Descripción

The LTC4001 is a 2A Li-Ion battery charger intended for 5V wall adapters. It utilizes a 1.5MHz synchronous buck converter topology to reduce power dissipation during charging. Low power dissipation, an internal MOSFET and sense resistor allow a physically small charger that can be embedded in a wide range of handheld applications. The LTC4001 includes complete charge termination circuitry, automatic recharge and a ±1% 4.2V float voltage. Input short-circuit protection is included so no blocking diode is required.

Battery charge current, charge timeout and end-of-charge indication parameters are set with external components. Additional features include shorted cell detection, temperature qualified charging and overvoltage protection. The LTC4001 is available in a low profile (0.75mm) 16-lead (4mm × 4mm) QFN package.

 

CARACTERÍSTICAS

Low Power Dissipation

2A Maximum Charge Current

No External MOSFETs, Sense Resistor or Blocking Diode Required

Remote Sensing at Battery Terminals

Programmable Charge Termination Timer

Preset 4.2V Float Voltage with ±0.5% Accuracy

Programmable Charge Current Detection/Termination

Automatic Recharge

Thermistor Input for Temperature Qualified Charging

Compatible with Current Limited Wall Adapters

Low Profile 16-Lead (4mm × 4mm) QFN Package

 

Aplicaciones

Handheld Battery-Powered Devices

Handheld Computers

Charging Docks and Cradles

Digital Cameras

Smart Phones

 

Operación

The LTC4001 is a constant current, constant voltage Li-Ion battery charger based on a synchronous buck architecture. Low power dissipation makes continuous high rate (2A) battery charging practical. The battery DC charge current is programmed by a resistor RPROG (or a DAC output current) at the PROG pin. The final battery float voltage is internally set to 4.2V.

Overvoltage, Chip Overtemperature and Short-Circuit Current Protection

The LTC4001 includes overvoltage, chip overtemperature and several varieties of short-circuit protection.

A comparator turns off both chargers (high rate and trickle) if battery voltage exceeds the float voltage by approximately 5%. This may occur in situations where the battery is accidentally disconnected while battery charging is underway.

A comparator continuously monitors on-chip temperature and will shut off the battery charger when chip temperature exceeds approximately 160°C.  Battery charging will be enabled again when temperature drops to approximately 150°C.

Short-circuit protection is provided in several different ways. First, a hard short on the battery terminals will cause the charge to enter trickle charge mode, limiting charge current to the trickle charge current (typically 50mA). Second, PWM charging is prevented if the high rate charge current is programmed far above the 2A maximum recommended charge current (via the PROG pin). Third, an overcurrent comparator monitors the peak inductor current.

 

INFORMACIÓN SOBRE APLICACIONES

Soft-Start and Compensation Capacitor Selection

The LTC4001 has a low current trickle charger and a PWM-based high current charger. Soft-start is used whenever the high rate charger is initially turned on, preventing high start-up current. Soft-start ramp rate is set by the internal 12.8µA pull-up current and an external capacitor. The control range on the SS pin is approximately 0.3V to 1.6V. With a 0.1µF capacitor, the time to ramp up to maximum duty cycle is approximately 10ms.

The external capacitor on the SS pin also sets the compensation for the current control loop and the float voltage control loop. A minimum capacitance of 10nF is required.

 

Input and Output Capacitors

The LTC4001 uses a synchronous buck regulator to provide high battery charging current. A 10µF chip ceramic capacitor is recommended for both the input and output capacitors because it provides low ESR and ESL and can handle the high RMS ripple currents. However, some high Q capacitors may produce high transients due to self resonance under some start-up conditions, such as connecting the charger input to a hot power source. For more information, refer to Application Note 88.

EMI considerations usually make it desirable to minimize ripple current in the battery leads, and beads or inductors may be added to increase battery impedance at the 1.5MHz switching frequency. Switching ripple current splits between the battery and the output capacitor depending on the ESR of the output capacitor and the battery impedance. If the ESR of the output capacitor is 0.1Ω and the battery impedance is raised to 2Ω with a bead or inductor, only 5% of the ripple current will flow in the battery. Similar techniques may also be applied to minimize EMI from the input leads.