NCP4308
www.onsemi.com
25
Figure 52. Equivalent Schematic of Gate Drive Circuitry
P
DRV_IC
+
1
2
@ C
g_ZVS
@ V
CLAMP
2
@ f
SW
@ ǒ
R
DRV_SINK_EQ
R
DRV_SINK_EQ
) R
G_EXT
) R
g_int
Ǔ) C
g_ZVS
@ V
CLAMP
@ f
SW
@
ǒ
V
CC
* V
CLAMP
Ǔ
)
1
2
@ C
g_ZVS
@ V
CLAMP
2
@ f
SW
@ ǒ
R
DRV_SOURCE_EQ
R
DRV_SOURCE_EQ
) R
G_EXT
) R
g_int
Ǔ
(eq. 10)
Where:
R
DRV_SINK_EQ
is the NCP4308x driver low side switch
equivalent resistance (0.5 W)
R
DRV_SOURCE_EQ
is the NCP4308x driver high side switch
equivalent resistance (1.2 W)
R
G_EXT
is the external gate resistor (if used)
R
g_int
is the internal gate resistance of the
MOSFET
Step 3 − IC Consumption Calculation:
In this step, power dissipation related to the internal IC
consumption is calculated. This power loss is given by the
I
CC
current and the IC supply voltage. The I
CC
current
depends on switching frequency and also on the selected min
t
ON
and t
OFF
periods because there is current flowing out
from the min t
ON
and t
OFF
pins. The most accurate method
for calculating these losses is to measure the I
CC
current
when C
DRV
= 0 nF and the IC is switching at the target
frequency with given MIN_TON and MIN_TOFF adjust
resistors. IC consumption losses can be calculated as:
P
CC
+ V
CC
@ I
CC
(eq. 11)
Step 4 − IC Die Temperature Arise Calculation:
The die temperature can be calculated now that the total
internal power losses have been determined (driver losses
plus internal IC consumption losses). The package thermal
resistance is specified in the maximum ratings table for a
35 mm thin copper layer with no extra copper plates on any
pin (i.e. just 0.5 mm trace to each pin with standard soldering
points are used).
The DIE temperature is calculated as:
T
DIE
+
ǒ
P
DRV_IC
) P
CC
Ǔ
@ R
qJ−A
) T
A
(eq. 12)
Where:
P
DRV_IC
is the IC driver internal power dissipation
P
CC
is the IC control internal power
dissipation
R
q
JA
is the thermal resistance from junction to
ambient
T
A
is the ambient temperature
