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LT1512I Datasheet

  • LT1512I

  • Linear Technology [SEPIC Constant-Current/ Constant-Voltage...

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LT1512
APPLICATIONS INFORMATION
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碌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 鈥渉ot switched鈥?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
V
IN
1
4
R4
R1
L1A
2 WINDING
INDUCTOR
L1B
R3
C4 R2
2
3
C3
C2B C2A
S/S
D1
R5
C5
V
C
V
SW
I
FB
GND S
GND
GND
GND
1512 F04a
R1 R2
FB
S/S
R5
V
IN
U1
C1
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)
Figure 4. LT1512 Suggested Layouts for Critical Thermal and Electrical Paths
8
U
W
U
U
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:
I
(V
+
V )(1.1)
I
COUP(RMS)
=
CHRG IN BAT
2(V
IN
)
(1.1 is a fudge factor to account for inductor ripple current
and other losses)
With I
CHRG
= 0.5A, V
IN
= 15V and V
BAT
= 8.2V, I
COUP
= 0.43A
The recommended capacitor is a 2.2碌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碌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
GND
+V
IN
4
L1B
3
D1
C2
1
L1A
2
V
BATT
+
V
BATT
C1
C3
+
C5
R3
C4
R4
S/S
1512 F04b
b. Single-Sided Altenative Layout

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