LTC3250 Datasheet, PDF (7/12 Pages) Linear Technology – High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converter

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OPERATIO (Refer to Simplified Block Diagram)
current. For an ideal 2 to 1 step-down charge pump the
power efficiency is given by:
η ≡ ^POUT = ^{VOUT •IOUT} = ^2VOUT
P_IN
V_IN • _21IOUT
V_IN
The switching losses and quiescent current of the
LTC3250-1.5/LTC3250-1.2 are designed to minimize effi-
ciency loss over the entire output current range, causing
only a couple % error from the theoritical efficiency. For
example with V_IN = 3.6V, I_OUT = 100mA and V_OUT regulat-
ing to 1.5V the measured efficiency is 80.6% which is in
close agreement with the theoretical 83.3% calculation.
V_OUT Capacitor Selection
The ESR and value of capacitors used with the LTC3250-
1.5/LTC3250-1.2 determine several important parameters
such as regulator control loop stability, output ripple, and
charge pump strength.
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.
To reduce output noise and ripple, it is suggested that a
low ESR (<0.1Ω) ceramic capacitor (4.7µF or greater) be
used for C_OUT. Tantalum and aluminum capacitors are not
recommended because of their high ESR.
Both ESR and value of the C_OUT can significantly affect the
stability of the LTC3250-1.5/LTC3250-1.2. As shown in
the block diagram, the LTC3250-1.5/LTC3250-1.2 use 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. Thus 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 2.5µF of capacitance over all condi-
tions (see “Ceramic Capacitor Selection Guidelines” sec-
tion).
Likewise excessive ESR on the output capacitor will tend
to degrade the loop stability of the LTC3250-1.5/LTC3250-
1.2. The closed-loop output resistance is designed to be
LTC3250-1.5/LTC3250-1.2
0.15Ω for the LTC3250-1.5 and 0.12Ω for the
LTC3250-1.2. For a 250mA load current change the output
voltage will change by about 37mV for the LTC3250-1.5
and by 30mV for the LTC 3250-1.2. If the ESR of the output
capacitor is greater than the closed-loop-output imped-
ance the part will cease to roll-off in a simple one-pole
fashion and poor load transient response or instability
could result. Ceramic capacitors typically have excep-
tional ESR performance and combined with a tight board
layout should yield excellent stability and load transient
performance.
Further output noise reduction can be achieved by filtering
the LTC3250-1.5/LTC3250-1.2 output through a very small
series inductor as shown in Figure 1. A 10nH inductor will
10nH
(TRACE INDUCTANCE)
V_OUT
V_OUT
LTC3250-1.5/
LTC3250-1.2
4.7µF
0.22µF
GND
3250 F01
Figure 1. 10nH Inductor Used for
Additional Output Noise Reduction
reject the fast output transients, thereby presenting a
nearly constant output voltage. For economy the 10nH
inductor can be fabricated on the PC board with about 1cm
(0.4") of PC board trace.
V_IN Capacitor Selection
The constant frequency architecture used by the
LTC3250-1.5/LTC3250-1.2 makes input noise filtering
much less demanding than conventional charge pump
regulators. On a cycle by cycle basis, the LTC3250-1.5/
LTC3250-1.2 input current will go from I_OUT/2 to 0mA.
Lower ESR will reduce the voltage steps caused by chang-
ing 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 1µF or
greater ceramic capacitor be used for C_IN (see “Ceramic
Capacitor Selection Guidelines” section). Aluminum and
tantalum capacitors are not recommended because of
their high ESR.
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