LTC3221_15 Datasheet, PDF (9/12 Pages) Linear Technology – Micropower, Regulated Charge Pump

LTC3221/
LTC3221-3.3/LTC3221-5
APPLICATIO S I FOR ATIO
be expressed by the following equation:
R_OL(MIN)
≡
2V_IN – V_OUT
I_OUT
≅
1
f_OSC • C_FLY
where f_OSC is the switching frequency (600kHz) and C_FLY
is the value of the flying capacitor. The charge pump will
typically be weaker than the theoretical limit due to ad-
ditional switch resistance. However, for very light load ap-
plications, the above expression can be used as a guideline
in determining a starting capacitor value.
Ceramic Capacitors
Capacitors of different materials lose their capacitance
with higher temperature and voltage at different rates.
For example, a ceramic capacitor made of X7R material
will retain most of its capacitance from –40°C to 85°C,
whereas, a Z5U or Y5V style capacitor will lose considerable
capacitance over that range. Z5U and Y5V capacitors may
also have a very strong voltage coefficient causing them
to lose 50% or more of their capacitance when the rated
voltage is applied. Therefore when comparing different
capacitors, it is often more appropriate to compare the
amount of achievable capacitance for a given case size
rather than discussing the specified capacitance value.
For example, over rated voltage and temperature condi-
tions, a 1µF 10V Y5V ceramic capacitor in a 0603 case
may not provide any more capacitance than a 0.22µF 10V
X7R capacitor available in the same 0603 case. In fact,
for most LTC3221-3.3/LTC3221-5/LTC3221 applications,
these capacitors can be considered roughly equivalent. The
capacitor manufacturer’s data sheet should be consulted
to determine what value of capacitor is needed to ensure
0.6µF at all temperatures and voltages.
Table 1 shows a list of ceramic capacitor manufacturers
and how to contact them.
Table 1. Ceramic Capacitor Manufacturers
AVX
www.avxcorp.com
Kemet
www.kemet.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay
www.vishay.com
Programming the LTC3221 Output Voltage (FB Pin)
While the LTC3221-3.3/LTC3221-5 versions have internal
resistive dividers to program the output voltage, the pro-
grammable LTC3221 may be set to an arbitrary voltage via
an external resistive divider. Figure 3 shows the required
voltage divider connection.
V_OUT ⁶
LTC3221
R1
V_OUT
=
1.23V
(1
+
R1 ₎
R2
C1
3
FB
R2
4
GND
C_OUT
3221 F03
Figure 3. Programming the Adjustable LTC3221
The voltage divider ratio is given by the expression:
R1 ₌ V_{OUT – 1}
R2 1.23V
Since the LTC3221 employs a voltage doubling charge
pump, it is not possible to achieve output voltages greater
than twice the available input voltage. The V_IN supply
range required for regulation is given by the following
expression:
Maximum V_IN < V_OUT + 0.6
( ) _{Minimum VIN =}
V_OUT + I_OUT • R_OL
2
or 1.8V;
whichever is higher
Where R_OL is the effective open-loop output resistance and
I_OUT is the maximum load current. V_IN cannot be higher
than V_OUT by more than 0.6V, or else the line regulation
is poor. Also, V_IN has to be higher than the minimum
operating voltage of 1.8V.
The sum of the voltage divider resistors can be made large
to keep the quiescent current to a minimum. Any standing
current in the output divider (given by 1.23/R2) will be
reflected by a factor of 2 in the input current. A reasonable
resistance value should be such that the standing current
is in the range of 10µA to 100µA when V_OUT is regulated.
3221f
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