LTC1624CS datasheet(12/28 Pages) LINER

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LTC1624

with COUT, causing a rapid drop in VOUT. No regulator can

deliver enough current to prevent this problem if the load

switch resistance is low and it is driven quickly. The only

solution is to limit the rise time of the switch drive so that

the load rise time is limited to approximately (25 • CLOAD).

Thus a 10

µF capacitor would require a 250µs rise time,

limiting the charging current to about 200mA.

Automotive Considerations: Plugging into the

Cigarette Lighter

As battery-powered devices go mobile there is a natural

interest in plugging into the cigarette lighter in order to

conserve or even recharge battery packs during operation.

But before you connect, be advised: you are plugging into

the supply from hell. The main battery line in an automo-

bile is the source of a number of nasty potential transients,

including load dump, reverse battery and double battery.

Load dump is the result of a loose battery cable. When the

cable breaks connection, the field collapse in the alternator

can cause a positive spike as high as 60V which takes

several hundred milliseconds to decay. Reverse battery is

just what it says, while double battery is a consequence of

tow-truck operators finding that a 24V jump start cranks

cold engines faster than 12V.

The network shown in Figure 4 is the most straightforward

approach to protect a DC/DC converter from the ravages

of an automotive battery line. The series diode prevents

current from flowing during reverse battery, while the

transient suppressor clamps the input voltage during load

dump. Note that the transient suppressor should not

conduct during double battery operation, but must still

clamp the input voltage below breakdown of the converter.

Although the LTC1624 has a maximum input voltage of

APPLICATIONS INFORMATION

36V, most applications will be limited to 30V by the

MOSFET BVDSS.

Modifying Burst Mode Operation

The LTC1624 automatically enters Burst Mode operation

at low output currents to boost efficiency. The point when

continuous mode operation changes to Burst Mode op-

eration scales as a function of maximum output current.

The output current when Burst Mode operation com-

mences is approximately 8mV/RSENSE (8% of maximum

output current).

With the additional circuitry shown in Figure 5 the LTC1624

can be forced to stay in continuous mode longer at low

output currents. Since the LTC1624 is not a fully synchro-

nous architecture, it will eventually start to skip cycles as

the load current drops low enough. The point when the

minimum on-time (450ns) is reached determines the load

current when cycle skipping begins at approximately 1%

of maximum output current. Using the circuit in Figure 5

the LTC1624 will begin to skip cycles but stays in regula-

tion when IOUT is less than IOUT(MIN):

OUT MIN

ON MIN

OUT

()













−

() +









where tON(MIN) = 450ns, f = 200kHz.

The transistor Q1 in the circuit of Figure 5 operates as a

current source developing an 18mV offset across the

1000pF

100

Ω

18mV

COUT

RSENSE

CIN

VOUT

MBRS340T3

2N2222

VIN

1624 F05

(VOUT – 0.7V)

180

µA

*R =

VIN

LTC1624

SENSE–

–

Figure 5. Modifying Burst Mode Operation

Figure 4. Plugging into the Cigarette Lighter

LTC1624

VIN

50A IPK

RATING

12V

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

1624 F04

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