high power applications, typically, the I2xR loss in the primary MOSFETs is

significantly higher than the switching loss. To minimize the I2xR loss, either the primary MOSFETs are paralleled

or MOSFETs with low RDS (on) are employed. Both these scenarios increase the total gate charge to be driven by

the controller IC. An increase in the gate charge increases the FET transition time and hence increases the

switching losses. Therefore, to keep the total losses within a manageable limit the transition time must be

reduced.

Generally, during the Miller capacitance charge/discharge the total available driver current is lower during the

turnoff process than during the turnon process and often it is enough to speed-up the turnoff time to achieve the

efficiency and thermal goals. This can be achieved simply by employing a PNP device, as shown in Figure 30,

from gate to source of the power FET. During the turnon process, when the LO1 goes high, the current is

sourced through the diode D1 and the BJT Q1 provides the path for the turnoff current. Q1 should be located as

close to the power FET as possible so that the turnoff current has the shortest possible path to the ground and

does not have to pass through the controller.

Figure 30. Circuit to Speed-Up the Turnoff Process

Depending on the gate charge characteristics of the primary FET, if it is required to speed up both the turnon and

the turnoff time, a bipolar totem pole structure as shown in Figure 31 can be used. When LO1 goes high, the

gate to source current is sourced through the NPN transistor Q1 and similar to the circuit shown in Figure 30

when LO1 goes low, the PNP transistor Q2 expedites the turnoff process.

Submit Documentation Feedback

Product Folder Links: LM5045

LM5045 Datasheet(PDF) 31 Page - Texas Instruments