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參數(shù)資料
| 型號(hào)： | LTC3226IUD#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHED CAPACITOR REGULATOR, PQCC16 |
| 封裝： | 3 X 3 MM, LEAD FREE, PLASTIC, MO-220WEED-2, QFN-16 |
| 文件頁數(shù)： | 5/16頁 |
| 文件大?。?/td> | 431K |
| 代理商： | LTC3226IUD#PBF |
LTC3226
13
3226f
APPLICATIONS INFORMATION
For the LTC3226 charge pump, the sum of the switch
resistances is approximately 2.5Ω in a typical applica-
tion where VIN = 3.3V and VCPO = 5V. For CFLY = 1F and
fOSC = 1MHz, the effective open-loop output resistance
of the charge pump can be approximated from the above
equation as follows:
ROL 22.5+
1
1MHz1F
= 6
Maximum Available Charge Current
In the absence of any internal current limit, the maximum
available current out of a charge pump in 2x mode can be
calculated from the charge pump input and output voltage
and the effective open-loop output resistance ROL using
the following equation:
ICHRG =
2VIN – VCPO
ROL
For example, if the LTC3226 charge pump (ROL 6Ω)
has to charge a supercapacitor to 5V from 2.5V input,
the charge current available when VCPO = 4.8V can be
calculated as follows:
ICHRG =
2 2.5V – 4.8V
6
= 33.3mA
So even if the charge pump input current limit is pro-
grammed for 315mA (RPROG = 33.2k), the actual charge
current will be considerably less than 157.5mA (half of
programmed limit) in 2x mode for very low input supply.
For VIN = 2.5V, the CPO voltage above which the charge
current will decrease from the programmed value of
157.5mA (RPROG = 33.2k) can be calculated from the
previous equation as follows:
VCPO = (2 2.5V – 157.5mA 6Ω) = 4.055V
Choosing the LDO Output Capacitor
In the event VINfallsbelowtheprogrammedPFIthreshold,
the PFI comparator turns off the charge pump and turns
on the internal LDO to supply the load from the backed-
up supercapacitor storage. However, due to the delay
associated with the PFI comparator and LDO circuitry, it
could be up to 2μs before the LDO is capable of supplying
the load demand at VOUT. In order to prevent VOUT from
drooping too much during this transition, a 47μF ceramic
output capacitance, COUT, delay, Δt, and load current,
ILOAD, the drop in VOUT, V, can be calculated using the
following equation:
ILOAD =COUT
V
t
For example, if VOUT can not tolerate more than 100mV
drop under a maximum load of 2A during this transition,
the minimum capacitance required at the LDO output can
be calculated using the above equation as follows:
COUT(MIN) = 2A
2s
100mV
= 40F
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