LTC1430A View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1430A' PDF : 24 Pages View PDF
LTC1430A
APPLICATI S I FOR ATIO
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 0.05Ω 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 0.007Ω output capacitor ESR.
Sanyo OS-CON part number 10SA220M (220µF/10V)
capacitors feature 2.3A allowable ripple current at 85°C
and 0.035Ω ESR; three in parallel at the input and six at the
output will meet the above requirements.
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 350µA) allows the use of
relatively large filter resistors and correspondingly small
filter capacitors. 100Ω 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
VCC
PVCC2 PVCC1
PVCC
INTERNAL
CIRCUITRY
LTC1430A (16-LEAD)
G1
Q1
L1
G2
Q2
VOUT
+
COUT
1430 F08
Figure 8. 16-Lead Power Supplies
VCC/PVCC2
PVCC1
PVCC
INTERNAL
CIRCUITRY
LTC1430A (8-LEAD)
G1
Q1
L1
G2
Q2
VOUT
+
COUT
1430 F09
Figure 9. 8-Lead Power Supplies
12
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