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參數(shù)資料
| 型號： | UCC2956 |
| 廠商： | Texas Instruments, Inc. |
| 英文描述： | Switch Mode Lithium-Ion Battery Charger Controller |
| 中文描述： | 開關(guān)式鋰離子電池充電控制器 |
| 文件頁數(shù)： | 8/10頁 |
| 文件大?。?/td> | 424K |
| 代理商： | UCC2956 |
8
UCC2956
UCC3956
APPLICATION INFORMATION (cont.)
100% charged. If the battery is not taken from the
charger at time t4, the charger will continue charging.
The timer will expire and the charge cycle will terminate
at time t5.
A typical value of current used to indicate near full
charge is 1/10 of the bulk current value. This current level
is established by setting the appropriate voltage on IMIN.
IMIN is tied to an internal comparator along with the out-
put of the current sense amplifier.
sense amplifier voltage becomes greater than the voltage
on IMIN, the internal state machine indicates near full
charge by setting STAT0 and STAT1 to logic level
highs. In the application circuit of Figure 1, resistors RS4
and RS5 determine the voltage at IMIN. With RS4 at 11k
and RS5 at 10k, near full charge is indicated at 120mA.
When the current
VIMIN= 4.1
RS5
RS4+ RS5
INEAR_FULL =2.05-VIMIN
5RSENSE
The UCC3956 timer has a 14 bit counter that allows long
over-charge times with reasonable component values.
As stated above, the charger will continue charging the
battery until the timer expires (unless the battery is pulled
from the charger). Referring to Figure 2, the timer starts
at time t2 and expires at time t5. The frequency of the
timer can be determined as follows:
fTIMER=
0.06
RSETCTO
With a 14 bit counter the time-out period in minutes be-
comes:
TIMEOUT = 4550CTORSET
In the application circuit, a value of 0.15
μ
F is used for
CTO to give 120 minutes of overcharge (more than twice
the bulk charge time). When the timer expires, CHG is
pulled low by an internal buffer and the charge cycle ter-
minates. If tied to a bi-directional port, CHG can be read
by a microprocessor.
Inductor Sizing
For good efficiency, the inductor should be sized to give
continuous current in the bulk charge state. For a buck
converter, duty cycle in continuous mode is given by:
D=VBATTERY+ VSCHOTTKY
VINPUT+ VSCHOTTKY
Allowing a 25% ripple in the bulk current will give a rea-
sonable value of inductance. The inductor value can be
calculated as follows:
L =4(VINPUT -VBAT)D
IBULK fOSC
A 150
μ
H inductor is used in the application circuit.
Current Control Loop
The UCC3956 features an outer voltage loop and an in-
ner average current loop. The virtues of average current
mode control are well documented in Reference [1]. A
simplified block diagram of the feedback elements is pro-
vided in Figure 3. The network for the current amplifier
could be as simple as a single capacitor, providing a
dominant pole response, which may be adequate for a
battery charger application. The current amplifier network
of Figure 3 provides improved transient performance.
The component values for CF3, CF4, and RF4 will be se-
lected to give a constant gain from approximately
f
OSC
/10 to f
OSC
. At frequencies below f
OSC
/10, the net-
work gain will increase at 20dB/decade, giving a high DC
gain. The network will attenuate at 20dB/decade above
the switching frequency, giving noise immunity.
A feedback design that optimizes transient response will
have the amplified inductor current down-slope approach
the PWM saw-tooth slope [1]. This occurs by designing
the total loop gain to cross unity at 1/3 to 1/6 of the
switching frequency. The applications circuit is designed
to cross unity gain at 1/10 of the switching frequency
(10kHz), with a 12V nominal input. The power stage
small signal gain can be approximated by:
GPOWER_STAGE =
VINRSENSE
SL+ RSENSE+ ESR
Referring to Figure 3, the current sense amplifier pro-
-60
-40
-20
0
20
40
60
80
10
100
1000
10000
100000
1000000
FREQUENCY
G
POWER
STAGE
FEEDBACK
GAIN
Figure 4a. Current Loop Power Stage
and Feedback Gain
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