LT4256 LINER [Linear Technology], LT4256 Datasheet - Page 10

• Manufacturer Part Number: LT4256
• Description: UV, OV and Reverse Supply Protection Controller Low Operating Current: 125?A
• Manufacturer: LINER [Linear Technology]

LTC4365
APPLICATIONS INFORMATION
Figure 4 shows the timing associated with the UV pin.
Once a UV fault propagates through the UV comparator
(t
current sink discharges the GATE pin. As V
GATE pin tracks V
Figure 5 shows the timing associated with the OV pin.
Once an OV fault propagates through the OV comparator
(t
current sink discharges the GATE pin. As V
GATE pin tracks V
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
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.
10
FAULT
FAULT
FAULT
GATE
FAULT
GATE
Figure 4. UV Timing (OV < (V
Figure 5. OV Timing (UV > (V
UV
OV
), the FAULT output is asserted low and a 50mA
), the FAULT output is asserted low and a 50mA
V
V
UV
OV
t
GATE(FAST)
t
GATE(FAST)
OUT
OUT
RECOVERY
.
.
t
FAULT
t
EXTERNAL N-CHANNEL MOSFET
FAULT
EXTERNAL N-CHANNEL MOSFET
OV
UV
TURNS OFF
) before the load is again
TURNS OFF
– V
+ V
V
V
UV
OVHYST
UVHYST
OV
t
RECOVERY
t
+ V
RECOVERY
– V
UVHYST
OVHYST
t
FAULT
t
FAULT
), SHDN > 1.2V)
), SHDN > 1.2V)
OUT
OUT
falls, the
falls, the
4365 F04
4365 F05
Procedure for Selecting UV/OV External Resistor Values
The following 3-step procedure helps select the resistor
values for the resistive divider of Figure 3. This procedure
minimizes UV and OV offset errors caused by leakage
currents at the respective pins.
The example of Figure 3 uses standard 1% resistor values.
The following parameters were selected:
1. Choose maximum tolerable offset at the UV pin,
2. Select the desired V
3. Select the desired V
V
I
UV
OV
UV
OS(UV)
V
the UV pin, I
to V
presence of R3, the actual offset at UV will be slightly
lower:
the value of R3:
the values of R1 and R2:
TH
TH
= 10nA
OS(UV)
R3 = 2 •
R1=
R2 =
R1+R2 =
= 3.5V
= 18V
OS(UV)
= 3mV
. Divide by the worst case leakage current at
V
V
OS(UV)
I
OS(UV)
2 • OV
divided by 10nA. Note that due to the
I
UV
UV
V
UV
V
OS(UV)
I
OS(UV)
(10nA). Set the sum of R1 + R2 equal
UV
I
UV
– R1
TH
+R3
IN
IN
• UV
UV trip threshold, UV
OV trip threshold, OV
(
TH
– 0.5V
)
TH
TH
. Find
. Find
4365f