10
LTC1430
INTERNAL
CIRCUITRY
V
CC
/PV
CC2
LTC1430 (8-LEAD)
PV
CC1
M1
L1
M2
G1
G2
PV
CC
C
OUT
V
OUT
LTC1430 • F07
+
Figure 7. 8-Lead Power Supplies
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 LTC1430 requires four supply voltages to
operate: PV
CC
for the main power input, PV
CC1
and PV
CC2
for MOSFET gate drive and a clean, low ripple V
CC
for the
LTC1430 internal circuitry (Figure 6). In many applica-
tions, PV
CC
and PV
CC2
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 M2. This can be the 5V system supply if a logic
level MOSFET is used for M2. V
CC
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 V
CC
.
For both versions of the LTC1430, PV
CC1
must be higher
than PV
CC
by at least one external MOSFET V
GS(ON)
to fully
enhance the gate of M1. This higher voltage can be
provided with a separate supply (typically 12V) which
should power up after PV
CC
, or it can be generated with a
simple charge pump (Figure 4). The charge pump consists
of a 1N4148 diode from PV
CC
to PV
CC1
and a 0.1µF
capacitor from PV
CC1
to the switching node at the drain of
M2. This circuit provides 2PV
CC
– V
F
to PV
CC1
while M1 is
ON and PV
CC
– V
F
while M1 is OFF where V
F
is the ON
voltage of the 1N4148 diode. Ringing at the drain of M2
can cause transients above 2PV
CC
at PV
CC1
; if PV
CC
is
higher than 7V, a 12V zener diode should be included from
PV
CC1
to PGND to prevent transients from damaging the
circuitry at PV
CC2
or the gate of M1.
More complex charge pumps can be constructed with the
16-lead versions of the LTC1430 to provide additional
voltages for use with standard threshold MOSFETs or very
low PV
CC
voltages. A tripling charge pump (Figure 5) can
provide 2PV
CC
and 3PV
CC
voltages. These can be con-
nected to PV
CC2
and PV
CC1
respectively, allowing stan-
dard threshold MOSFETs to be used with 5V at PV
CC
or 5V
logic level threshold MOSFETs to be used with 3.3V at
PV
CC
. V
CC
can be driven from the same potential as PV
CC2
,
allowing the entire system to run from a single 3.3V
supply. Tripling charge pumps require the use of Schottky
diodes to minimize forward drop across the diodes at
start-up. The tripling charge pump circuit will tend to
rectify any ringing at the drain of M2 and can provide well
more than 3PV
CC
at PV
CC1
; all tripling (or higher multiply-
ing factor) circuits should include a 12V zener clamp diode
D
Z
to prevent overvoltage at PV
CC1
.
INTERNAL
CIRCUITRY
V
CC
LTC1430 (16-LEAD)
PV
CC2
PV
CC1
M1
L1
M2
G1
G2
PV
CC
C
OUT
V
OUT
LTC1430 • F06
+
Figure 6. 16-Lead Power Supplies
The 8-lead versions of the LTC1430 have the PV
CC2
and
V
CC
pins tied together inside the package (Figure 7). This
pin, brought out as V
CC
/PV
CC2
, has the same low ripple
requirements as the 16-lead part, but must also be able to
supply the gate drive current to M2. This can be obtained
by using a larger RC filter from the PV
CC
pin; 22Ω and 10µF
work well here. The 10µF capacitor must be VERY close to
the part (preferably right underneath the unit) or output
regulation may suffer.
APPLICATIO S I FOR ATIO
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