LTC1421 datasheet(11/24 Pages) LINER

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LTC4223-1/LTC4223-2

422312f

APPLICATIONS INFORMATION

Several conditions must be met before the external and

internal switches are allowed to turn on. First VCC and

the input supplies (12VIN, AUXIN) must exceed their

undervoltage lockout thresholds. Next TIMER must be

<0.2V and ⎯E⎯N must be pulled low.

Once these conditions are met, a debounce timing cycle is

initiated when AUXON or 12ON pin is toggled from low to

high. These two control pins turn on/off the 3.3V auxiliary

and 12V supplies. At the end of the debounce cycle, the

ON pins and fault status are checked. If both ON pins are

high and fault is cleared, the 3.3V auxiliary supply starts

up ﬁrst followed by the 12V supply. Note that the turn-on

delay for the AUXON and 12ON pins is 15μs and 30μs.

Figure 2 shows the two supplies turning on in sequence

after ⎯E⎯N goes low.

By default, the internal pass transistor turns on ﬁrst if

both ON pins are high and start-up conditions met. The

output is current limited at 240mA by its internal ACL

ampliﬁer as the load current charging the output capaci-

tor increases. This causes the TIMER to ramp up with a

10μA pull-up. Normally the AUXOUT voltage exceeds its

power-good threshold before TIMER time-out and then

⎯A⎯U⎯X⎯P⎯G⎯O⎯O⎯D pulls low.

Once AUXOUT signals power is good and the TIMER pin

returns to <0.2V, the external MOSFET is then allowed to

turn on by charging up the GATE with a 10μA current source

(Figure 2). The voltage at the GATE pin rises with a slope

equal to 10μA/CG and the supply inrush current ﬂowing

into the load capacitor CL1 (see Figure 14) is limited to:

µA

INRUSH

= 1 10

•

The 12V output follows the GATE voltage when the MOSFET

turns on. If the voltage across the current sense resistor

RS becomes too high, the inrush current is limited by the

internal current limit circuitry. Once the output, 12VOUT

exceeds its power-good threshold, ⎯1⎯2⎯P⎯G⎯O⎯O⎯D also pulls

low.

If only the 12ON pin is high at the end of debounce cycle,

the external MOSFET turns on ﬁrst. After that, if AUXON

pulls high, the internal switch turns on only after the 12V

output signals power is good and TIMER <0.2V.

Figure 2. Normal Power-Up Sequence

Table 1. 12V and Auxiliary Supply Turn-Off Conditions

CONDITION

RESULT

CLEARED BY

AUX

12V

AUXON Goes

Low

Turns Off

No Effect

AUXON High

12ON Goes Low

No Effect

Turns Off

12ON High

⎯E⎯N Goes High

Turns Off

⎯E⎯N Low

UVLO on VCC

Turns Off

VCC > UVLO

UVLO on AUXIN

Turns Off

No Effect

AUXIN > UVLO

UVLO on 12VIN

No Effect

Turns Off

12VIN > UVLO

AUX Overcurrent

Fault

Turns Off

AUXON and 12ON Low,

⎯E⎯N High-to-Low, UVLO

on VCC

12V Overcurrent

Fault

No Effect

Turns Off

12ON Low, ⎯E⎯N High-to-

Low, UVLO on VCC

Thermal

Shutdown

Turns Off

AUXON and 12ON Low,

⎯E⎯N High-to-Low, UVLO

on VCC, Temperature <

120°C

20ms/DIV

12PGOOD

5V/DIV

AUXOUT

5V/DIV

TIMER

2V/DIV

12VOUT

5V/DIV

AUXPGOOD

5V/DIV

422312 F02

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