LTC4211
14
4211fc
For more information www.linear.com/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 resistive 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 resistive divider sets the circuit to turn on when V
CC
reaches around 79% of its final value. If a different turn on
V
CC
voltage is desired change the resistive divider values
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
CC
to restart the system.
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.
Figure 4. FB Comparator Glitch Filter Time
vs Feedback Transient Voltage
Figure 3. ON Pin Sets the Undervoltage
Lockout Voltage Externally
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
TIMER
at a rate given by Equation 1:
C
TIMER
Charge-Up Rate =
2µA
C
TIMER
(1)
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:
t
TIMER
= 1.236V •
C
TIMER
2µA
(2)
As a design aid, the LTC4211’s timer period as a function
of the C
TIMER
using standard values from 3.3nF to 0.33µF
is shown in Table 1.
V
IN
3.3V
R1
10k
R2
10k
ON PIN
(a) 3.3V
IN
V
IN
5V
R1
20k
R2
10k
ON PIN
(b) 5V
IN
V
IN
12V
R1
61.9k
R2
10k
ON PIN
(c) 12V
IN
4211 F03
FB TRANSIENT (mV)
0
GLITCH FILTER TIME (µs)
150
200
250
160
100
50
0
40 80 12020 18060 100 140 200
T
A
= 25°C
