L6747C datasheet(10/15 Pages) STMICROELECTRONICS

10 / 15 page

Device description and operation

L6747C

10/15

Doc ID 17127 Rev 1

To prevent the bootstrap capacitor from overcharging as a consequence of large negative

spikes, an external series RBOOT resistor (in the range of few ohms) may be required in

series with the BOOT pin.

Figure 5.

Bootstrap capacitance design

4.4

Power dissipation

The L6747C embeds high current drivers for both high-side and low-side MOSFETs. It is

therefore important to consider the power that the device is going to dissipate in driving

them in order to avoid exceeding the maximum junction operating temperature.

Two main factors contribute to device power dissipation: bias power and driver power.

●

Device power (PDC) depends on the static consumption of the device through the

supply pins and is easily quantifiable as follows:

●

Driver power is the power needed by the driver to continuously switch the external

MOSFETs ON and OFF. It is a function of the switching frequency and total gate

charge of the selected MOSFETs. It can be quantified considering that the total power

PSW dissipated to switch the MOSFETs is influenced by three main factors: external

gate resistance (when present), intrinsic MOSFET resistance, and intrinsic driver

resistance. This last factor is the important one to be determined to calculate the device

power dissipation.

The total power dissipated to switch the MOSFETs is:

When designing an application based on the L6747C it is recommended to take into

consideration the effect of external gate resistors on the power dissipated by the driver.

External gate resistors help the device to dissipate the switching power since the same

power PSW is shared between the internal driver impedance and the external resistor,

resulting in a general cooling of the device.

Referring to Figure 6, a classic MOSFET driver can be represented by a push-pull output

stage with two different MOSFETs: a P-MOSFET to drive the external gate high, and an N-

MOSFET to drive the external gate low (with their own RDS(on): Rhi_HS, Rlo_HS, Rhi_LS,

Rlo_LS). The external power MOSFET can be represented in this case as a capacitance

(CG_HS, CG_LS) that stores the gate-charge (QG_HS, QG_LS) required by the external power

0.0

0.5

1.0

1.5

2.0

2.5

10203040

5060708090

100

High-Side MOSFET Gate Charge [nC]

Cboot = 47nF

Cboot = 100nF

Cboot = 220nF

Cboot = 330nF

Cboot = 470nF

500

1000

1500

2000

2500

0.0

0.2

0.4

0.6

0.8

1.0

Boot Cap Delta Voltage [V]

Qg = 10nC

Qg = 25nC

Qg = 50nC

Qg = 100nC

PVCC

⋅

GHS

PVCC

⋅

GLS

VCC

⋅

()

⋅

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