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LTC4211

4211f

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 VCC

reaches around 79% of its final value. If a different turn on

VCC voltage is desired change the resistor divider value

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 VCC to restart the

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

OPERATIO

FEEDBACK TRANSIENT (mV)

150

200

250

160

4211 F03

100

120

180

100

140

200

TA = 25°C

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 2

µA current source is then connected to the

TIMER pin to charge CTIMER at a rate given by Equation 1:

Charge -Up Rate

TIMER

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:

TIMER

1 236

.•

(2)

As a design aid, the LTC4211’s timer period as a function

of the CTIMER using standard values from 3.3nF to 0.33µF

is shown in Table 1.

VIN

3.3V

10k

ON PIN

(a) 3.3VIN

VIN

20k

10k

ON PIN

(b) 5VIN

VIN

12V

61.9k

10k

ON PIN

4211 F04

pin. The relationship between glitch filter time and the

feedback transient voltage is shown in Figure 4.

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