LT1513/LT1513-2
U
W U U
APPLICATIONS INFORMATION
Programmed Charging Current
exceed 60µA to maintain a sharp constant voltage to
constant current crossover characteristic. ICHARGE can
also be controlled by a PWM input. Assuming the signal is
aCMOSrail-to-railoutputwithasourceimpedanceofless
than a few hundred ohms, effective ISET is VCC multiplied
by the PWM ratio. ICHARGE has good linearity over the
entire 0% to 100% range.
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, IFB acts as a virtual
ground. The ISET voltage across R5 is balanced by the
voltage across R4 in the ratio R4/R5.
Charging current is given by:
Voltage Mode Loop Stability
(V )(R4/R5)– I
ISET
FBVOS
I
=
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 VC pin of the chip. Figure 6 shows the
simplified model for the voltage loop. The error amplifier is
modeled as a transconductance stage with gm = 1500µmho
CHARGE
R3
IFB input current is small and can normally be ignored, but
IFB 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, ISET
from 0V to 5V corresponds to an ICHARGE of 0A to 1A
+37/–62mA. C4 and R4 smooth the switch current wave-
form.Duringconstant-currentoperation,thevoltagefeed-
back network loads the FB pin, which is held at VREF by the
IFB amplifier. It is recommended that this load does not
LT1513-2
I
FB
L1B
R5
249k
R4
10k
I
SET
C4
0.1µF
R3
0.2Ω
1513 F05
Figure 5
MODULATOR SECTION
I
P
I
P
4(V
IN
)
V1
g
=
=
m
V1
V + V
IN BAT
C **
P
3pF
V
V
= DC INPUT VOLTAGE
BAT
R1*
71.5k
IN
= DC BATTERY VOLTAGE
C1
C1
R
R **
P
1M
R
BAT
CAP
FB
0.1Ω
≈0.15Ω
–
EACH
V
C
+
+
C1
22µF
EACH
BATTERY
EA
+
R5
R2
12.5k
g
R
G
330k
m
330Ω
1500µmho
1.245V
C5
0.1µF
1513 F06
*
**
FOR 8.4V BATTERY. ADJUST VALUE OF R1 FOR ACTUAL BATTERY VOLTAGE
AND C MODEL PHASE DELAY IN THE MODULATOR
R
P
P
THIS IS A SIMPLIFIED AC MODEL FOR THE LT1513 IN CONSTANT-
VOLTAGE MODE. RESISTOR AND CAPACITOR NUMBERS
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.
CORRESPOND TO THOSE USED IN FIGURE 1. R AND C MODEL
P
P
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.
Figure 6. Constant-Voltage Small-Signal Model
sn1513 1513fas
10
Linear Systems [ Linear Systems ]
