Datasheet
LT1512
9
1512fb
For more information www.linear.com/LT1512
applicaTions inForMaTion
especially low. RMS ripple current in the input capacitor is
less than 0.1A with L = 33µH and less than 0.2A with L =
15µH. A low ESR 22µF, 25V solid tantalum capacitor (AVX
type TPS or Sprague type 593D) is adequate for most appli
-
cations with the following caveat. Solid tantalum capacitors
can be destroyed with a very high turn-on surge current
such as would be generated if a low impedance input source
were “hot switched” to the charger input. If this condition
can occur, the input capacitor should have the highest pos
-
sible voltage rating, at least twice the surge input voltage if
possible. Consult with the capacitor manufacturer before
a
final choice is made. A 2.2µF ceramic capacitor such as
the one used for the coupling capacitor can also be used.
These capacitors do not have a turn-on surge limitation.
The input capacitor must be connected directly to the V
IN
pin and the ground plane close to the LT1512.
Output Capacitor
It is assumed as a worst case that all the switching out
-
put ripple current from the battery charger could flow in
the output capacitor. This is a desirable situation if it is
necessar
y 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 22µF, 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 equal to input voltage. RMS ripple current
in the coupling capacitor has a maximum value of about
0.5A at full charging current. A conservative formula to
calculate this is:
I
IVV
V
COUP RMS
CHRG IN BAT
IN
()
()
(.)
()
=
+ 11
2
(1.1 is a fudge factor to account for inductor ripple current
and other losses)
2 WINDING
INDUCTOR
L1A
1512 F04a
L1B
R4
R1
1 4
32
C4 R2
D1
V
IN
GND
GND
V
BATT
R3
C2AC2B
C3
C5
C1
U1
R5
S/S
1
2
3
4
V
C
FB
I
FB
S/S
V
SW
GND
GND S
V
IN
R5
C4
R3 R4
S/S
1512 F04b
C5
R2
C1
D1
C2
C3
R1
V
BATT
+ +
+V
IN
GND
L1B
L1A
R4
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)
b. Single-Sided Alternative Layout
Figure 4. LT1512 Suggested Layouts for Critical Thermal and Electrical Paths