9
LTC1911-1.5/LTC1911-1.8
1911f
To reduce output noise and ripple, it is suggested that a
low ESR (≤0.1Ω) ceramic capacitor (10µF
or greater) be
used for C
OUT
. Tantalum and Aluminum capacitors are not
recommended because of their high ESR (equivalent
series resistance).
Both the style and value of C
OUT
can significantly affect the
stability of the LTC1911. As shown in the Block Diagram,
the part uses a control loop to adjust the strength of the
charge pump to match the current required at the output.
The error signal of this loop is stored directly on the output
charge storage capacitor. The charge storage capacitor
also serves to form the dominant pole for the control loop.
To prevent ringing or instability it is important for the
output capacitor to maintain at least 4µF of capacitance
over all conditions (See Ceramic Capacitor Selection
Guidelines).
Likewise excessive ESR on the output capacitor will tend
to degrade the loop stability of the LTC1911. The closed-
loop output resistance of the part is designed to be 0.13Ω.
For a 250mA load current change, the output voltage will
change by about 33mV. If the output capacitor has 0.13Ω
or more of ESR, the closed-loop frequency response will
cease to roll-off in a simple 1-pole fashion and poor load
transient response or instability could result. Ceramic
capacitors typically have exceptional ESR performance,
and combined with a tight board layout, should yield
excellent stability and load transient performance.
V
IN
Capacitor Selection
The constant frequency architecture used by the
LTC1911 makes input noise filtering much less demand-
ing than with conventional regulated charge pumps. De-
pending on the mode of operation the input current of the
LTC1911 can vary from I
OUT
to 0mA on a cycle-by-cycle
basis. Lower ESR will reduce the voltage steps caused by
changing input current, while the absolute capacitor value
will determine the level of ripple. For optimal input noise
and ripple reduction, it is recommended that a low ESR
ceramic capacitor be used for C
IN
. A tantalum capacitor
may be used for C
IN
but the higher ESR will lead to more
input noise. The LTC1911 will operate with capacitors
APPLICATIO S I FOR ATIO
WUUU
less than 1µF but the increasing input noise will feed
through to the output causing degraded performance.
For best performance a 1µF
or greater capacitor is sug-
gested for C
IN
. Aluminum capacitors are not recom-
mended because of their high ESR.
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or
aluminum should never be used for the flying capacitors
since their voltage can reverse upon start-up of the
LTC1911. 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 for the flying capacitor to have at least 0.4µF of
capacitance over operating temperature with a 2V bias
(See Ceramic Capacitor Selection Guidelines). If only
100mA or less of output current is required the flying
capacitor minimum can be reduced to 0.15µF.
Ceramic Capacitor Selection Guidelines
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 capaci-
tance over that range (60% to 80% loss typ). Z5U and Y5V
capacitors may also have a very strong voltage coefficient
causing them to lose an additional 60% 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 capaci-
tance for a given case size rather than discussing the
specified capacitance value. For example, over rated volt-
age and temperature conditions, a 4.7µF, 10V, Y5V ce-
ramic capacitor in a 0805 case may not provide any more
capacitance than a 1µF, 10V, X7R available in the same
0805 case. In fact, over bias and temperature range, the
1µF, 10V, X7R will provide more capacitance than the
4.7µF, 10V, Y5V. The capacitor manufacturer’s data sheet
should be consulted to determine what value of capacitor
