Datasheet

LTC4227
15
422712fa
For more information www.linear.com/LTC4227
applicaTions inForMaTion
to be greater than 10V. The gate drive is limited to not
more than 14V. This allows the use of logic-level threshold
N-channel MOSFETs and standard N-channel MOSFETs
above 7V. An external Zener diode can be used to clamp
the potential from the MOSFET ’s gate to source if the rated
breakdown voltage is less than 14V.
The maximum allowable drain-source voltage, BV
DSS
,
must be higher than the supply voltages as the full sup
-
ply voltage can appear across the MOSFET. If an input or
output is connected to ground, the full supply voltage will
appear across the MOSFET. The R
DS(ON)
should be small
enough to conduct the maximum load current, and also
stay within the MOSFET’s power rating.
CPO Capacitor Selection
The recommended value of the capacitor, C
CP
, between
the CPO and IN pins is approximately 10× the input capaci
-
tance, C
ISS
, of the ideal diode MOSFET. A larger capacitor
takes a correspondingly longer time to charge up by the
internal charge pump. A smaller capacitor suffers more
voltage drop during a fast gate turn-on event as it shares
charge with the MOSFET gate capacitance.
Supply Transient
Protection
When the capacitances at the input and output are very
small, rapid changes in current during an input or output
short-circuit event can cause transients that exceed the
24V absolute maximum ratings of the IN and OUT pins.
To minimize such spikes, use wider traces or heavier
trace plating to reduce the power trace inductance. Also,
bypass locally with a 10µF electrolytic and 0.1µF ceramic,
or alternatively clamp the input with a transient voltage
suppressor (Z1, Z2). A 10Ω, 0.1µF snubber damps the
response and eliminates ringing (See Figure 9).
Design Example
As a design example for selecting components, consider
a 12V system with a 7.6A maximum load current for the
two supplies (see Figure 1).
First, select the appropriate value of the current sense re
-
sistor, R
S
, for the 12V supply. Calculate the sense resistor
value based on the maximum load current and the lower
limit for the circuit breaker threshold, ∆V
SENSE(CB)(MIN)
:
R
S
=
∆V
SENSE(CB)(MIN)
I
LOAD(MAX)
=
47.5mV
7.6A
= 6.25mΩ
Choose a 6mΩ sense resistor with a 1% tolerance. The
minimum and maximum circuit breaker trip current is
calculated as follows:
I
TRIP(MIN)
=
∆V
SENSE(CB)(MIN)
R
S(MAX)
=
47.5mV
6.06mΩ
= 7.8A
I
TRIP(MAX)
=
∆V
SENSE(CB)(MAX)
R
S(MIN)
=
52.5mV
5.94mΩ
= 8.8A
For proper operation, I
TRIP(MIN)
must exceed the maximum
load current with margin, so R
S
= 6mΩ should suffice for
the 12V supply.
Next, calculate the R
DS(ON)
of the ideal diode MOSFET to
achieve the desired forward drop at maximum load. Assum
-
ing a forward drop, ∆V
FWD
of 60mV across the MOSFET :
R
DS(ON)
≤
∆V
FWD
I
LOAD(MAX)
=
60mV
7.6A
= 7.9mΩ
The SiR462DP offers a good choice with a maximum
R
DS(ON)
of 7.9mΩ at V
GS
= 10V. The input capacitance,
C
ISS
, of the SiR462DP is about 1155pF. Slightly exceeding
the 10× recommendation, a 0.1µF capacitor is selected for
C
CP1
and C
CP2
at the CPO pins.
Next, verify that the thermal ratings of the selected Hot
Swap MOSFET, Si7336ADP, are not exceeded during
power-up or an output short.
Assuming the MOSFET dissipates power due to inrush
current charging the load capacitor, C
L
, at power-up, the
energy dissipated in the MOSFET is the same as the energy
stored in the load capacitor, and is given by:
E
CL
=
1
2
• C
L
• V
IN
2