LTC1647 LINER [Linear Technology], LTC1647 Datasheet - Page 14

• Manufacturer Part Number: LTC1647
• Description: Hot Swap Controller
• Manufacturer: LINER [Linear Technology]

LTC4211
OPERATION
UNDERVOLTAGE LOCKOUT
The LTC4211’s power-on reset circuit initializes the start-
up procedure and ensures the chip is in the proper state if
the input supply voltage is too low. If the supply voltage
falls below 2.18V, the LTC4211 is in undervoltage lockout
(UVLO) mode, and the GATE pin is pulled low. Since the
UVLO circuitry uses hysteresis, the chip restarts after the
supply voltage rises above 2.3V and the ON pin goes high.
In addition, users can utilize the ON comparator (COMP1)
or the FAULT comparator (COMP6) to effectively program
a higher undervoltage lockout level. Figure 3 shows how
the external resistor divider at the ON pin programs the
system’s undervoltage lockout voltage. The system will
enter the plug-in cycle after the ON pin rises above 1.316V.
The resistor divider sets the circuit to turn on when V
reaches around 79% of its final value. If a different turn on
V
accordingly. Alternatively, the FAULT comparator can be
used to configure the external undervoltage lockout level.
If the FAULT comparator is used for this purpose, the
system will wait for the input voltage to increase above
the level set by the user before starting the second timing
cycle. Also, if the input voltage drops below the set level
in normal operating mode, the user must cycle the ON pin
or V
14
GLITCH FILTER FOR RESET
The LTC4211 has a glitch filter to prevent transients on the
FB pin from generating a system reset. The relationship
between glitch filter time and the FB transient voltage is
shown in Figure 4.
CC
CC
voltage is desired change the resistor divider values
to restart the system.
3.3V
V
IN
(a) 3.3V
R1
10k
R2
10k
Figure 3. ON Pin Sets the Undervoltage
Lockout Voltage Externally
ON PIN
IN
V
5V
IN
(b) 5V
R1
20k
R2
10k
ON PIN
IN
12V
V
IN
(c) 12V
R1
61.9k
R2
10k
4211 F03
ON PIN
IN
CC
SYSTEM TIMING
System timing for the LTC4211 is generated at the TIMER
pin (see the Block Diagram). If the LTC4211’s internal
timing circuit is off, an internal N-channel FET connects
the TIMER pin to GND. If the timing circuit is enabled,
an internal 2μA current source is then connected to the
TIMER pin to charge C
When the TIMER pin voltage reaches COMP4’s threshold
of 1.236V, the TIMER pin is reset to GND. Equation 2 gives
an expression for the timer period:
As a design aid, the LTC4211’s timer period as a function
of the C
is shown in Table 1.
C
t
TIMER
TIMER
TIMER
250
200
150
100
Figure 4. FB Comparator Glitch Filter Time
vs Feedback Transient Voltage
= 1.236V •
50
Charge-Up Rate =
0
0
using standard values from 3.3nF to 0.33μF
T
A
20
= 25°C
40
60
FB TRANSIENT (mV)
C
TIMER
TIMER
2µA
80
100
at a rate given by Equation 1:
120
C
140
TIMER
2µA
160
180
4211 F04
200
4211fb
(1)
(2)