LT1512C LINER [Linear Technology], LT1512C Datasheet - Page 8

• Manufacturer Part Number: LT1512C
• Description: SEPIC Constant-Current/ Constant-Voltage Battery Charger
• Manufacturer: LINER [Linear Technology]

LT1512
APPLICATIONS
Output Capacitor
It is assumed as a worst case that all the switching output
ripple current from the battery charger could flow in the
output capacitor. This is a desirable situation if it is neces-
sary to have very low switching ripple current in the battery
itself. Ferrite beads or line chokes are often inserted in series
with the battery leads to eliminate high frequency currents
that could create EMI problems. This forces all the ripple
current into the output capacitor. Total RMS current into the
capacitor has a maximum value of about 0.5A, and this is
handled with a 22 F, 25V capacitor shown in Figure 1. This is
an AVX type TPS or Sprague type 593D surface mount solid
tantalum unit intended for switching applications. Do not
substitute other types without ensuring that they have
adequate ripple current ratings. See Input Capacitor section
for details of surge limitation on solid tantalum capacitors if
the battery may be “hot switched” to the output of the
charger.
Coupling Capacitor
C2 in Figure 1 is the coupling capacitor that allows a SEPIC
converter topology to work with input voltages either higher
or lower than the battery voltage. DC bias on the capacitor is
8
a. Double-Sided (Vias Connect to the Backside of Ground Plane.
Dash Lines Indicate Interconnects on Backside. Demo Board
Uses This Layout, Except that R5 Has Been Added to Increase
Phase Margin)
GND
S/S
V
IN
R5
L1A
C5
2
1
U
2 WINDING
INDUCTOR
C3
U1
INFORMATION
Figure 4. LT1512 Suggested Layouts for Critical Thermal and Electrical Paths
U
C2B
4
3
L1B
C2A
W
R4
R3
C1
D1
C4 R2
R1
U
1512 F04a
V
GND
BATT
V
equal to input voltage. RMS ripple current in the coupling
capacitor has a maximum value of about 0.5A at full charg-
ing current. A conservative formula to calculate this is:
(1.1 is a fudge factor to account for inductor ripple current
and other losses)
With I
The recommended capacitor is a 2.2 F ceramic type from
Marcon or Tokin. These capacitors have extremely low ESR
and high ripple current ratings in a small package. Solid
tantalum units can be substituted if their ripple current rating
is adequate, but typical values will increase to 22 F or more
to meet the ripple current requirements.
Diode Selection
The switching diode should be a Schottky type to minimize
both forward and reverse recovery losses. Average diode
current is the same as output charging current , so this will
be under 1A. A 1A diode is recommended for most applica-
tions, although smaller devices could be used at reduced
BATT
I
COUP RMS
CHRG
(
R1
= 0.5A, V
R2
)
D1
b. Single-Sided Altenative Layout
+
R5
C1
I
CHRG IN
C2
C5
IN
= 15V and V
(
V
4
1
2
(
V
IN
GND
V
)
BAT
L1B
L1A
BAT
+V
C4
IN
)( . )
C3
= 8.2V, I
1 1
R3
2
3
+
R4
R4
COUP
1512 F04b
S/S
= 0.43A