LTC3261
APPLICATIONS INFORMATION
Effective Open-Loop Output Resistance
⎛
⎞
IOUT
COUT
1
f
OSC
VRIPPLE(P-P)
where f
≈
•
– tON
⎜
⎝
⎟
⎠
Theeffectiveopen-loopoutputresistance(R )ofacharge
OL
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 ), value of the flying capacitor (C ), the nonoverlap
is the oscillator frequency t is the on-time
of the oscillator (1μs) typical and C
output capacitor.
OSC
ON
is the value of the
OUT
OSC
FLY
time, the internal switch resistances (R ) and the ESR of
S
Just as the value of C
controls the amount of output
OUT
the external capacitors.
ripple,thevalueofC controlstheamountofripplepresent
IN
Typical R values as a function of temperature are shown
at the input (V ) pin. The amount of bypass capacitance
OL
IN
in Figure 2
required at the input depends on the source impedance
driving V . For best results it is recommended that V
IN
IN
60
V
V
V
= 32V
= 25V
= 12V
f
= 500kHz
IN
IN
IN
OSC
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.
55
50
45
40
35
30
25
20
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.
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3261 F02
Figure 2. Typical ROL vs Temperature
For very light load applications, the flying capacitor may
be reduced to save space or cost. For example, a 0.2μF
capacitormightbesufficientforloadcurrentsupto20mA.
A smaller flying capacitor leads to a larger effective open-
loop resistance (R ) and thus limits the maximum load
current that can be delivered by the charge pump.
Input/Output Capacitor Selection
The style and value of capacitors used with the LTC3261
determineseveralimportantparameterssuchasregulator
control loop stability, output ripple, charge pump strength
andminimumturn-ontime.Toreducenoiseandripple,itis
recommendedthatlowESRceramiccapacitorsbeusedfor
thechargepumpoutput.Thechargepumpoutputcapacitor
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
OL
Ceramic Capacitors
Ceramic capacitors of different materials lose their capaci-
tancewithhighertemperatureandvoltageatdifferentrates.
For example, a capacitor made of X5R or X7R material will
retainmostofitscapacitancefrom–40°Cto85°Cwhereasa
Z5UorY5Vstylecapacitorwillloseconsiderablecapacitance
over that range. Z5U and Y5V capacitors may also have a
poorvoltagecoefficientcausingthemtolose60%ormoreof
theircapacitancewhentheratedvoltageisapplied.Therefore
when comparing different capacitors, it is often more ap-
propriate to compare the amount of achievable capacitance
for a given case size rather than discussing the specified
mode, the value of C
directly controls the amount of
OUT
outputrippleforagivenloadcurrent.Increasingthesizeof
will reduce the output ripple at the expense of higher
C
OUT
minimum turn-on time. The peak-to-peak output ripple at
the V
pin is approximately given by the expression:
OUT
capacitance value. The capacitor manufacture’s data sheet
3261f
8
Linear Systems [ Linear Systems ]
