ISL6596_14 Datasheet, PDF (8/11 Pages) Intersil Corporation – Synchronous Rectified MOSFET Driver

ISL6596
Power Dissipation
Package power dissipation is mainly a function of the
switching frequency (F_SW), the output drive impedance, the
external gate resistance, and the selected MOSFET’s
internal gate resistance and total gate charge. Calculating
the power dissipation in the driver for a desired application is
critical to ensure safe operation. Exceeding the maximum
allowable power dissipation level will push the IC beyond the
maximum recommended operating junction temperature of
+125°C. The maximum allowable IC power dissipation for
the SO8 package is approximately 800mW at room
temperature, while the power dissipation capacity in the DFN
package, with an exposed heat escape pad, is much higher.
See “Layout Considerations” on page 9 for thermal transfer
improvement suggestions. When designing the driver into an
application, it is recommended that the following calculation
is used to ensure safe operation at the desired frequency for
the selected MOSFETs. The total gate drive power losses
due to the gate charge of MOSFETs and the driver’s internal
circuitry and their corresponding average driver current can
be estimated with Equations 2 and 3, respectively:
P_{Qg_TOT} = P_{Qg_Q1} + P_{Qg_Q2} + I_Q • VCC
P _{Q g _Q1}
=
Q_{-----G-----1----}•_-----V_----C_-----C_-----²_-
V_GS1
•
F_S
W
•
N_Q1
P _{Q g _Q2}
=
Q_{-----G-----2----}•_-----V_----C_-----C_-----²_-
V_GS2
•
F_S
W
•
N_Q2
(EQ. 2)
I_VCC
=
⎛
⎜
⎝
Q_{-----G-----1----}•_-----N_----Q-----1-
V_GS1
+
Q_{-----G---V--2--G--}•_--S---N_{-2---Q-----2-⎠⎟⎞}
• VCC • F_SW + I_Q
(EQ. 3)
where the gate charge (Q_G1 and Q_G2) is defined at a
particular gate to source voltage (V_GS1and V_GS2) in the
corresponding MOSFET datasheet; I_Q is the driver’s total
quiescent current with no load at both drive outputs; N_Q1
and N_Q2 are the number of upper and lower MOSFETs,
respectively. The I_Q V_CC product is the quiescent power of
the driver without capacitive load and is typically negligible.
The total gate drive power losses are dissipated among the
resistive components along the transition path. The drive
resistance dissipates a portion of the total gate drive power
losses, the rest will be dissipated by the external gate
resistors (R_G1 and R_G2, should be a short to avoid
interfering with the operation shoot-through protection
circuitry) and the internal gate resistors (R_GI1 and R_GI2) of
MOSFETs. Figures 3 and 4 show the typical upper and lower
gate drives turn-on transition path. The power dissipation on
the driver can be roughly estimated as:
P_DR = P_{DR_UP} + P_{DR_LOW} + I_Q • VCC
P _{D R _UP}
=
⎛
⎜
⎝
-------------^R-----^H----^I--¹--------------
R_HI1 + R_EXT1
+
-_R----_L---_O-----₁^R----+-^L---^O-_R----¹-_E----_X----_T---₁- _{⎠⎟⎞}
•
P_{-----Q----g----_--Q-----1-}
2
(EQ. 4)
P _{D R _LOW}
=
⎛
⎜
⎝
-------------^R-----^H----^I--²--------------
R_HI2 + R_EXT2
+
R^-----_L^---_O^-----₂^{R----+-L---O-}R^----2-_E^----_X^----_T^---₂^- _{⎠⎟⎞}
• ^P-----^Q----^g----^_--^Q-----²-
2
R_EXT2
=
R_G1
+
R_{-----G-----I-1--}
N_Q1
R_EXT2
=
R_G2
+
R_{-----G-----I-2--}
N_Q2
VCC
BOOT
D
R_HI1
R_LO1
UGATE
PHASE
C_GD
G
R_G1
R_GI1
C_GS
S
C_DS
Q1
FIGURE 3. TYPICAL UPPER-GATE DRIVE TURN-ON PATH
VCC
R_HI2 ^LGATE
R_LO2
GND
C_GD
G
R_G2
R_GI2
C_GS
S
D
C_DS
Q2
FIGURE 4. TYPICAL LOWER-GATE DRIVE TURN-ON PATH
8
FN9240.1
January 22, 2010