ltc1751 Linear Technology Corporation, ltc1751 Datasheet - Page 8

• Manufacturer Part Number: ltc1751
• Description: Micropower, Regulated Charge Pump Dc/dc Converters
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or
aluminum should never be used for the flying capacitor
since its voltage can reverse upon start-up of the LTC1751.
Low ESR ceramic capacitors should always be used for
the flying capacitor.
The flying capacitor controls the strength of the charge
pump. In order to achieve the rated output current, it is
necessary to have at least 0.6 F of capacitance for the
flying capacitor. Capacitors of different materials lose their
capacitance with higher temperature and voltage at differ-
ent 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. The capacitor
manufacturer’s data sheet should be consulted to deter-
mine what value of capacitor is needed to ensure 0.6 F at
all temperatures and voltages.
Generally an X7R ceramic capacitor is recommended for
the flying capacitor with a minimum value of 1 F. For very
low load applications, it may be reduced to 0.01 F-0.68 F.
A smaller flying capacitor delivers less charge per clock
cycle to the output capacitor resulting in lower output
ripple. The output ripple is reduced at the expense of
maximum output current and efficiency.
The theoretical minimum output resistance of a voltage
doubling charge pump is given by:
Where f if the switching frequency and C is the value of the
flying capacitor. (Using units of MHz and F is convenient
since they cancel each other.) Note that the charge pump
will typically be weaker than the theoretical limit due to
additional switch resistance. However, for light load appli-
cations, the above expression can be used as a guideline
in determining a starting capacitor value.
LTC1751/LTC1751-3.3/LTC1751-5
8
R
OUT MIN
(
)
2
V
IN
U
I
OUT
–
V
OUT
U
fC
1
W
U
Below is a list of ceramic capacitor manufacturers and
how to contact them:
Output Ripple
Low frequency regulation mode ripple exists due to the
hysteresis in the sense comparator and propagation
delays in the charge pump control circuits. The amplitude
and frequency of this ripple are heavily dependent on the
load current, the input voltage and the output capacitor
size. For large V
tial because the increased strength of the charge pump
causes fast edges that may outpace the regulation cir-
cuitry. In some cases, rather than bursting, a single
output cycle may be enough to boost the output voltage
into or possibly beyond regulation. In these cases the
average output voltage will climb slightly. For large input
voltages a larger output capacitor will ensure that burst-
ing always occurs, thus mitigating possible DC problems.
Generally the regulation ripple has a sawtooth shape
associated with it.
A high frequency ripple component may also be present
on the output capacitor due to the charge transfer action
of the charge pump. In this case, the output can display a
voltage pulse during the output-charging phase. This
pulse results from the product of the charging current and
the ESR of the output capacitor. It is proportional to the
input voltage, the value of the flying capacitor and the ESR
of the output capacitor.
For example, typical combined output ripple for an
LTC1751-5 with V
75mV
output capacitor and/or larger output current load will
result in higher ripple due to higher output voltage slew
rates.
P-P
Taiyo Yuden
with a low ESR 10 F output capacitor. A smaller
Murata
Vishay
Kemet
AVX
IN
the ripple voltage can become substan-
IN
= 3V under maximum load is
www.avxcorp.com
www.t-yuden.com
www.murata.com
www.vishay.com
www.kemet.com