LTC1430
U
W U U
APPLICATIO S I FOR ATIO
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.
For both versions of the LTC1430, PVCC1 must be higher
than PVCC by at least one external MOSFET VGS(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 PVCC, or it can be generated with a
simplechargepump(Figure4).Thechargepumpconsists
of a 1N4148 diode from PVCC to PVCC1 and a 0.1µF
capacitor from PVCC1 to the switching node at the drain of
M2. This circuit provides 2PVCC – VF to PVCC1 while M1 is
ON and PVCC – VF while M1 is OFF where VF is the ON
voltage of the 1N4148 diode. Ringing at the drain of M2
can cause transients above 2PVCC at PVCC1; if PVCC is
higherthan7V, a12Vzenerdiodeshouldbeincludedfrom
PVCC1 to PGND to prevent transients from damaging the
circuitry at PVCC2 or the gate of M1.
Input Supply Considerations/Charge Pump
The 16-lead LTC1430 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
LTC1430 internal circuitry (Figure 6). In many applica-
tions, PVCC and PVCC2 can be tied together and fed from
a common high power supply, provided that the supply
voltageishighenoughtofullyenhancethegateofexternal
MOSFET M2. This can be the 5V system supply if a logic
level MOSFET is used for M2. 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.
More complex charge pumps can be constructed with the
16-lead versions of the LTC1430 to provide additional
voltagesforusewithstandardthresholdMOSFETsorvery
low PVCC voltages. A tripling charge pump (Figure 5) can
provide 2PVCC and 3PVCC voltages. These can be con-
nected to PVCC2 and PVCC1 respectively, allowing stan-
dard threshold MOSFETs to be used with 5V at PVCC or 5V
logic level threshold MOSFETs to be used with 3.3V at
V
CC
PV
CC2
PV
CC1
PV
CC
G1
G2
M1
PVCC. VCC canbedrivenfromthesamepotentialasPVCC2
,
L1
INTERNAL
CIRCUITRY
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 3PVCC at PVCC1; all tripling (or higher multiply-
ingfactor)circuitsshouldincludea12Vzenerclampdiode
V
OUT
+
M2
C
OUT
LTC1430 (16-LEAD)
LTC1430 • F06
Figure 6. 16-Lead Power Supplies
The 8-lead versions of the LTC1430 have the PVCC2 and
VCC pins tied together inside the package (Figure 7). This
pin, brought out as VCC/PVCC2, 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
byusingalargerRCfilterfromthePVCC pin;22Ωand10µF
work well here. The 10µF capacitor must be VERY close to
the part (preferably right underneath the unit) or output
regulation may suffer.
DZ to prevent overvoltage at PVCC1
.
PV
CC1
PV
CC
V
CC
/PV
CC2
G1
G2
M1
L1
INTERNAL
CIRCUITRY
V
OUT
+
M2
C
OUT
LTC1430 (8-LEAD)
LTC1430 • F07
Figure 7. 8-Lead Power Supplies
10
Linear [ Linear ]
