BQ24702_16 Datasheet, PDF (21/37 Pages) Texas Instruments – MULTICHEMISTRY BATTERY CHARGER CONTROLLER

www.ti.com
bq24702
bq24703
SLUS553E – MAY 2003 – REVISED OCTOBER 2007
Selecting an Output Capacitor
For this application the output capacitor is used primarily to shunt the output ripple current away from the battery.
The output capacitor should be sized to handle the full output ripple current as defined as:
ǒ Ǔ _{Ic (RMS) +}
VIN * VBAT
FS L
D
ARMS
(8)
Selecting an Input Capacitor
The input capacitor is used to shunt the converter ripple current on the input lines. The capacitor(s) must have a
ripple current (RMS) rating of:
Ǹ _{IRMS + [Ichg (1–D)]}² _{D ) [Ichg D]}² _{(1–D) ARMS}
(9)
In addition to shunting the converter input ripple when the PWM is operating, the input capacitor also acts as part
of an LC filter, where the inductance component is defined by the ac adapter cable inductance and board trace
inductance from adapter connector to filter capacitor. Overshoot conditions can be observed at V_CC line during
fast load transients when the adapter powers the load or when the adapter is hot-plugged .
Increasing the input capacitor value decreases the overshoot at V_CC. Avoid overshoot voltages at V_CC in excess
of the absolute maximum ratings for that pin.
Compensating the Loop
For the bq24702/bq24703 used as a buck converter, the best method of compensation is to use a Type II
compensation network from the output of the transconductance amplifiers (COMP pin) to ground (GND) as
shown in Figure 2. A Type II compensation adds a pole-zero pair and an additional pole at dc.
The Type II compensation network places a zero at
ǒ Ǔ FZ
+
1
2
1
Hz
p RCOMP CZ
(10)
and a pole at
ǒ Ǔ FP
+
1
2
p
1
RCOMP
CP
Hz
(11)
For this battery charger application the following component values: C_Z = 4.7 μF, C_P = 150 pF, and R_COMP = 100
Ω, provides a closed loop response with more than sufficient phase margin, as long as the LC pole [1/2 × PI ×
sqrt (l×c)] is set below 10 kHz. The SRP/SRN filter (R19, R21, C8) and ACP/ACN filter (R13/R15/C3) are
required to filter noise associated with the PWM switching. To avoid adding secondary poles to the PWM closed
loop system those filters should be set with cutoff frequencies higher than 1 kHz.
SELECTOR OPERATION
The bq24702/bq24703 allows the host controller to manually select the battery as the system's main power
source, without having to remove adapter power. This allows battery conditioning through smart battery learn
cycles. In addition, the bq24702/bq24703 supports autonomous supply selection during fault conditions on either
supply. The selector function uses low R_DS(on) P-channel MOSFETs for reduced voltage drops and longer battery
run times.
NOTE:
Selection of battery power whether manual or automatic results in the suspension of
battery charging.
Copyright © 2003–2007, Texas Instruments Incorporated
Submit Documentation Feedback
21
Product Folder Link(s): bq24702 bq24703