LM3647 Datasheet, PDF (8/17 Pages) National Semiconductor (TI) – Universal Battery Charger for Li-Ion, Ni-MH and Ni-Cd Batteries

nection between RESET and SYSOK is optional but highly
recommended for safe operation of the LM3647.
The resistor R7 keeps the transistor Q2 off until the LM3647
has been powered up and is in control of the circuitry.
4.3.5 Dimensioning the RCIN circuitry
The RC-circuitry is designed to time the charger so it charges
and regulates at the correct frequency. The values of the R
and C are important, because a change in the RC-value
gives a higher or lower operating frequency which affects
charge quality. The capacitance should be a ceramic type
and preferably an NP0 type, which gives the least frequency
deviation with temperature change.
4.3.7 BUZZER output circuitry
The buzzer-circuitry turns the transistor Q3 on when the
buzzer should sound. If the current consumption for the buzz-
er is lower than 0.3mA then the buzzer may be directly con-
nected to the BUZZER-pin. Please note that the BUZZER-
pin does not generate a PWM-signal, such buzzers must
have their own drive-circuitry. If an electromagnetic buzzer is
used, then the transistor may need a reverse-biased diode to
protect it from harmful voltage spikes.
Q3
4.3.6 Dimensioning the DISCHARGE circuitry
The discharge-circuitry controls the discharge rate during the
discharge phase (if used). The discharge output turns on the
transistor Q2, and a current flows from the battery through
the discharge resistor R8. The current through R8 depends
on battery voltage and the value of R8. This value depends
on the maximum discharge rate for the battery-pack. The ap-
proximate value can be calculated using the formula below:
R8
≈
-M-----a---x---i--m-----u---m-----B----a---t--t--e---r---y---V----o---l---t--a---g---e-
MaximumDisch arg eRate
4.3.8 PWM filter circuitry
The PWM-pin can either output a fast PWM-signal, or a slow
on/off output (for controlling external constant current source,
Ni-Cd/Ni-MH mode only).
Fast PWM-mode:
The RC-network R6, C9 and R5, C1||C2 lowpass-filters the
PWM-signal from the LM3647 to a DC-level that is fed into
the operational amplifier. The resistor R22 is required to pre-
vent DC-output before the LM3647 has control of the RC-net-
works.
8
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