LTC4211 LINER [Linear Technology], LTC4211 Datasheet - Page 13

• Manufacturer Part Number: LTC4211
• Description: Hot Swap Controller with Multifunction Current Control
• Manufacturer: LINER [Linear Technology]

OPERATIO
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 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. The FAULT comparator can be the alternative
for external undervoltage lockout setting. 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
system.
GLITCH FILTER FOR RESET
The LTC4211 has a glitch filter to prevent RESET from
generating a system reset if there are transients on the FB
CC
voltage is desired change the resistor divider value
3.3V
V
IN
(a) 3.3V
R1
10k
R2
10k
Figure 3. ON Pin Sets the Undervoltage
Lockout Voltage Externally
ON PIN
IN
U
V
5V
IN
(b) 5V
R1
20k
R2
10k
ON PIN
IN
12V
V
IN
(c) 12V
CC
R1
61.9k
R2
10k
4211 F04
to restart the
ON PIN
IN
CC
pin. The relationship between glitch filter time and the
feedback transient voltage is shown in Figure 4.
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 2 A 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
Figure 4. FB Comparator Glitch Filter Time
vs Feedback Transient Voltage
250
200
100
Charge -Up Rate
150
50
1 236
0
using standard values from 3.3nF to 0.33 F
.
0
T
A
20
= 25 C
V
40
•
FEEDBACK TRANSIENT (mV)
C
TIMER
60
2
TIMER
80
A
100
at a rate given by Equation 1:
120
C
2 A
TIMER
140
160
180
4211 F03
LTC4211
200
13
4211f
(1)
(2)