Datasheet

LTC4365
11
Rev. B
For more information www.analog.com
Figure 5 shows the timing associated with the UV pin.
Once a UV fault propagates through the UV comparator
(t
FAULT
), the FAULT output is asserted low and a 50mA
current sink discharges the GATE pin. As V
OUT
falls, the
GATE pin tracks V
OUT
.
APPLICATIONS INFORMATION
Figure 6 shows the timing associated with the OV pin.
Once an OV fault propagates through the OV comparator
(t
FAULT
), the FAULT output is asserted low and a 50mA
current sink discharges the GATE pin. As V
OUT
falls, the
GATE pin tracks V
OUT
.
Procedure for Selecting UV/OV External Resistor Values
The following 3-step procedure helps select the resistor
values for the resistive divider of Figure 4. This procedure
minimizes UV and OV offset errors caused by leakage
currents at the respective pins.
1. Choose maximum tolerable offset at the UV pin,
V
OS(UV)
. Divide by the worst case leakage current at
the UV pin, I
UV
(10nA). Set the sum of R1 + R2 equal
to V
OS(UV)
divided by 10nA. Note that due to the
presence of R3, the actual offset at UV will be slightly
lower:
R1 + R2 =
V
OS(UV)
I
UV
2. Select the desired V
IN
UV trip threshold, UV
TH
. Find
the value of R3:
R3 =
V
OS(UV)
I
UV
•
UV
TH
– 0.5V
0.5V
⎛
⎝
⎜
⎞
⎠
⎟
3. Select the desired V
IN
OV trip threshold, OV
TH
. Find
the values of R1 and R2:
R1 =
V
OS(UV)
I
UV
⎛
⎝
⎜
⎞
⎠
⎟
+ R3
OV
TH
• 0.5V
R2 =
V
OS(UV)
I
UV
– R1
The example of Figure 4 uses standard 1% resistor values.
The following parameters were selected:
V
OS(UV)
= 3mV
I
UV
= 10nA
UV
TH
= 5V
OV
TH
= 18V
4365 F05
FAULT
GATE
t
FAULT
t
GATE(FAST)
V
UV
V
UV
+ V
UVHYST
t
FAULT
t
RECOVERY
EXTERNAL N-CHANNEL MOSFET
TURNS OFF
UV
4365 F06
FAULT
GATE
t
FAULT
t
GATE(FAST)
V
OV
V
OV
– V
OVHYST
t
FAULT
t
RECOVERY
EXTERNAL N-CHANNEL MOSFET
TURNS OFF
OV
Figure 5. UV Timing (OV < (V
OV
– V
OVHYST
), SHDN > 1.2V)
Figure 6. OV Timing (UV > (V
UV
+ V
UVHYST
), SHDN > 1.2V)
When both the UV and OV faults are removed, the external
MOSFET is not immediately turned on. The input supply
must remain within the user selected power good window
for at least 36ms (t
RECOVERY
) before the load is again
connected to the supply. This recovery timeout period
filters noise (including line noise) at the input supply and
prevents chattering of power at the load. For applications
that require faster turn-on after a fault, the LTC4365-1
provides a 1ms recovery timeout period.
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