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LTC1430A

APPLICATI

I FOR ATIO

Input Supply Considerations/Charge Pump

The 16-lead LTC1430A requires four supply voltages to

operate: PVCC for the main power input, PVCC1 and PVCC2

for MOSFET gate drive and a clean, low ripple VCC for the

LTC1430A internal circuitry (Figure 8). In many applica-

tions, PVCC and PVCC2 can be tied together and fed from a

common high power supply, provided that the supply

voltage is high enough to fully enhance the gate of external

MOSFET Q2. This can be the 5V system supply if a logic

level MOSFET is used for Q2. VCC can usually be filtered

with an RC from this same high power supply; the low

quiescent current (typically 350

µA) allows the use of

relatively large filter resistors and correspondingly small

filter capacitors. 100

Ω and 4.7µF usually provide ad-

equate filtering for VCC.

The 8-lead version of the LTC1430A has the PVCC2 and VCC

pins tied together inside the package (Figure 9). This pin,

brought out as VCC/PVCC2, has the same low ripple re-

quirements as the 16-lead part, but must also be able to

supply the gate drive current to Q2. This can be obtained

The output capacitor in a buck converter sees much less

ripple current under steady-state conditions than the input

capacitor. Peak-to-peak current is equal to that in the

inductor, usually a fraction of the total load current. Output

capacitor duty places a premium not on power dissipation

but on low ESR. During an output load transient, the

output capacitor must supply all of the additional load

current demanded by the load until the LTC1430A can

adjust the inductor current to the new value. ESR in the

output capacitor results in a step in the output voltage

equal to the ESR value multiplied by the change in load

current. A 5A load step with a 0.05

Ω ESR output capacitor

will result in a 250mV output voltage shift; this is a 7.6%

output voltage shift for a 3.3V supply! Because of the

strong relationship between output capacitor ESR and

output load transient response, the output capacitor is

usually chosen for ESR, not for capacitance value; a

capacitor with suitable ESR will usually have a larger

capacitance value than is needed to control steady-state

output ripple.

Electrolytic capacitors rated for use in switching power

supplies with specified ripple current ratings and ESR can

be used effectively in LTC1430A applications. OS-CON

electrolytic capacitors from Sanyo give excellent perfor-

mance and have a very high performance/size ratio for an

electrolytic capacitor. Surface mount applications can use

either electrolytic or dry tantalum capacitors. Tantalum

capacitors must be surge tested and specified for use in

switching power supplies; low cost, generic tantalums are

known to have very short lives followed by explosive

deaths in switching power supply applications. AVX TPS

series surface mount devices are popular tantalum capaci-

tors that work well in LTC1430A applications. A common

way to lower ESR and raise ripple current capability is to

parallel several capacitors. A typical LTC1430A applica-

tion might require an input capacitor with a 5A ripple

current capacity and 2% output shift with a 10A output

load step, which requires a 0.007

Ω output capacitor ESR.

Sanyo OS-CON part number 10SA220M (220

µF/10V)

capacitors feature 2.3A allowable ripple current at 85

°C

and 0.035

Ω ESR; three in parallel at the input and six at the

output will meet the above requirements.

Figure 9. 8-Lead Power Supplies

INTERNAL

CIRCUITRY

VCC/PVCC2

LTC1430A (8-LEAD)

PVCC1

PVCC

COUT

VOUT

1430 F09

Figure 8. 16-Lead Power Supplies

INTERNAL

CIRCUITRY

VCC

LTC1430A (16-LEAD)

PVCC2

PVCC1

PVCC

COUT

VOUT

1430 F08

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