LTC3588-2
12
35882fc
For more information www.linear.com/LTC3588-2
APPLICATIONS INFORMATION
effect during this time. For applications where the output
must reach regulation on a single UVLO cycle, the energy
required to charge the output capacitor must be taken into
account when sizing C
IN
.
The duration for which the regulator sleeps depends on
the load current and the size of the output capacitor. The
sleep time decreases as the load current increases and/or
as the output capacitor decreases. The DC sleep hysteresis
window is ±16mV around the programmed output volt-
age. Ideally this means that the sleep time is determined
by the following equation:
t
SLEEP
= C
OUT
I
LOAD
This is true for output capacitors on the order of 100µF
or larger, but as the output capacitor decreases towards
10µF delays in the internal sleep comparator along with
the load current may result in the V
OUT
voltage slewing
past the ±16mV thresholds. This will lengthen the sleep
time and increase V
OUT
ripple. A capacitor less than 10µF
is not recommended as V
OUT
ripple could increase to an
undesirable level.
If transient load currents above 100mA are required then a
larger capacitor can be used at the output. This capacitor
will be continuously discharged during a load condition and
the capacitor can be sized for an acceptable drop in V
OUT
:
C
OUT
= (I
LOAD
– I
BUCK
)
LOAD
V
+
– V
–
Here V
OUT
+
is the value of V
OUT
when PGOOD goes high
and V
OUT
–
is the desired lower limit of V
OUT
. I
BUCK
is the
average current being delivered from the buck converter,
typically I
PEAK
/2.
A standard surface mount ceramic capacitor can be used
for C
OUT
, though some applications may be better suited
to a low leakage aluminum electrolytic capacitor or a
supercapacitor. These capacitors can be obtained from
manufacturers such as Vishay, Illinois Capacitor, AVX,
or CAP-XX.
Inductor
The buck is optimized to work with a 22µH inductor. Induc-
tor values greater than 22µH may yield benefits in some
applications. For example, a larger inductor will benefit
high voltage applications by increasing the on-time of the
PMOS switch and improving efficiency by reducing gate
charge loss. Choose an inductor with a DC current rating
greater than 350mA. The DCR of the inductor can have
an impact on efficiency as it is a source of loss. Trade-offs
between price, size, and DCR should be evaluated. Table 3
lists several inductors that work well with the LTC3588-2.
Table 3. Recommended Inductors for LTC3588-2
INDUCTOR
TYPE
L
(µH)
MAX
I
DC
(mA)
MAX
DCR
(Ω)
SIZE in mm
(L × W × H)
MANU-
FACTURER
A997AS-220M 22 390 0.440 4.0 × 4.0 × 1.8 Toko
LPS5030-223MLC 22 700 0.190 4.9 × 4.9 × 3.0 Coilcraft
LPS4012-473MLC 47 350 1.400 4.0 × 4.0 × 1.2 Coilcraft
SLF7045T 100 500 0.250 7.0 × 7.0 × 4.8 TDK
V
IN2
and CAP Capacitors
A 1μF capacitor should be connected between V
IN
and
CAP and a 4.7µF capacitor should be connected between
V
IN2
and GND. These capacitors hold up the internal rails
during buck switching and compensate the internal rail
generation circuits.
Additional Applications with Piezo Inputs
The versatile
LTC3588-2 can be used in a variety of con-
figurations. Figure 6 shows a single piezo source powering
two LTC3588-2s simultaneously, providing capability for
multiple rail systems. As the piezo provides input power
both V
IN
rails will initially come up together, but when
one output starts drawing power, only its corresponding
V
IN
will fall as the bridges of each LTC3588-2 provide
isolation. Input piezo energy will then be directed to this
lower voltage capacitor until both V
IN
rails are again
equal. This configuration is expandable to any number
of LTC3588-2s powered by a single piezo as long as the
piezo can support the sum total of the quiescent currents
from each LTC3588-2.
