LTC3260
11
3260fa
applicaTions inForMaTion
Effective Open-Loop Output Resistance
The effective open-loop output resistance (R
OL
) of a charge
pump is a very important parameter which determines the
strength of the charge pump. The value of this parameter
depends on many factors such as the oscillator frequency
(f
OSC
), value of the flying capacitor (C
F LY
), the nonoverlap
time, the internal switch resistances (R
S
) and the ESR of
the external capacitors.
Typical R
OL
values as a function of temperature are shown
in Figure 4
minimum turn-on time. The peak-to-peak output ripple at
the V
OUT
pin is approximately given by the expression:
V
RIPPLE(P-P)
≈
I
OUT
C
OUT
1
f
OSC
– t
ON
where C
OUT
is the value of the output capacitor, f
OSC
is the
oscillator frequency and t
ON
is the on-time of the oscillator
(1µs typical).
Just as the value of C
OUT
controls the amount of output
ripple, the value of C
IN
controls the amount of ripple present
at the input (V
IN
) pin. The amount of bypass capacitance
required at the input depends on the source impedance
driving V
IN
. For best results it is recommended that V
IN
be bypassed with at least 2µF of low ESR capacitance. A
high ESR capacitor such as tantalum or aluminum will
have higher input noise than a low ESR ceramic capacitor.
Therefore, a ceramic capacitor is recommended as the
main bypass capacitance with a tantalum or aluminum
capacitor used in parallel if desired.
Flying Capacitor Selection
The flying capacitor controls the strength of the charge
pump. A 1µF or greater ceramic capacitor is suggested
for the flying capacitor for applications requiring the full
rated output current of the charge pump.
For very light load applications, the flying capacitor may
be reduced to save space or cost. For example, a 0.2µF
capacitor might be sufficient for load
currents up to 20mA.
A
smaller flying capacitor leads to a larger effective open-
loop resistance (R
OL
) and thus limits the maximum load
current that can be delivered by the charge pump.
Ceramic Capacitors
Ceramic capacitors of different materials lose their capaci-
tance with higher temperature and voltage at different rates.
For example, a capacitor made of X5R or 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
Figure 4. Typical R
OL
vs Temperature
Input/Output Capacitor Selection
The style and value of capacitors used with the LTC3260
determine several important parameters such as regulator
control loop stability, output ripple, charge pump strength
and minimum turn-on time. To reduce noise and ripple,
it is recommended that low ESR ceramic capacitors be
used for the charge pump and LDO outputs. All capacitors
should retain at least 2µF of capacitance over operating
temperature and bias voltage. Tantalum and aluminum
capacitors can be used in parallel with a ceramic capacitor
to increase the total capacitance but should not be used
alone because of their high
ESR. In constant frequency
mode,
the value of C
OUT
directly controls the amount of
output ripple for a given load current. Increasing the size of
C
OUT
will reduce the output ripple at the expense of higher
TEMPERATURE (°C)
–50
0
EFFECTIVE OPEN-LOOP RESISTANCE (Ω)
10
20
30
40
0 50
100
150
3620 F04
50
60
–25 25
75
125
V
IN
= 32V
V
IN
= 25V
V
IN
= 12V
f
OSC
= 500kHz
