Battery Charging

The electrical specification gives the idle voltages produced by the acceptable chargers at the DC connector input (CHRGR+). The absolute maximum input voltage is 30V, due to the transient suppressor that is protecting the charger input. At phone end there is no difference between a plug-in charger and a desktop charger because the DC-jack pins and bottom connector charging pads are connected internally in the System connector. The control-line of three terminal chargers is connected to GND on the PCB. Charger ground is also connected directly to GND on the PCB.

The purpose of the Capacitor on the CTIM pin is to reduce Output Current slew-rate of the CHAPS. This is to minimize switching noise in the Audio circuits. The CTIM-pin is also controlled by the MAD-signal CCUT. When CTIM is shorted (CCUT = T), the CHAPS stops charging. During detection of accessories the charging is stopped to prevent, that 'high' charge current will disturb the sensible A/D-measurement. Charging block diagram is shown in Figure overleaf.

Nokia3310 Battery Connector Pin Details

Figure 8: Charging Block Diagram

Start-up Charging

When a charger is connected, the CHAPS is supplying a startup current minimum of 130mA to the phone. The startup current provides initial charging to a phone with an empty battery. Startup circuit charges the battery until the battery voltage level reaches 3.0V (+/- 0.1V) and the CCONT releases the PURX reset signal and program execution starts. Charging mode is changed from startup charging to PWM charging that is controlled by the MCU software.

If the battery voltage reaches 3.55V (3.75V maximum) before the program has taken control over the charging, the startup current is switched off.

The startup current is switched on again when the battery voltage has dropped 100mV (nominal).

Table 8: Startup Characteristics

Parameter

Symbol

Min

Typ

Max

Unit

VOUT Start- up mode cutoff limit

Vstart

3.45

3.55

3.75

V

VOUT Start- up mode hysteresis NOTE: Cout 2 4.7 uF

Vstarthys

8O

100

200

mV

Start-up regulator output current VOUT 2 OV... Vstart

Istart

13O

165

200

mA

Battery over-voltage protection

The CHAPS includes an over-voltage protection circuit (input pin VBAT), which purpose is to protect the phone from damage caused by too high Battery voltage. Different cutoffvoltages (VLIM1 orVLIM2) for two different battery types (Li or Ni) are selected by the MAD via CHAPS-input pin LIM according to the table below.

Table 9: VLIM Characteristics

Parameter

Symbol

LIM input

Min

Typ

Max

Unit

Output voltage cutoff limit (during transmission or

Li-battery)

VLIM 1

LOW

4.4

4.6

4.8

V

Output voltage cutoff limit (Ni-battery)

VLIM2

High

4.8

5.0

5.2

V

The internal power switch is immediately turned OFF, if the voltage at CHAPS-input pin VBAT rises above the selected limit VLIM.

When the internal power switch has turned OFF because of an overvoltage detection, it stays OFF until the Charger-voltage drops below Vpor (CHAPS Power On Reset Threshold) (Vpor min. = 2.8V, Vpor max.= 3.12V). It is necessary to reconnect the charger to reset the VLIM protection.

The VBAT input of the CHAPS is connected through a 120R resistor to the battery voltage at a sense point close to the battery (see fig.9). A 1uF Capacitor is placed at the VBAT pin close to the CHAPS and acts with the 120R resistor as a filter to reduce influences of fast overvoltage transients caused by the Charger Switch or when connecting the charger.

The newer battery types from the battery vendors are getting 'better and better', which means that the charge voltage of a fully charged battery now is higher than when the VLIM threshold was specified.

This means that the difference between VLIM and the actually battery voltage is decreasing and the risk ofOvervoltage protection occurduring a normal charge is likely to occur.

A voltage divider (120R + 3K3) on the VBAT input of the CHAPS is implemented to solve this problem. The voltage divider is only active during charging which is controlled by a transistor activated by the MAD signal CHAR_CTRL.

The voltage divider reduces the Sense voltage by approx. 175mV +/-25mV, which gives new values for VLIM as shown below :

VLIM2min = 4.8+0.175-0.025 = 4.95V VLIM2may = 5.2+0.175+0.025 = 5.4V

Battery removal during charging

Output over-voltage protection is also needed in case the main battery is removed when charger connected or charger is connected before the battery is connected to the phone.

With a charger connected, if VOUT exceedsVLIM, CHAPS turns switch OFF until the charger input has sunken below Vpor (Vpor min. = 2.8V, Vpor max.= 3.12V). MCU software will stop the charging (turn off PWM) when it detects that battery has been removed.

1. Battery removed, (standard) charger connected, VOUT rises (follows charger

2. VOUT exceeds limit VLIM(X), switch is turned immediately

3. VOUT falls (because no battery) , also VCH<Vpor (standard chargers full-output). When VCH > Vpor and VOUT < VLIM(X) -> switch turned on again (also is still HIGH) and VOUT again exceeds

4. Software sets PWM = LOW -> CHAPS does not enter PWM

5. PWM low -> Startup mode, startup current flows until Vstart limit

6. VOUT exceeds limit Vstart, Istart is

7. VCH falls below

Figure 9: Output Overvoltage protection - Battery Removed

IMOKIA NHM-5NX

PAMS Technical Documentation System Module

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