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LTC4010EFE Datasheet(PDF) 11 Page - Linear Integrated Systems |
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LTC4010EFE Datasheet(HTML) 11 Page - Linear Integrated Systems |
11 / 20 page 11 LTC4010 4010p current threshold for CC such that the desired average current through RSENSE is maintained. The current com- parator output does this by switching the state of the SR latch at the appropriate time. At the beginning of each oscillator cycle, the PWM clock sets the SR latch and the external P-channel MOSFET is switched on (N-channel MOSFET switched off) to refresh the current carried by the external inductor. The inductor current and voltage drop across RSENSE begin to rise linearly. During normal operation, the PFET is turned off (NFET on) during the cycle by CC when the voltage difference across RSENSE reaches the peak value set by the output of EA. The inductor current then ramps down linearly until the next rising PWM clock edge. This closes the loop and maintains the desired average charge current in the external inductor. Low Dropout Charging After charging is initiated, the LTC4010 does not require that VCC remain at least 500mV above BAT because situations exist where low dropout charging might occur. OPERATIO In one instance, parasitic series resistance may limit PWM headroom (between VCC and BAT) as 100% charge is reached. A second case can arise when the DC adapter selected by the end user is not capable of delivering the current programmed by RSENSE, causing the output volt- age of the adapter to collapse. While in low dropout, the LTC4010 PWM runs near 100% duty cycle with a fre- quency that may not be constant and can be less than 550kHz. The charge current will drop below the pro- grammed value to avoid generating audible noise, so the actual charge delivered to the battery may depend prima- rily on the LTC4010 charge timer. Internal Die Temperature The LTC4010 provides internal overtemperature detec- tion to protect against electrical overstress, primarily at the FET driver outputs. If the die temperature rises above this thermal limit, the LTC4010 stops switching and indicates a fault as previously discussed. APPLICATIO S I FOR ATIO External DC Source The external DC power source should be connected to the charging system and the VCC pin through a power diode acting as an input rectifier. This prevents catastrophic system damage in the event of reverse-voltage polarity at the DC input. The LTC4010 automatically senses when this input drives the VCC pin above BAT. The open-circuit voltage of the DC source should be between 5.5V and 34V, depending on the number of cells being charged. In order to avoid low dropout operation, ensure 100% capacity at charge termination, and allow reliable detection of battery insertion, removal or overvoltage, the following equation can be used to determine the minimum full-load voltage that should be produced at VCC when the external DC power source is connected. VCC(MIN) = (n • 2V) + 0.3V where n is the number of series cells in the battery pack. The LTC4010 will properly charge over a wide range of VCC and BAT voltage combinations. Operating the LTC4010 in low dropout or with VCC much greater than BAT will force the PWM frequency to be much less than 550kHz. The LTC4010 disables charging and sets a fault if a large VCC to BAT differential would cause generation of audible noise. Load Control Proper current load control is an important consideration when fast charging nickel cells. This control ensures that the system load remains powered at all times, but that normal system operation and associated load transients do not adversely affect fast charge termination. The input protecton detailed in the previous paragraph is an integral part of the necessary load control. The battery should also be connected to the raw system supply by some rectifying means, thus forming a switch that |
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