LTC4364-1/LTC4364-2
17
436412f
Another way to limit supply transients above 100V at the
V
CC
pin is to use a Zener diode and a resistor, D1 and R4,
as shown in Figure 1. D1 clamps voltage spikes at the V
CC
pin while R4 limits the current through D1 to a safe level
during the surge. In the negative direction, D1 along with
R4 clamps the V
CC
pin near GND. The inclusion of R4 in
series with the V
CC
pin increases the minimum required
supply voltage due to the extra voltage drop across the
resistor, which is determined by the supply current of the
LTC4364 and the leakage current of D1. 2.2k adds about
1V to the minimum operating voltage.
For sustained, elevated suppy voltages, the power dissipa-
tion of R4 becomes unacceptable. This can be resolved
by using an external NPN transistor (Q1 in Figure 7) as
a buffer. To protect Q1 against supply reversal, block the
collector of Q1 with a series diode or tie it to the cathode
of D3 and D4 in Figure 1.
Transient suppressor D3 in Figure 1 clamps the input
voltage to 200V for voltage transients higher than 200V,
to prevent breakdown of M1. It also blocks forward con-
duction in D4. D4 limits the SOURCE pin voltage to 24V
below GND when the input goes negative. C
OUT
helps
absorb the inductive energy at the output upon a sudden
input short, protecting the OUT and SENSE pins.
Output Port Protection
In applications where the output is on a connector, as
shown in Figure 14, if the output is plugged into a supply
that is higher than the input, the ideal diode MOSFET, M2,
turns off to open the backfeeding path. In the case where
the output port is plugged into a supply that is below GND,
the SOURCE pin is pulled below GND through the body
diode of M2. The LTC4364 responds to this condition by
shorting the HGATE pin to the SOURCE pin, turning M1
off and shutting down the current path from V
IN
to V
OUT
.
Design Example
As a design example, consider an application with the
following specifications: V
IN
= 8V to 14V DC with a peak
transient of 200V and decay time constant τ of 1ms, V
OUT
≤ 27V, minimum current limit I
LIM(MIN)
at 4A, low-battery
detection at 6V, input overvoltage level at 60V, and 1ms
of overvoltage early warning (Figure 1).
Selection of CMZ5945B for D1 will limit the voltage at
the V
CC
pin to less than 71V during the 200V surge. The
minimum required voltage at the V
CC
pin is 4V when V
IN
is
at 6V; the maximum supply current for LTC4364 is 750μA.
The maximum value for R4 to ensure proper operation is:
R4 =
0.75mA
= 2.7k
Select 2.2k for R4 to accommodate all conditions.
With the minimum Zener voltage at 64V, the peak current
through R4 into D1 is then calculated as:
I
D1(PK)
=
= 62mA
which can be handled by the CMZ5945B with a peak power
rating of 200W at 10/1000μs.
With a bypass capacitance of 0.1μF (C1), along with R4
of 2.2k, high voltage transients up to 250V with a pulse
width less than 20μs are filtered out at the V
CC
pin.
Next, calculate the resistive divider value to limit V
OUT
to
27V during an overvoltage event:
V
REG
=
1.25V • R7+ R8
R8
= 27V
APPLICATIONS INFORMATION
V
CC
LTC4364
GND
436412 F07
C1
100nF
Q1
PZTA42
V
IN
200V
D1
CMZ5945B
68V
R4
22k
1/4W
Figure 7. Buffering V
CC
to Extend Input Supply Range
Output Bypassing
The OUT and SENSE pins can withstand up to 100V above
and 20V below GND. In all applications the output must
be bypassed with at least 22μF low ESR electrolytic (C
OUT
in Figure 1) to stabilize the voltage and current limiting
loops, and to minimize capacitive feedthrough of input
transients. Total ceramic bypassing of up to one-tenth
the total electrolytic capacitance is permissible without
compromising performance.
