10

LT1513/LT1513-2

sn1513 1513fas

Programmed Charging Current

LT1513-2 charging current can be programmed with a DC

voltage source or equivalent PWM signal, as shown in

voltage across R4 in the ratio R4/R5.

Charging current is given by:

I

VR

R

I

R

CHARGE

ISET

FBVOS

=

()(

/

)–

45

3

wide range of program currents. The voltage across R3 at

maximum charge current can be increased to reduce

+37/– 62mA. C4 and R4 smooth the switch current wave-

form. During constant-current operation, the voltage feed-

exceed 60

µA to maintain a sharp constant voltage to

also be controlled by a PWM input. Assuming the signal is

a CMOS rail-to-rail output with a source impedance of less

entire 0% to 100% range.

Voltage Mode Loop Stability

The LT1513 operates in constant-voltage mode during the

final phase of charging lithium-ion and lead-acid batteries.

This feedback loop is stabilized with a series resistor and

simplified model for the voltage loop. The error amplifier is

Figure 6. Constant-Voltage Small-Signal Model

1M

1500

µmho

MODULATOR SECTION

=

V1

V1

FB

R1*

71.5k

≈0.15Ω

EACH

0.1

Ω

C1

C1

BATTERY

1513 F06

C1

22

µF

EACH

R2

12.5k

1.245V

EA

330k

R5

330

Ω

* FOR 8.4V BATTERY. ADJUST VALUE OF R1 FOR ACTUAL BATTERY VOLTAGE

C5

0.1

µF

3pF

+

+

THIS IS A SIMPLIFIED AC MODEL FOR THE LT1513 IN CONSTANT-

VOLTAGE MODE. RESISTOR AND CAPACITOR NUMBERS

THE PHASE DELAY IN THE MODULATOR. C3 IS 3pF FOR A 10

µH

INDUCTOR. IT SHOULD BE SCALED PROPORTIONALLY FOR OTHER

INDUCTOR VALUES (6pF FOR 20

µH). THE MODULATOR IS A

TRANSCONDUCTANCE WHOSE GAIN IS A FUNCTION OF INPUT AND

BATTERY VOLTAGE AS SHOWN.

AS SHOWN, THIS LOOP HAS A UNITY-GAIN FREQUENCY OF

ABOUT 250Hz. UNITY-GAIN WILL MOVE OUT TO SEVERAL

KILOHERTZ IF BATTERY RESISTANCE INCREASES TO SEVERAL

OHMS. R5 IS NOT USED IN ALL APPLICATIONS, BUT IT GIVES

BETTER PHASE MARGIN IN CONSTANT-VOLTAGE MODE WITH

HIGH BATTERY RESISTANCE.

Figure 5

C4

0.1

µF

R3

0.2

Ω

1513 F05

L1B

R5

249k

R4

10k

LT1513-2