ltc3240-3.3 Linear Technology Corporation, ltc3240-3.3 Datasheet - Page 9

• Manufacturer Part Number: ltc3240-3.3
• Description: 3.3v/2.5v Step-up/ Step-down Charge Pump Dc/dc Converter
• Manufacturer: Linear Technology Corporation

In charge pump mode the value of C
amount of output ripple for a given load current. Increasing
the size of C
of higher minimum turn-on time. The peak-to-peak output
ripple is approximately given by the expression:
where f
and C
Also, the value and style of the output capacitor can sig-
nifi cantly affect the stability of the LTC3240. As shown
in the Block Diagram, the LTC3240 uses a linear 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 storage capacitor.
This output capacitor also serves to form the dominant
pole of the control loop. To prevent ringing or instability
on the LTC3240, it is important to maintain at least 2µF
of capacitance over all conditions.
Excessive ESR on the output capacitor can degrade the
loop stability of the LTC3240. The closed-loop output
resistance of the LTC3240 is designed to be 0.5Ω. For a
100mA load current change, the output voltage will change
by about 50mV. If the output capacitor has 0.5Ω or more
of ESR, the closed-loop frequency response will cease to
roll off in a simple one-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 very good stability
and load transient performance.
Just as the value of C
ripple, the value of C
present at the input pin (V
input current to the LTC3240 is relatively constant during
the input charging phase and the output charging phase
but drops to zero during the nonoverlap times. Since the
nonoverlap time is small (~25ns), these missing notches
result in a small perturbation on the input power supply
line. A higher ESR capacitor such as tantalum will have
higher input noise than a low ESR ceramic capacitor.
Therefore, ceramic capacitors are again recommended
for their exceptional ESR performance.
APPLICATIO S I FOR ATIO
V
RIPPLE P P
OUT
OSC
is the value of the output capacitor.
(
OUT
is the oscillator frequency (typically 1.2MHz)
−
)
will reduce the output ripple at the expense
≅
2
U
f
OSC
OUT
IN
I
OUT
•
controls the amount of ripple
U
controls the amount of output
IN
C
OUT
) in charge pump mode. The
OUT
W
directly controls the
U
Further input noise reduction can be achieved by power-
ing the LTC3240 through a very small series inductor as
shown in Figure 3. A 10nH inductor will reject the fast
current notches, thereby presenting a nearly constant
current load to the input power supply. For economy, the
10nH inductor can be fabricated on the PC board with
about 1cm (0.4") of PC board trace.
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or alumi-
num should never be used for the fl ying capacitor since
its voltage can reverse upon start-up of the LTC3240.
Low ESR ceramic capacitors should always be used for
the fl ying capacitor.
The fl ying capacitor controls the strength of the charge
pump. A 1µF or greater ceramic capacitor is suggested
for the fl ying capacitor. For the LTC3240-3.3 operating
at an input voltage in the range 1.8V ≤ V
necessary to have at least 0.5µF of capacitance for the
fl ying capacitor in order to achieve the maximum rated
current of 40mA.
For very light load applications, the fl ying capacitor may
be reduced to save space or cost. From the fi rst order
approximation of R
Resistance” section, the theoretical minimum output
resistance of a voltage doubling charge pump can be
expressed by the following equation:
where f
is the value of the fl ying capacitor. The charge pump
will typically be weaker than the theoretical limit due
R
OL MIN
LTC3240-3.3/LTC3240-2.5
V
IN
(
OSC
)
is the switching frequency (1.2MHz) and C
=
Figure 3. 10nH Inductor Used for
Additional Input Noise Reduction
2
V
1cm OF WIRE
IN
0.22µF
I
OUT
10nH
OL
–
V
in the “Effective Open-Loop Output
OUT
≅
2.2µF
f
OSC
•
1
C
2
1
FLY
V
GND
IN
LTC3240-3.3/
LTC3240-2.5
IN
≤ 2.5V, it is
3240 F03
3240fb
9
FLY