7

LT1512

LT1512

L1 A

L1 B

GND

FB

1512 F03

SHUTDOWN

D2

1N4148

CONNECT D2 ANODE HERE IF FULLY CHARGED

BATTERY VOLTAGE IS GREATER THAN 3.5V AND

Q1 MUST BE TURNED OFF IN SHUTDOWN WITH

CONNECT D2 ANODE HERE FOR FULLY

CHARGED BATTERY VOLTAGE LESS

THAN 3.5V. Q1 WILL NOT BE TURNED OFF

S/S

C2

C6

470pF

•

R1

R5

470k

R2

R3

Q1

2N7002

+

D1

Figure 3. Eliminating Divider Current

Maximum Input Voltage

Maximum input voltage for the circuit in Figure 1 is partly

determined by battery voltage. A SEPIC converter has a

maximum switch voltage equal to input voltage plus output

voltage. The LT1512 has a maximum input voltage of 30V

and a maximum switch voltage of 40V, so this limits

Shutdown and Synchronization

The dual function S/S pin provides easy shutdown and

synchronization. It is logic level compatible and can be

pulled high or left floating for normal operation. A logic low

on the S/S pin activates shutdown, reducing input supply

current to 12

µA.Tosynchronizeswitching,drivetheS/Spin

between 600kHz and 800kHz.

Inductor Selection

L1A and L1B are normally just two identical windings on one

core, although two separate inductors can be used. A typical

value is 33

µH,whichgivesabout0.25Apeak-to-peakinduc-

tor current. Lower values will give higher ripple current,

which reduces maximum charging current. 15

µH can be

used if charging currents are at least 20% lower than the

values shown in the maximum charging current graph.

Higher inductance values give slightly higher maximum

charging current, but are larger and more expensive. A low

losstoroidcoresuchasKoolM

is recommended. Series resistance should be less than

0.1

Ωforeachwinding.“Opencore”inductors,suchasrods

or barrels are not recommended because they generate

large magnetic fields which may interfere with other elec-

tronics close to the charger.

Input Capacitor

The SEPIC topology has relatively low input ripple current

compared to other topologies and higher harmonics are

especially low. RMS ripple current in the input capacitor is

less than 0.1A with L = 33

µH and less than 0.2A with

L = 15

µH. A low ESR 22µF, 25V solid tantalum capacitor

(AVX type TPS or Sprague type 593D) is adequate for most

applications with the following caveat. Solid tantalum ca-

pacitors 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 possible voltage rating, at least twice the surge input

voltage if possible. Consult with the capacitor manufacturer

before a final choice is made. A 2.2

µFceramiccapacitorsuch

as the one used for the coupling capacitor can also be used.

These capacitors do not have a turn-on surge limitation. The

the ground plane close to the LT1512.

KoolM

µ is a registered trademark of Magnetics, Inc.

Metglas is a registered trademark of AlliedSignal Inc.