LT1513/LT1513-2
APPLICATIONS INFORMATION
Programmed Charging Current
LT1513-2 charging current can be programmed with a DC
voltage source or equivalent PWM signal, as shown in
Figure 5. In constant-current mode, I
FB
acts as a virtual
ground. The I
SET
voltage across R5 is balanced by the
voltage across R4 in the ratio R4/R5.
Charging current is given by:
I
CHARGE
=
( V
ISET
)( R4 / R5 )– I
FBVOS
R3
I
FB
input current is small and can normally be ignored, but
I
FB
offset voltage must be considered if operating over a
wide range of program currents. The voltage across R3 at
maximum charge current can be increased to reduce
offset errors at lower charge currents. In Figure 5, I
SET
from 0V to 5V corresponds to an I
CHARGE
of 0A to 1A
+37/– 62mA. C4 and R4 smooth the switch current wave-
form. During constant-current operation, the voltage feed-
back network loads the FB pin, which is held at V
REF
by the
I
FB
amplifier. It is recommended that this load does not
MODULATOR SECTION
V1
C
P
**
3pF
R
P
**
1M
V
C
EA
R5
330Ω
C5
0.1µF
R
G
330k
g
m
1500µmho
I
4(V
IN
)
g
m
=
P
=
V1 V
IN
+ V
BAT
V
IN
= DC INPUT VOLTAGE
V
BAT
= DC BATTERY VOLTAGE
FB
I
P
R1*
71.5k
* FOR 8.4V BATTERY. ADJUST VALUE OF R1 FOR ACTUAL BATTERY VOLTAGE
** R
P
AND C
P
MODEL PHASE DELAY IN THE MODULATOR
THIS IS A SIMPLIFIED AC MODEL FOR THE LT1513 IN CONSTANT-
VOLTAGE MODE. RESISTOR AND CAPACITOR NUMBERS
CORRESPOND TO THOSE USED IN FIGURE 1. R
P
AND C
P
MODEL
THE PHASE DELAY IN THE MODULATOR. C3 IS 3pF FOR A 10µH
INDUCTOR. IT SHOULD BE SCALED PROPORTIONALLY FOR OTHER
INDUCTOR VALUES (6pF FOR 20µH). 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 6. Constant-Voltage Small-Signal Model
10
U
–
+
W
U
U
exceed 60µA to maintain a sharp constant voltage to
constant current crossover characteristic. I
CHARGE
can
also be controlled by a PWM input. Assuming the signal is
a CMOS rail-to-rail output with a source impedance of less
than a few hundred ohms, effective I
SET
is V
CC
multiplied
by the PWM ratio. I
CHARGE
has good linearity over the
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
capacitor on the V
C
pin of the chip. Figure 6 shows the
simplified model for the voltage loop. The error amplifier is
modeled as a transconductance stage with g
m
= 1500µmho
LT1513-2
I
FB
R5
249k
I
SET
C4
0.1µF
R3
0.2Ω
1513 F05
R4
10k
L1B
Figure 5
C1
C1
R
CAP
≈0.15Ω
EACH
C1
22µF
EACH
R
BAT
0.1Ω
BATTERY
+
R2
12.5k
+
1.245V
1513 F06
LINER [ LINEAR TECHNOLOGY ]
