Datasheet

LTC3388-1/LTC3388-3
15
338813f
If transient load currents above 50mA 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 of at least
22H. This value represents a suitable trade-off between
size and efficiency for typical applications. A larger induc-
tor 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
APPLICATIONS INFORMATION
current rating greater than 200mA. The DCR of the induc-
tor can have an impact on efficiency as it is a source of
loss. Trade-offs between price, size, and DCR should be
evaluated. Table 2 lists several inductors that work well
with the LTC3388-1/LTC3388-3.
Table 2. Recommended Inductors for LTC3388-1/LTC3388-3
INDUCTOR TYPE
L
(μH)
MAX
I
DC
(mA)
MAX
DCR
(Ω)
SIZE in mm
(L × W × H)
MANU-
FACTURER
CDRH2D18/LDNP 22 300 0.320
3.2 × 3.2 × 2.0
Sumida
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 × 4.5
TDK
V
IN2
and CAP Capacitors
A 1F capacitor should be connected between V
IN
and
CAP and a 4.7F 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.
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.
338813 F06
EN
STBY
V
IN
V
IN2
CAP
D1
D0
EN
STBY
PGOOD
SW
V
OUT
PGOOD
LTC3388-1
GND
2.2µF
6V
2.7V TO 5.5V
10µF
6V
V
OUT
1.2V
22µH
Figure 6. Smallest Solution Size 1.2V
Low Input Voltage Power Supply