7

ICS1708

Clock Input: RC Pin

The RC pin is used to set the frequency of the internal clock. Alternatively, an external 1 MHz clock may be used. Normally an external

resistor is connected between this pin and VDD. An external capacitor is connected between this pin and ground (VSS). The frequency of

the internal clock is typically 1 MHz with a 15k

MHz clock. Operating the clock at a lower frequency will proportionally change all time durations. Operating the clock at a frequency

significantly lower than 1 MHz, without adjusting the charge current accordingly, will lessen the effectiveness of the fast charge timer and

lower the accuracy of the controller. Operating the clock at a frequency greater than 1 MHz will also change all time durations and, without

adjusting the charge current accordingly, may cause termination to occur due to the fast charge timer expiring rather than by the battery

reaching full charge.

The clock may be driven by a 1 MHz external 0 to 5V pulse provided the duty cycle is between 10% and 60%. The clock input impedance is

typically 1k

Voltage Input: VIN Pin

The battery voltage is normalized by an external resistor divider network to one cell. The electrochemical potential of one cell is about 1.2V.

For example, if the battery consists of six cells in series, the voltage at the VIN pin should be set to equal to the total battery voltage divided

by six. This is accomplished with two resistors, as shown in Figure 7.

Figure 7: Resistor divider network at the VIN pin

To determine resistor values, count the number of cells to be charged in series. Then choose either R1 or R2 and solve for the other resistor

using:

R1 = R2 * (# of cells -1) or R2 =

R1

___

(# of cells -1)

R2 value should be kept below 27K so that if a 0.047

µF capacitor is used in parallel with it, more than five RC time constants

occur before data acquisition. Consult the factory for applications that require use of higher resistance voltage dividers.

Circuit Voltage Reference: OPREF pin 6

The OPREF pin has an internal 85k

(no battery) voltage reference. The purpose of this voltage reference is to detect the removal of the battery from the charging system. The

voltage at this pin is compared to the voltage at the VIN pin for the first 4mssec after the current source is turned on by CHG pin 1. If the

voltage at VIN is greater than the voltage set on OPREF, the ICS1708 assumes the battery has been removed and the ICS1708 enters the

polling detect mode.

For proper operation, the voltage on OPREF pin 6 is set below the (divided down) no battery voltage at VIN pin 7 produced by the current

source (when on) and above the maximum normalized voltage at VIN pin 7 with a battery present. The OPREF pin voltage must not exceed

2.3V or it will prevent the start of fast charge. If the voltage on OPREF exceeds 4V, the controller will shutdown and power to the ICS1708

must be removed and restored to restart the ICS1708.

R1

R2

VIN pin

# of cells