4

dc2150af

DEMO MANUAL DC2150A

CIRCUIT OPERATION

D4 additionally provides reverse polarity protection,

preventing negative input voltages from showing up at

the output. (The LTC4366 surge stopper in front of D4 is

capable of withstanding reverse voltages and the –250V

spike without additional protection.)

An optional bidirectional TVS (Transient Voltage Suppres-

sor) is present at the input to provide extra protection. Its

150V breakdown voltage does not affect circuit operation

below 100V. For applications where a TVS is not desirable

at the input, this optional component can be removed.

STD-1275D spike tests in Figures 3 and 4 shows high

frequency ringing which is a measurement artifact of the

large currents that flow in supply and ground traces when

the 0.1µF test circuit capacitor is discharged directly at the

circuitinputwithallresistancesandinductancesminimized.

±7V Ripple

Satisfying the ripple specification of MIL-STD-1275D

requires several more components. Diode D4 in combi-

nation with capacitors C1-C12 form an AC rectifier. This

rectified signal appears at the RIPCAP node. The LT4363

in combination with sense resistor R3 limits the maximum

current to 5A (typical). If the rising edge of the input ripple

waveform attempts to pull up the output capacitor with

more than 5A, the LT4363 momentarily limits the current

by pulling down on Q4’s gate. To quickly restore the gate

voltage, the small charge pump formed by components

D5, D6, C24, C25, and C26 supplements the LT4363’s

internalchargepumptoquicklypullupMOSFETQ4’sgate.

Even still, the available load current must be reduced to

2.8A during this ripple condition. Figure 5 shows that the

output remains powered during ripple testing.

Finally,thermalprotectionisimplementedbycomponents

R25,R26,R27,Q6-1,Q6-2,Q5-1andthermistorR29.Ifthe

temperatureatQ4’sheatsinkexceeds105°C,theLT4363’s

UV pin is pulled down by Q5-1 to force off MOSFET Q4

and limit its maximum temperature.

Starting Mode Initial Engagement Surge

Itshouldalsobenotedthatwiththespecifiedcomponents,

this circuit is only guaranteed to work down to a minimum

of 8V during the starting mode initial engagement surge

rather than the minimum 6V specified in MIL-STD-1275D.

DC2150A Assembly Options

Assembly options DC2150A-B and DC2150A-D eliminate

capacitors C1-C12 to reduce total solution cost. In those

assembly options, C25, C27, R31, R32, D7-1, and D7-2

inject a small current into the LT4363’s TIMER on each

rising edge of the input ripple (which appears at the

SOURCE1 node). In the face of ripple, those components

will force the LT4363 to timeout quickly and turn off Q4.

10ms/DIV

5V/DIV

5V/DIV

1A/DIV

21V

20.3V

34V

35V

28V

27.15V

2.715A