LTC3204-5_15 Datasheet, PDF (9/12 Pages) Linear Technology – Low Noise Regulated Charge Pump in 2 2 DFN

LTC3204-3.3/LTC3204-5/
LTC3204B-3.3/LTC3204B-5
APPLICATIO S I FOR ATIO
where f_OSC is the oscillator frequency (typically
1.2MHz) and C_OUT is the value of output charge storage
capacitor.
Also, the value and style of the output capacitor can signifi-
cantly affect the stability of the LTC3204-3.3/LTC3204-5/
LTC3204B-3.3/LTC3204B-5. As shown in the Block
Diagram, the LTC3204-3.3/LTC3204-5/LTC3204B-
3.3/LTC3204B-5 use 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 out-
put capacitor also serves to form the dominant pole of
the control loop. To prevent ringing or instability on the
LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5,
it is important to maintain at least 1µF of capacitance over
all conditions.
Excessive ESR on the output capacitor can degrade the loop
stability of the LTC3204-3.3/LTC3204-5/LTC3204B-3.3/
LTC3204B-5. The closed loop output resistance of the
LTC3204-5 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 good board layout should yield very good stability
and load transient performance.
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 pin (V_IN). The input current to the LTC3204-3.3/
LTC3204-5/LTC3204B-3.3/LTC3204B-5 will be relatively
constant during the input charging phase or the output
charging phase but will drop to zero during the nonoverlap
times. Since the nonoverlap time is small (~25ns), these
missing notches will result in only a small perturbation
on the input power supply line. Note that a higher ESR
capacitor such as tantalum will have higher input noise
due to the voltage drop in the ESR. Therefore, ceramic
capacitors are again recommended for their exceptional
ESR performance.
Further input noise reduction can be achieved by powering
the LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5
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.
1cm OF WIRE
10nH
2 _VIN
V_IN
0.22µF
2.2µF
LTC3204-3.3/
LTC3204-5
1
GND
32005 F03
Figure 3. 10nH Inductor Used for
Additional Input Noise Reduction
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or
aluminum should never be used for the flying capaci-
tor since its voltage can reverse upon start-up of the
LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5.
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 1µF of capacitance for the fly-
ing capacitor.
For very light load applications, the flying capacitor may be
reduced to save space or cost. From the first order approxi-
mation of R_OL in the section “Effective Open-Loop Output
Resistance,” the theoretical minimum output resistance
of a voltage doubling charge pump can be expressed by
the following equation:
R_0L(MIN)
=
2V_IN – V_OUT
I_OUT
≅
1
f _OSC• C_FLY
where f_OSC is the switching frequency (1.2MHz) and C_FLY
is the value of the flying capacitor. The charge pump
will typically be weaker than the theoretical limit due to
additional switch resistance. However, for very light load
applications, the above expression can be used as a guide-
line in determining a starting capacitor value.
3204fa
9