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LTC4219

4219f

APPLICATIONS INFORMATION

Power Good Indication

In addition to setting the foldback current limit threshold,

the FB pin is used to determine a power good condition.

The LTC4219-12 and LTC4219-5 use an internal resis-

tive divider on the OUT pin to drive the FB pin. On the

LTC4219-12, the PG comparator indicates logic high when

OUT pin rises above 10.5V. If the OUT pin subsequently falls

below 10.3V, the comparator toggles low. On the LTC4219-

5 the PG comparator drives high when the OUT pin rises

above 4.35V and low when OUT falls below 4.27V.

Once the PG comparator is high, the GATE pin voltage is

monitored with respect to the OUT pin. Once the GATE

minus OUT voltage exceeds 4.2V, the PG pin goes low.

This indicates to the system that it is safe to load the OUT

pin while the MOSFET is completely turned “on”. The PG

pin goes high when the GATE is commanded off (using

the EN1, EN2 or SENSE pins) or when the PG comparator

drives low.

Design Example

Consider the following design example (Figure 5): VIN =

12V, IMAX = 5A. IINRUSH = 100mA, CL = 330μF, VPWRGD =

10.5V.

The inrush current is deﬁned by the current required to

charge the output capacitor using the ﬁxed 0.3V/ms GATE

charge up rate. The inrush current is deﬁned as:

IINRUSH = CL •

0.3V

⎛

⎝⎜

⎞

⎠⎟

= 330µF •

0.3V

⎛

⎝⎜

⎞

⎠⎟

= 100mA

As mentioned previously, the charge up time is the out-

put voltage (12V) divided by the output rate of 0.3V/ms

resulting in 40ms. The peak power dissipation of 12V at

100mA (or 1.2W) is within the SOA of the pass MOSFET for

40ms (see MOSFET SOA curve in the Typical Performance

Characteristics section).

Next the power dissipated in the MOSFET during overcur-

rent must be limited. The active current limit uses a timer

to prevent excessive energy dissipation in the MOSFET.

The worst-case power dissipation occurs when the voltage

versus current proﬁle of the foldback current limit is at the

maximum. This occurs when the current is 6.1A and the

voltage is one half of the 12V or 6V. See the Current Limit

Sense Voltage vs FB Voltage in the Typical Performance

Characteristics section to view this proﬁle. In order to

survive 36W, the MOSFET SOA dictates a maximum time

of 10ms (see SOA graph). Use the internal 2ms timer

invoked by tying the TIMER pin to INTVCC.

Figure 5. 5A, 12V Card Resident Application

ADC

0.1μF

12V

VOUT

12V

20k

4219 F05

10k

12V

10k

12V

330μF

VDD

OUT

GND

IMON

LTC4219DHC-12

INTVCC

TIMER

EN1

EN2

FLT

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