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UC2909 Fiches technique(PDF) 6 Page - Texas Instruments |
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UC2909 Fiches technique(HTML) 6 Page - Texas Instruments |
6 / 14 page 6 UC2909 UC3909 APPLICATION INFORMATION A Block Diagram of the UC3909 is shown on the first page, while a Typical Application Circuit is shown in Fig- ure 2. The circuit in Figure 2 requires a DC input voltage between 12V and 40V. The UC3909 uses a voltage control loop with average current limiting to precisely control the charge rate of a lead-acid battery. The small increase in complexity of average current limiting is offset by the relative simplicity of the control loop design. CONTROL LOOP Current Sense Amplifier This amplifier measures the voltage across the sense re- sistor RS with a fixed gain of five and an offset voltage of 2.3V. This voltage is proportional to the battery current. The most positive voltage end of RS is connected to CS- ensuring the correct polarity going into the PWM com- parator. CSO = 2.3V when there is zero battery current. RS is chosen by dividing 350mV by the maximum allow- able load current. A smaller value for RS can be chosen to reduce power dissipation. Maximum Charge Current, Ibulk, is set by knowing the maximum voltage error amplifier output, VOH = 5V, the maximum allowable drop across RS, and setting the re- sistors RG1 and RG2 such that; (1) RG RG V VLOGIC CA V VV V V RS RS RS 1 2 55 52 3 5 27 185 = • = • = • = –– – . . .2 •• IRS BULK The maximum allowable drop across RS is specified to limit the maximum swing at CSO to approximately 2.0V to keep the CSO amplifier output from saturating. No charge/load current: VCSO = 2.3V, Max charge/load current: Vmax(CSO) = 2.3V–2.0V = 0.3V Voltage Error Amplifier: The voltage error amplifier (VEA) senses the battery voltage and compares it to the 2.3V – 3.9mV/°C thermis- tor generated reference. Its output becomes the current command signal and is summed with the current sense amplifier output. A 5.0V voltage error amplifier upper clamp limits maximum load current. During the trickle charge state, the voltage amplifier output is opened (high impedance output) by the charge enable comparator. A trickle bias current is summed into the CA– input which sets the maximum trickle charge current. The VEA, VOH = 5V clamp saturates the voltage loop and consequently limits the charge current as stated in Equation 1. During the trickle bias state the maximum allowable charge current (ITC) is similarly determined: (2) ITC IRG RS TRICK CONTROL = • • _ 1 5 ITRCK_CONTROL is the fixed control current into CA–. ITRCK_CONTROL is 10µA when RSET = 11.5k. See RSET pin description for equation. Current Error Amplifier The current error amplifier (CA) compares the output of the current sense amplifier to the output of the voltage error amplifier. The output of the CA forces a PWM duty cycle which results in the correct average battery current. With integral compensation, the CA will have a very high DC current gain, resulting in effectively no average DC current error. For stability purposes, the high frequency gain of the CA must be designed such that the magni- tude of the down slope of the CA output signal is less than or equal to the magnitude of the up slope of the PWM ramp. CHARGE ALGORITHM Refer to Figure 3 in UC3906 Data Sheet in the data book. A) Trickle Charge State STAT0 = STAT1 = STATLV = logic 0 When CHGNB is less than VREF (2.3V – 3.9mV/°C), STATLV is forced low. This decreases the sense voltage divider ratio, forcing the battery to overcharge (VOC). (3) () VOC VREF RS RS RS RS RS RS =• ++ () () || 123 4 34 || During the trickle charge state, the output of the voltage error amplifier is high impedance. The trickle control cur- rent is directed into the CA– pin setting the maximum trickle charge current. The trickle charge current is de- fined in Equation 2. B) Bulk Charge State STAT1 = STATLV = logic 0, STAT0 = logic 1 As the battery charges, the UC3909 will transition from trickle to bulk charge when CHGENB becomes greater than 2.3V. The transition equation is (4) VT VREF RS RS RS RS RS RS RS =• ++ + (|| ) (|| ) 1234 234 STATLV is still driven low. |
Numéro de pièce similaire - UC2909 |
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Description similaire - UC2909 |
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