Datasheet LT1512 (Analog Devices) - 5

LT1512
pin FuncTions VC:
The compensation pin is primarily used for frequency
S/S:
This pin can be used for shutdown and/or synchro- compensation, but it can also be used for soft starting and nization. It is logic level compatible, but can be tied to VIN current limiting. It is the output of the error amplifier and if desired. It defaults to a high ON state when floated. A the input of the current comparator. Peak switch current logic low state will shut down the charger to a micropower increases from 0A to 1.8A as the VC voltage varies from state. Driving the S/S pin with a continuous logic signal of 1V to 1.9V. Current out of the VC pin is about 200µA when 600kHz to 800kHz will synchronize switching frequency the pin is externally clamped below the internal 1.9V clamp to the external signal. Shutdown is avoided in this mode level. Loop frequency compensation is performed with a with an internal timer. capacitor or series RC network from the VC pin directly to
V
the ground pin (avoid ground loops).
IN:
The input supply pin should be bypassed with a low ESR capacitor located right next to the IC chip. The
FB:
The feedback pin is used for positive output voltage grounded end of the capacitor must be connected directly sensing. This pin is the inverting input to the voltage to the ground plane to which the GND pin is connected. error amplifier. The R1/R2 voltage divider connected to
GND S, GND:
The LT1512 uses separate ground pins for FB defines Li-Ion float voltage at full charge, or acts as a switch current (GND) and the control circuitry (GND S). voltage limiter for NiCd or NiMH applications. Input bias This isolates the control ground from any induced volt- current is typically 300nA, so divider current is normally age created by fast switch currents. Both pins should be set to 100µA to swamp out any output voltage errors due tied directly to the ground plane, but the external control to bias current. The noninverting input of this amplifier is circuit components such as the voltage divider, frequency tied internally to a 1.245V reference. The grounded end of compensation network and I the output voltage divider should be connected directly to FB bypass capacitor should be connected directly to the GND S pin or to the ground plane the LT1512 ground pin (avoid ground loops). close to the point where the GND S pin is connected.
IFB:
The current feedback pin is used to sense charging
V
current. It is the input to a current sense amplifier that
SW:
The switch pin is the collector of the power switch, carrying up to 1.5A of current with fast rise and fall times. controls charging current when the battery voltage is below Keep the traces on this pin as short as possible to mini- the programmed voltage. During constant-current opera- mize radiation and voltage spikes. In particular, the path tion, the IFB pin regulates at –100mV. Input resistance of in Figure 1 which includes Sw to C2, D1, C1 and around this pin is 5kΩ, so filter resistance (R4, Figure 1) should be to the LT1512 ground pin should be as short as possible less than 50Ω. The 24Ω, 0.22µF filter shown in Figure 1 is to minimize voltage spikes at switch turn-off. used to convert the pulsating current in the sense resistor to a smooth DC current feedback signal. 1512fc For more information www.linear.com/LT1512 5 Document Outline Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Related Parts