LTC1435ACS datasheet(14/20 Pages) LINER

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LTC1435A

APPLICATIONS INFORMATION

By powering EXTVCCfromanoutput-derivedsource,the

additional VIN current resulting from the driver and

control currents will be scaled by a factor of

Duty Cycle/Efficiency. For example, in a 20V to 5V ap-

plication, 10mA of INTVCC current results in approxi-

mately 3mA of VIN current. This reduces the midcurrent

loss from 10% or more (if the driver was powered di-

rectly from VIN) to only a few percent.

3. I2R losses are predicted from the DC resistances of the

MOSFET, inductor and current shunt. In continuous

mode the average output current flows through L and

RSENSE, but is “chopped” between the topside main

MOSFET and the synchronous MOSFET. If the two

MOSFETs have approximately the same RDS(ON), then

the resistance of one MOSFET can simply be summed

with the resistances of L and RSENSE to obtain I2R

losses. For example, if each RDS(ON) = 0.05Ω,

RL = 0.15Ω, and RSENSE = 0.05Ω, then the total resis-

tance is 0.25

Ω. This results in losses ranging from 3%

to 10% as the output current increases from 0.5A to

2A. I2R losses cause the efficiency to drop at high output

currents.

4. Transition losses apply only to the topside MOSFET(s),

and only when operating at high input voltages (typically

20V or greater). Transition losses can be estimated from:

Transition Loss = 2.5 (VIN)1.85(IMAX)(CRSS)(f)

Other losses, including CIN and COUT ESR dissipative

losses, Schottky conduction losses during dead-time,

and inductor core losses, generally account for less than

2% total additional loss.

Checking Transient Response

The regulator loop response can be checked by looking at

the load transient response. Switching regulators take sev-

eral cycles to respond to a step in DC (resistive) load cur-

rent. When a load step occurs, VOUT immediately shifts by

an amount equal to (

∆ILOAD)(ESR), where ESR is the ef-

fective series resistance of COUT. ∆ILOAD also begins to

charge or discharge COUTwhichgeneratesafeedbackerror

signal. The regulator loop then acts to return VOUT to its

steady-state value. During this recovery time VOUT can be

monitored for overshoot or ringing, which would indicate

a stability problem. The ITH external components shown in

the Figure 1 circuit will provide adequate compensation for

most applications.

A second, more severe transient is caused by switching in

loads with large (>1

µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

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 9 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

Figure 9. Automotive Application Protection

1435A F09

50A IPK RATING

LTC1435A

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

VIN

12V

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