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LTC4007EGN Datasheet(PDF) 11 Page - Linear Technology |
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LTC4007EGN Datasheet(HTML) 11 Page - Linear Technology |
11 / 24 page 11 LTC4007 4007fa The peak inductor current, at which ICMP resets the SR latch, is controlled by the voltage on ITH. ITH is in turn controlled by several loops, depending upon the situation at hand. The average current control loop converts the voltage between CSP and BAT to a representative current. Error amp CA2 compares this current against the desired current programmed by RPROG at the PROG pin and adjusts ITH until: V R VV A k k REF PROG CSP BAT = +µ Ω Ω –. • . . 11 67 3 01 301 therefore, I V R A k R CHARGE MAX REF PROG SENSE () –. • . =µ ⎛ ⎝⎜ ⎞ ⎠⎟ Ω 11 67 301 The voltage at BAT is divided down by an internal resistor divider and is used by error amp EA to decrease ITH if the divider voltage is above the 1.19V reference. When the charging current begins to decrease, the voltage at PROG will decrease in direct proportion. The voltage at PROG is then given by: VI R A k R k PROG CHARGE SENSE PROG =+ µ Ω () Ω •. • . • . 11 67 3 01 301 VPROG is plotted in Figure 2. The amplifier CL1 monitors and limits the input current, normally from the AC adapter to a preset level (100mV/ RCL). At input current limit, CL1 will decrease the ITH OPERATIO ICHARGE (% OF MAXIMUM CURRENT) 0 0 0.2 0.4 0.6 0.8 20 40 60 80 100 4007 F02 1.0 1.2 1.19V 0.309V Figure 2. VPROG vs ICHARGE voltage, thereby reducing charging current. The ICL indica- tor output will go low when this condition is detected and the FLAG indicator will be inhibited if it is not already LOW. If the charging current decreases below 10% to 15% of programmed current while engaged in input current lim- iting, BGATE will be forced low to prevent the charger from discharging the battery. Audible noise can occur in this mode of operation. An overvoltage comparator guards against voltage tran- sient overshoots (>7% of programmed value). In this case, both MOSFETs are turned off until the overvoltage condition is cleared. This feature is useful for batteries which “load dump” themselves by opening their protec- tion switch to perform functions such as calibration or pulse mode charging. PWM Watchdog Timer There is a watchdog timer that observes the activity on the BGATE and TGATE pins. If TGATE stops switching for more than 40 µs, the watchdog activates and turns off the top MOSFET for about 400ns. The watchdog engages to prevent very low frequency operation in dropout—a po- tential source of audible noise when using ceramic input and output capacitors. Charger Start-Up When the charger is enabled, it will not begin switching until the ITH voltage exceeds a threshold that assures initial current will be positive. This threshold is 5% to 15% of the maximum programmed current. After the charger begins switching, the various loops will control the current at a level that is higher or lower than the initial current. The duration of this transient condition depends upon the loop compensation, but is typically less than 100 µs. Thermistor Detection The thermistor detection circuit is shown in Figure 3. It requires an external resistor and capacitor in order to function properly. The thermistor detector performs a sample-and-hold func- tion. An internal clock, whose frequency is determined by |
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