LTC4417
23
4417f
applicaTions inForMaTion
Significant power is dissipated during the channel transi-
tion time. The SOA of the P-channel MOSFET should be
checked to make sure their SOA is not violated.
Worst case slew rate limited channel transition time
would occur when the lithium-ion batteries are running
low at 5.6V, and the supply connects while running 20%
high, at 14.4V. This results in a time of 25µs, as shown
in Equation (24).
dt =
35A
dt = 25µs
(24)
The IRF7324 thermal response curve at 25µs shows Z
θJA
to be approximately 0.18 for a single pulse. The Z
θJA
of
0.18 results in a maximum transient power dissipation of
694W at 25°C and 361W at 85°C. The external P-channel
MOSFETs will dissipate no more than 8.8V • 37A = 325W
during this period, below the available 361W at 85°C.
The initial soft-start period will also force the external
back-to-back MOSFETs to dissipate significant power. To
check the SOA during this period, start with Equation (9).
t
STARTUP
(ms) =
12V
5[V/ms]
t
STARTUP
(ms) = 2.4ms
(25)
I
MAXCAP
current of 500mA is calculated using Equation (10).
I
MAXCAP
= 100µF • 5[V/ms]
I
MAXCAP
= 500mA
(26)
The worst case soft-start power dissipation from Equa-
tion (11) is:
P
SS
(W) = 12V • 500mA
P
SS
(W) = 6W
(27)
The soft-start power dissipation of 6W is well below the
calculated transient power dissipation (P
DM
) of 79.4W at
a T
C
of 25°C. An ambient temperature, T
A
, of 85°C results
in a P
DM
of 41.3W, indicating it is sufficient to handle the
2.4ms transient 6W power dissipation. A graphical check
with the manufacturer’s SOA curves confirms sufficient
operating margin.
Setting Operational Range
Assuming the 12V source has a tolerance of ±20%, the
input source has an operational undervoltage limit of
9.6V and an overvoltage limit of 14.4V. Ideally the UV1,
UV2 and UV3 and OV1, OV2 and OV3 thresholds would
be set to these limits. However, since the actual threshold
varies by 1.5% and resistor tolerances are 1%, OV and
UV limits must be adjusted to ±26% or 8.9V and 15.1V.
Further, instead of using the internal fixed 30mV, a UV
hysteresis of 200mV is set using an external hysteresis
current of 250nA.
The
design p
rocess starts with setting R
HYS
using
Equation (1).
R
HYS
=
= 252kΩ
(28)
The nearest standard value is 255kΩ.
Now set the UV hysteresis value using R3
R3 =
Desired Hysteresis
I
OVUV(HYS)
=
200mV
247nA
= 810kΩ
(29)
The nearest standard value is 806kΩ.
With R3 set, the remaining resistance can be determined
with
R1,2 =
UV
TH(FALLING)
– V
OVUV(THR)
=
806kΩ
= 102kΩ
(30)
R1 is
R1=
R1,2+R3
OV
TH(RISING)
=
102kΩ+ 806kΩ
15.1V
= 60.1kΩ
(31)
The nearest 1% standard value is: 60.4kΩ.
R2 is
R2 = R1,2 – R3 = 102kΩ – 60.4kΩ = 41.6kΩ (32)
The nearest 1% standard value is 41.2kΩ.
