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LTC3246 데이터시트(HTML) 12 Page - Analog Devices

부품명 LTC3246
상세내용  Wide VIN Range Buck-Boost Charge Pump with Watchdog Timer
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LTC3246 데이터시트(HTML) 12 Page - Analog Devices

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LTC3246
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3246fa
For more information www.linear.com/LTC3246
half of the total output current. The efficiency ( ) and chip
power dissipation (PD) in 2:1 are approximately:
POUT
PIN
=
VOUT •IOUT
VIN
1
2
IOUT
=
2VOUT
VIN
PD =
VIN
2
– VOUT IOUT
1:1 Step-Down Charge Pump Operation
When the input supply is less than about two times the
output voltage, but more than the programmed output
voltage, the LTC3246 will operate in 1:1 step-down mode.
This method of regulation is very similar to a linear regula-
tor. Charge is delivered directly from VIN to VOUT through
most of the oscillator period. The charge transfer is briefly
interruptedattheendoftheperiod.Whenin1:1step-down
mode, the input current will be approximately equal to the
total output current. Thus, efficiency ( ) and chip power
dissipation (PD) in 1:1 are approximately:
POUT
PIN
=
VOUT •IOUT
VIN •IOUT
=
VOUT
VIN
PD = VIN – VOUT IOUT
1:2 Step-Up Charge Pump Operation
When the input supply is less than the output voltage,
the LTC3246 will operate in 1:2 step-up mode. Charge
transfer happens in two phases. On the first phase, the
flying capacitor (CFLY) is connected between VIN and GND.
On this phase, CFLY is charged up. On the second phase,
the flying capacitor (CFLY) is connected between VIN and
VOUT and the charge stored on CFLY during the first phase
is transferred to VOUT. When in 1:2 step-up mode, the
input current will be approximately twice the total output
current. Thus, efficiency ( ) and chip power dissipation
(PD) in 1:2 are approximately:
POUT
PIN
=
VOUT •IOUT
VIN • 2IOUT
=
VOUT
2VIN
PD = 2VIN – VOUT IOUT
VOUT Ripple and Capacitor Selection
The type and value of capacitors used with the LTC3246
determine output ripple and charge pump strength. The
value of COUT directly controls the amount of output ripple
for a given load current. Output ripple decreases with
output capacitance until about 20µF, at which point output
peak to peak ripple remains more or less constant. See
Figure 3 for graph of output ripple vs output capacitance.
Figure3. Typical VOUT Ripple Voltage vs COUT Capacitance
0
5
10
15
20
25
30
35
40
0
25
50
75
100
125
150
175
200
3246 TA01b
BOOST, 500mA
BOOST, 50mA
BUCK, 500mA
BUCK, 50mA
LDO, 500mA
LDO, 50mA
COUT CAPACITANCE (µF)
To reduce output noise and ripple, it is suggested that a
low ESR (equivalent series resistance < 0.1) ceramic
capacitor (10µF or greater) be used for COUT. For optimal
performance, it is best to increase COUT for low VOUT as
the ripple becomes a larger percentage of the regulation
voltage degrading performance. Tantalum and aluminum
capacitors can be used in parallel with a ceramic capacitor
to increase the total capacitance but are not recommended
to be used alone because of their high ESR.
VOUT Overvoltage Protection
An internal comparator monitors the voltage at VOUT and
will prevent charge transfer in the event that VOUT exceeds
the overvoltage threshold (5.9V typ.). Overvoltage protec-
tion is added as a safety feature to prevent damage to the
part in the event of a fault such as VOUTS/ADJ pin shorted
to ground or not connected to VOUT. Charge transfer will
start once the output falls to about 5.75V.
APPLICATIONS INFORMATION


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