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參數(shù)資料
| 型號： | LM3647IMX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 電源管理 |
| 英文描述： | 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO20 |
| 封裝： | PLASTIC, SOIC-20 |
| 文件頁數(shù)： | 13/14頁 |
| 文件大?。?/td> | 271K |
| 代理商： | LM3647IMX |
4.0 Functional Description (Continued)
4.3.3 Charge Current
Charge current selection depends on the type of power
supply used. The LM3647 supports two types: current feed-
back and constant current. The PWM pin is used to control
the power supply and depending on the supply type, it either
produces a simple on/off signal (PWM slow, constant current
supply) or a PWM signal that is filtered externally to a DC
voltage to regulate the charge voltage in a current feedback
power supply (PWM fast). (SEL3 = V
CC, if SEL2 = GND or
V
CC; automatic if SEL2 = open.)
4.3.3.1 Current Feedback Power Supply
A current feedback type power supply must be used if charg-
ing Li-Ion batteries. When a current feedback power supply
is used, the charge-current is selected by setting the current
sensing resistor and the gain of the differential amplification
stage. The current sensing resistor (R5) should be dimen-
sioned such that a voltage drop over it is not too small,
because the signal will be more susceptible to noise and
offsets in the amplification-stage. The resistance should not
be too large either (especially in high-current applications),
because this will only generate more heat from the compo-
nent. A suitable value is one that develops about 50 mV
across the resistor when maximum current flows through it.
10131811
The current-sensing signal is amplified, inverted and cen-
tered on the 2.5V reference by the operational amplifier and
fed into the CS pin on the LM3647. The amplification stage
must be dimensioned by setting the appropriate ratio be-
tween R1 (R2) and R3 (R4). The figure above is dimen-
sioned for a maximum current of about 1.1A. This was
calculated using the following formula:
Example:
R3 = 5.1 k
,R1=100 k, R5 = 0.047,V
REF = 2.5V
→ MaxCurrent ≈ 1.09 Ampere
With a current feedback power supply the PWM pin must be
configured for “fast” mode (SEL3 = V
CC; if SEL2 = GND or
V
CC; automatic if SEL2 = open).
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
prevent DC-output before the LM3647 has control of the
RC-networks.
10131813
4.3.3.2 Constant Current Power Supply
A constant current power supply can only be used with
Ni-Cd/Ni-MH batteries, it can not be used to charge Li-Ion
batteries. With a constant current power supply (NiCd/NiMH
batteries only: SEL2 = GND or V
CC) the PWM pin must be
configured for “slow” mode (SEL3 = GND). In that mode the
PWM-pin turns the external current source on and off at a
rate of 0.1 Hz. (This example is just one of many possible
design solutions.) The PWM-pin (SLOW_PWM) turns the
transistor Q1 on and off. When the transistor is off the
current source is on, and when the transistor is on the
current source is off (V_OUT at approximate 0.7V). The
value of R1 depends on the size of the charge current (see
formula):
LM3647
www.national.com
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