BQ24640 Datasheet, PDF (18/25 Pages) Texas Instruments – High-Efficiency Synchronous Switch-Mode Super Capacitor Charger

bq24640
SLUSA44 – MARCH 2010
www.ti.com
APPLICATION INFORMATION
INDUCTOR SELECTION
The bq24640 has 600kHz switching frequency to allow the use of small inductor and capacitor values. The
Inductor saturation current should be higher than the charging current (ICHG) plus half the ripple current
(IRIPPLE):
I_SAT ³ I_CHG + (1/2) I_RIPPLE
(6)
The inductor ripple current depends on input voltage (VIN), duty cycle (D = V_OUT/V_IN), switching frequency (fs)
and inductance (L):
I_RIPPLE =
V_IN
´ D ´ (1
¦s ´ L
-
D)
(7)
The maximum inductor ripple current happens with D = 0.5 or close to 0.5. Usually inductor ripple is designed in
the range of (20–40%) maximum charging current as a trade-off between inductor size and efficiency for a
practical design.
INPUT CAPACITOR
Input capacitor should have enough ripple current rating to absorb input switching ripple current. The worst case
RMS ripple current is half of the charging current when duty cycle is 0.5. If the converter does not operate at
50% duty cycle, then the worst case capacitor RMS current I_CIN occurs where the duty cycle is closest to 50%
and can be estimated by the following equation:
I_CIN = I_CHG ´ D × (1 - D)
(8)
Low ESR ceramic capacitor such as X7R or X5R is preferred for input decoupling capacitor and should be
placed to the drain of the high side MOSFET and source of the low side MOSFET as close as possible. Voltage
rating of the capacitor must be higher than normal input voltage level. 25V rating or higher capacitor is preferred
for 20V input voltage. The 20mF capacitance is suggested for typical of 3–4A charging current.
OUTPUT CAPACITOR
Output capacitor also should have enough ripple current rating to absorb output switching ripple current. The
output capacitor RMS current I_COUT is given:
I_COUT = ^IRIPPLE » 0.29 ´ I_RIPPLE
2´ 3
(9)
The output capacitor voltage ripple can be calculated as follows:
ΔV_O
=
1
8LC¦s²
æ
ççè
V_OUT
-
V_O2UT
V_IN
ö
÷÷ø
(10)
At certain input/output voltage and switching frequency, the voltage ripple can be reduced by increasing the
output filter LC.
The bq24640 has internal loop compensator. To get good loop stability, the resonant frequency of the output
inductor and output capacitor should be designed between 12 kHz and 17 kHz. The preferred ceramic capacitor
is 25V or higher rating, X7R or X5R.
POWER MOSFETs SELECTION
Two external N-channel MOSFETs are used for a synchronous switching charger. The gate drivers are internally
integrated into the IC with 6V of gate drive voltage. 30V or higher voltage rating MOSFETs are preferred for 20V
input voltage and 40V or higher rating MOSFETs are preferred for 20–28V input voltage.
Figure-of-merit (FOM) is usually used for selecting proper MOSFET based on a tradeoff between the conduction
loss and switching loss. For top side MOSFET, FOM is defined as the product of a MOSFET's on-resistance,
R_DS(ON), and the gate-to-drain charge, Q_GD. For bottom side MOSFET, FOM is defined as the product of the
MOSFET's on-resistance, R_DS(ON), and the total gate charge, Q_G.
18
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