I am wondering, if there is any charging only mode in LTC3553? I have gone through its datasheet but it is not clearly mentioned anywhere that what will be the status of battery charger and VOUT pin in different power modes.
Besides, it is mentioned that HPWR and SUSP pins have "conditional" internal pull-down resistors, what does it mean. Can we left these pins floating if we want a low level on this pin? Also, should we use an external pull-down resistor if we want to control the pin through microcontroller?
Moreover, as mentioned in the datasheet, a 100uF with 0.3 ohms resistor should be connected to the BAT terminal if there is no battery. In the case when the battery can be connected and disconnected, placing the capacitor and resistor with BAT terminal even in presence of battery will affect the chip performance? Also, with the 100 uF capacitor with 0.3 ohm resistor connected to the BAT pin and with battery disconnected, how will the CHRG pin behave when the VUSB pin will be connected? Will it indicate charging or not?
When the SUSP pin is pulled high the LTC3553 enters suspend mode to comply with the USB specification. In this mode, the power path between VBUS and VOUT is put in a high impedance state to reduce the VBUS input current to 15μA. The system load connected to VOUT is supplied through the ideal diode connected to BAT. The charger is disabled in SUSP mode. When in Standby Mode, the buck and LDO regulator quiescent current is reduced to very low levels, while still maintaining output voltage regulation. In this mode, the buck regulator is limited to 10mA maximum load current, and the LDO regulator’s response to line and load transients is slower. This does not affect the VOUT voltage or charger status.
The HPWR and SUSP pins do have a 4MΩ resistor and can float or be driven low by a microprocessor. An external resistor is not needed but will provide extra protection from any leakage currents if any develop on the application board over time or in the micro if not powered with voltages applied to GPIO pins.
The 100uF-0.3Ω resistor in parallel with the battery will not affect the operation of the charger because the battery impedance will dominate over the RC. When the battery is not present and the NTC resistor still connected or pulled low then the capacitor will quickly charge up to float. In float the /CHRG indicator goes to high impedance and the 4 hour safety timer starts. When the timer has elapsed then the 100uF cap discharges to the recharge threshold and then the cycle repeats.
If the NTC pin is pulled to VBUS then the charger will not operate and the charge indicator indicates a charger fault.
Thank you very much for your detailed response.
Wondering how easier it would have been if the NTC functionality could be disabled by pulling the NTC pin to VBUS and not to GND. Connecting batteries with and without NTC feature would have been so smooth. Please consider this when designing the new chips. Thank you.