LT1512
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Peak-to-Peak
Synchronization Voltage vs Temp
MINIMUM SYNCHRONIZATION VOLTAGE (V
P-P
)
f
SYNC
= 700kHz
FEEDBACK INPUT CURRENT (nA)
2.5
2.0
1.5
1.0
0.5
0
鈥?0 鈥?5
700
600
500
400
300
200
100
V
FB
= V
REF
NEGATIVE FEEDBACK INPUT CURRENT (碌A(chǔ))
3.0
0
25 50 75 100 125 150
TEMPERATURE (擄C)
1512 G04
PIN FUNCTIONS
V
C
:
The compensation pin is primarily used for frequency
compensation, but it can also be used for soft starting and
current limiting. It is the output of the error amplifier and
the input of the current comparator. Peak switch current
increases from 0A to 1.8A as the V
C
voltage varies from 1V
to 1.9V. Current out of the V
C
pin is about 200碌A(chǔ) when the
pin is externally clamped below the internal 1.9V clamp
level. Loop frequency compensation is performed with a
capacitor or series RC network from the V
C
pin
directly to
the ground pin
(avoid ground loops).
FB:
The feedback pin is used for positive output voltage
sensing. This pin is the inverting input to the voltage error
amplifier. The R1/R2 voltage divider connected to FB
defines Li-Ion float voltage at full charge, or acts as a
voltage limiter for NiCd or NiMH applications. Input bias
current is typically 300nA, so divider current is normally
set to 100碌A(chǔ) to swamp out any output voltage errors due
to bias current. The noninverting input of this amplifier is
tied internally to a 1.245V reference. The grounded end of
the output voltage divider should be connected directly to
the LT1512 ground pin (avoid ground loops).
I
FB
:
The current feedback pin is used to sense charging
current. It is the input to a current sense amplifier that
controls charging current when the battery voltage is
below the programmed voltage. During constant-current
operation, the I
FB
pin regulates at 鈥?100mV. Input resis-
tance of this pin is 5k鈩? so filter resistance (R4, Figure 1)
should be less than 50鈩? The 24鈩? 0.22碌F filter shown in
Figure 1 is used to convert the pulsating current in the
sense resistor to a smooth DC current feedback signal.
S/S:
This pin can be used for shutdown and/or synchroni-
zation. It is logic level compatible, but can be tied to V
IN
if
desired. It defaults to a high ON state when floated. A logic
low state will shut down the charger to a micropower state.
Driving the S/S pin with a continuous logic signal of
600kHz to 800kHz will synchronize switching frequency to
the external signal. Shutdown is avoided in this mode with
an internal timer.
V
IN
:
The input supply pin should be bypassed with a low
ESR capacitor located right next to the IC chip. The
grounded end of the capacitor must be connected directly
to the ground plane to which the GND pin is connected.
GND S, GND:
The LT1512 uses separate ground pins for
switch current (GND) and the control circuitry (GND S).
This isolates the control ground from any induced voltage
created by fast switch currents. Both pins should be tied
directly to the ground plane, but the external control
circuit components such as the voltage divider, frequency
compensation network and I
FB
bypass capacitor should
4
U W
Feedback Input Current
vs Temperature
800
0
Negative Feedback Input Current
vs Temperature
鈥?0
鈥?0
鈥?0
鈥?0
0
鈥?0 鈥?5
0
25 50 75 100 125 150
TEMPERATURE (擄C)
1512 G05
鈥?0
鈥?0 鈥?5
0
25 50 75 100 125 150
TEMPERATURE (擄C)
1512 G06
U
U
U
1
2
3
4
5
6
7
8
9
10
11
12
