Datasheet

LTC3588-1
13
35881fa
APPLICATIONS INFORMATION
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 ±12mV 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
= V
OUT+
− V
OUT–
()
I
LOAD
−I
BUCK
t
LOAD
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 an inductor in the range
of 10µH to 22µH, although inductor values outside this
range may yield benefi ts in some applications. For typical
applications, a value of 10µH is recommended. A larger
inductor will benefi t high voltage applications by increasing
the on-time of the PMOS switch and improving effi ciency
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 effi ciency as it is a source
of loss. Tradeoffs between price, size, and DCR should be
evaluated. Table 3 lists several inductors that work well
with the LTC3588-1.
Table 3. Recommended Inductors for LTC3588-1
INDUCTOR
TYPE
L
(μH)
MAX
I
DC
(mA)
MAX
DCR
(Ω)
SIZE in mm
(L × W × H)
MANU-
FACTURER
CDRH2D18/LDNP 10 430 0.180
3 × 3 × 2
Sumida
107AS-100M 10 650 0.145
2.8 × 3 × 1.8
Toko
EPL3015-103ML 10 350 0.301
2.8 × 3 × 1.5
Coilcraft
MLP3225s100L 10 1000 0.130
3.2 × 2.5 × 1.0
TDK
XLP2010-163ML 10 490 0.611
2.0 × 1.9 × 1.0
Coilcraft
SLF7045T 100 500 0.250
7.0 × 7.0 × 4.5
TDK
V
IN2
and CAP Capacitors
A 1F 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. In applications where the input source
is limited to less than 6V, the CAP pin can be tied to GND
and the V
IN2
pin can be tied to V
IN
as shown in Figure 6.
An optional 5.6V Zener diode can be connected to V
IN
to
clamp V
IN
in this scenario. The leakage of the Zener diode
below its Zener voltage should be considered as it may
be comparable to the quiescent current of the LTC3588-1.
This circuit does not require the capacitors on V
IN2
and
CAP, saving components and allowing a lower voltage
rating for the single V
IN
capacitor.
Figure 6. Smallest Solution Size 1.8V Low Voltage Input
Piezoelectric Power Supply
35881 F06
PZ1
V
IN
V
IN2
CAP
D1
D0
PZ2
PGOOD
SW
V
OUT
LTC3588-1
GND
10µF
6V
10µH
V
OUT
1.8V
PGOOD
10µF
6V
5.6V
(OPTIONAL)
MIDE V21BL