LTC1421 datasheet(14/40 Pages) LINER

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LTC4211

4211fb

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 VCC

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

VCC voltage is desired change the resistor 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 VCC 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 CTIMER at a rate given by Equation 1:

CTIMER Charge-Up Rate =

2µA

CTIMER

(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:

tTIMER = 1.236V •

CTIMER

2µA

(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 F03

FB TRANSIENT (mV)

150

200

250

160

4211 F04

100

120

180

100

140

200

TA = 25°C

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