Datasheet LT1512 (Analog Devices) - 8

LT1512
applicaTions inForMaTion
CONNECT D2 ANODE HERE FOR FULLY CONNECT D2 ANODE HERE IF FULLY CHARGED CHARGED BATTERY VOLTAGE LESS BATTERY VOLTAGE IS GREATER THAN 3.5V AND THAN 3.5V. Q1 WILL NOT BE TURNED OFF Q1 MUST BE TURNED OFF IN SHUTDOWN WITH IN SHUTDOWN IF VIN IS PRESENT VIN STILL ACTIVE L1 A • D2 C6 R5 1N4148 470pF 470k C2 D1 VIN VSW R1 + LT1512 L1 B SHUTDOWN S/S Q1 2N7002 GND FB R3 R2 1512 F03
Figure 3. Eliminating Divider Current
adequate drive to the transistor (even when the VIN voltage
Shutdown and Synchronization
is at its lowest operating point of 2.4V), the FET gate is The dual function S/S pin provides easy shutdown and driven wth a peak detected voltage via D2. Note that there synchronization. It is logic level compatible and can be are two connections for D2. The L1 A connection must be pulled high or left floating for normal operation. A logic used if the voltage divider is set for less than 3.5V (fully low on the S/S pin activates shutdown, reducing input charged battery). Gate drive is equal to battery voltage supply current to 12µA. To synchronize switching, drive plus input voltage. The disadvantage of this connection is the S/S pin between 600kHz and 800kHz. that Q1 will still be “on” if the VIN voltage is active and the charger is shut down via the S/S pin. The L1 B connection
Inductor Selection
allows Q1 to turn off when VIN is off or when shutdown is initiated, but the reduced gate drive (= V L1A and L1B are normally just two identical windings on BAT) is not adequate to ensure a Q1 on-state for fully charged battery voltages one core, although two separate inductors can be used. less than 3.5V. Do not substitute for Q1 unless the new A typical value is 33µH, which gives about 0.25A peak-to- device has adequate V peak inductor current. Lower values will give higher ripple GS maximum rating, especially if D2 is connected to L1A. C6 filters the gate drive and R5 current, which reduces maximum charging current. 15µH pulls the gate low when switching stops. can be used if charging currents are at least 20% lower than the values shown in the maximum charging current graph. Disconnecting the divider leaves only D1 diode leakage Higher inductance values give slightly higher maximum as a battery drain. See Diode Selection for a discussion charging current, but are larger and more expensive. A of diode leakage. low loss toroid core such as KoolMµ, Molypermalloy or Metglas is recommended. Series resistance should be
Maximum Input Voltage
less than 0.1Ω for each winding. “Open core” inductors, Maximum input voltage for the circuit in Figure 1 is partly such as rods or barrels are not recommended because determined by battery voltage. A SEPIC converter has a they generate large magnetic fields which may interfere maximum switch voltage equal to input voltage plus out- with other electronics close to the charger. put voltage. The LT1512 has a maximum input voltage of 30V and a maximum switch voltage of 40V, so this limits
Input Capacitor
maximum input voltage to 30V, or 40V – VBAT, whichever The SEPIC topology has relatively low input ripple current is less. Maximum VBAT = 40V – VIN. compared to other topologies and higher harmonics are 1512fc 8 For more information www.linear.com/LT1512 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