L6741 Datasheet, PDF (10/16 Pages) STMicroelectronics – High current MOSFET driver

Device description and operation
L6741
This last term is the important one to be determined to calculate the device power
dissipation.
The total power dissipated to switch the mosfets results:
P_SW = F_SW ⋅ (Q_GHS ⋅ PVCC + Q_GLS ⋅ VCC)
When designing an application based on L6741 it is recommended to take into
consideration the effect of external gate resistors on the power dissipated by the driver.
External gate resistors helps the device to dissipate the switching power since the same
power P_SW will be shared between the internal driver impedance and the external resistor
resulting in a general cooling of the device.
Referring to Figure 6, classical mosfet driver can be represented by a push-pull output stage
with two different mosfets: P-MOSFET to drive the external gate high and N-MOSFET to
drive the external gate low (with their own R_dsON: R_{hi_HS,} R_{lo_HS}, R_{hi_LS,} R_{lo_LS}). The
external power mosfet can be represented in this case as a capacitance (C_{G_HS}, C_{G_LS})
that stores the gate-charge (Q_{G_HS}, Q_{G_LS}) required by the external power MOSFET to
reach the driving voltage (PVCC for HS and VCC for LS). This capacitance is charged and
discharged at the driver switching frequency F_SW.
The total power Psw is dissipated among the resistive components distributed along the
driving path. According to the external Gate resistance and the power-MOSFET intrinsic
gate resistance, the driver dissipates only a portion of Psw as follow:
P_{SW – HS}
=
1_--
2
⋅
C_GHS
⋅
PVCC²
⋅
Fsw
⋅
⎛
⎝
-------------------------^R----^h---ⁱ--^H---^S--------------------------
R_hiHS + R_GateHS + R_iHS
+
R^-----_l^-_o^---_H^----_S^-----+-----R^--R--_G^{--l--oa---Ht--}_e^-S--_H^---_S^-----+-----R^-----_i^-_H^----_S^{-⎠⎞}
P_{SW – LS}
=
1_--
2
⋅
C_GLS
⋅
VCC²
⋅
Fsw
⋅
⎛
⎝
------------------------^R-----^h---ⁱ-^L---^S-------------------------
R_hiLS + R_GateLS + R_iLS
+
-_R----_l-_o---_L---_S-----+-----_R--^R--_G---^l-^oa---^Lt--_e-^S--_L---_S-----+----_R-----_i-_L---_S--_⎠⎞
The total power dissipated from the driver can then be determined as follow:
P = P_DC + P_{SW – HS} + P_{SW – LS}
Figure 6. Equivalent circuit for MOSFET drive.
VCC
PVCC
BOOT
RGATELS RILS
LGATE
CGLS
GND
LS DRIVER
LS MOSFET
RGATEHS RIHS
HGATE
CGHS
PHASE
HS DRIVER
HS MOSFET
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