LTC3250ES6-1.2#PBF Linear Technology, LTC3250ES6-1.2#PBF Datasheet - Page 7

• Manufacturer Part Number: LTC3250ES6-1.2#PBF
• Manufacturer: Linear Technology

Specifications of LTC3250ES6-1.2#PBF

• Lead Free Status / RoHS Status: Compliant

OPERATIO
current. For an ideal 2 to 1 step-down charge pump the
power efficiency is given by:
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
ing to 1.5V the measured efficiency is 80.6% which is in
close agreement with the theoretical 83.3% calculation.
V
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
ripple for a given load current. Increasing the size of C
will reduce the output ripple.
To reduce output noise and ripple, it is suggested that a
low ESR (<0.1 ) ceramic capacitor (4.7 F or greater) be
used for C
recommended because of their high ESR.
Both ESR and value of the C
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 2.5 F 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
OUT
Capacitor Selection
P
P
OUT
IN
OUT
. Tantalum and aluminum capacitors are not
OUT
IN
V
V
OUT
IN
= 3.6V, I
U
directly controls the amount of output
•
2
1
(Refer to Simplified Block Diagram)
•
I
I
OUT
OUT
OUT
OUT
= 100mA and V
2
can significantly affect the
V
V
OUT
IN
OUT
regulat-
OUT
0.15
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
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
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
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 1 F or
greater ceramic capacitor be used for C
Capacitor Selection Guidelines” section). Aluminum and
tantalum capacitors are not recommended because of
their high ESR.
IN
Capacitor Selection
LTC3250-1.5/LTC3250-1.2
for the LTC3250-1.5 and 0.12
Figure 1. 10nH Inductor Used for
Additional Output Noise Reduction
LTC3250-1.5/
LTC3250-1.2
V
GND
OUT
(TRACE INDUCTANCE)
4.7 F
10nH
0.22 F
V
3250 F01
OUT
IN
OUT
(see “Ceramic
/2 to 0mA.
for the
3250fa
7