MAX5920
If I
CB
is set to twice I
LOAD
, or 5A, V
DS
momentarily dou-
bles to ≤ 1.25V. If C
OUT
= 4000µF, transient-line input
voltage is ∆36V, the 5A charging-current pulse is:
Entering the data sheet transient-thermal-resistance
curves at 1ms provides a θJC = 0.9°C/W. P
D
= 6.25W,
so ∆t
JC
= 5.6°C. Clearly, this is not a problem.
Example 2:
I
LOAD
= 10A, efficiency = 98%, allowing V
DS
= 0.96V
but R
DS(ON)
≤ 96mΩ. An IRF530 in a D
2
PAK exhibits
R
DS(ON)
≤ 90mΩ at +25°C and ≤ 135mΩ at +80°C.
Power dissipation is 9.6W at +25°C or 14.4W at +80°C.
Junction-to-case thermal resistance is 1.9W/°C, so the
junction temperature rise would be approximately 5°C
above the +25°C case temperature. For higher efficien-
cy, consider IRL540NS with R
DS(ON)
≤ 44mΩ. This
allows η = 99%, P
D
≤ 4.4W, and T
JC
= +4°C (θ
JC
=
1.1°C/W) at +25°C.
Thermal calculations for the transient condition yield
I
CB
= 20A, V
DS
= 1.8V, t = 0.5ms, transient θ
JC
=
0.12°C/W, P
D
= 36W and ∆t
JC
= 4.3°C.
Layout Guidelines
Good thermal contact between the MAX5920A/
MAX5920B and the external MOSFET is essential for the
thermal-shutdown feature to operate effectively. Place
the MAX5920A/MAX5920B as close as possible to the
drain of the external MOSFET and use wide circuit-board
traces for good heat transfer (see Figure 15).
