Datasheet

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SOFTSTART
10
0
30
20
40
60
50
70
90
80
100
V
COMP
- Compensation Voltage - V
Percent Duty Cycle - %
PERCENT DUTY CYCLE
vs
COMPENSATION VOLTAGE
1.2 1.7 2.2 2.7 3.2
SETTING THE BATTERY CHARGE REGULATION VOLTAGE
V
BATTERY
+
(
R1 ) R2
)
V
BATSET
R2
V ) I
BATP
R1
(1)
bq24702
bq24703
SLUS553E – MAY 2003 – REVISED OCTOBER 2007
Softstart is provided to ensure an orderly start-up when the PWM is enabled. When the PWM controller is
disabled (ENABLE = Low), the 100- μ A current source pullup is disabled and the COMP pin is actively pulled
down to GND. Disabling the 100- μ A pullup reduces current drain when the PWM is disabled. When the
bq24702/bq24703 PWM is enabled (ENABLE = High), the COMP pin is released and the 100- μ A pullup is
enabled (refer to Figure 2 ). The voltage on the COMP pin increases as the pullup charges the external
compensation network connected to the COMP pin. As the voltage on the COMP pin increases the PWM duty
cycle increases linearly as shown in Figure 3 .
Figure 3.
As any one of the three controlling loops approaches the programmed limit, the g
m
amplifier begins to shunt
current away from the COMP pin. The rate of voltage rise on the COMP pin slows due to the decrease in total
current out of the pin, decreasing the rate of duty cycle increase. When the loop has reached the programmed
limit the g
m
amplifier shunts the entire bias current (100 μ A) and the duty cycle remains fixed. If any of the control
parameters tries to exceed the programmed limit, the g
m
amplifier shunts additional current from the COMP pin,
further reducing the PWM duty cycle until the offending parameter is brought into check.
The battery charge regulation voltage is programmed through the BATSET pin, if the internal precision reference
is not used. The BATSET input is a high-impedance input that is driven by either a keyboard controller DAC or
via a resistor divider from a precision reference (see Figure 4 ).
The battery voltage is fed back to the g
m
amplifier through a resistor divider network. The battery charge
regulation voltage can be defined as:
where I
BATP
= input bias current for pin BATP
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