Datasheet LTC4008 (Analog Devices) - 10

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OPERATION Input FET (LTC4008)
OFF The input FET circuit performs two functions. It enables TGATE ON the charger if the input voltage is higher than the CLP ON pin and provides the logic indicator of AC present on the BGATE tOFF ACP/SHDN pin. It controls the gate of the input FET to OFF keep a low forward voltage drop when charging and also TRIP POINT SET BY ITH VOLTAGE prevents reverse current flow through the input FET. INDUCTOR CURRENT If the input voltage is less than VCLP , it must go at least 4008 F01 170mV higher than VCLN to activate the charger. When this occurs the ACP/SHDN pin is released and pulled up with
Figure 1.
an external load to indicate that the adapter is present. The gate of the input FET is driven to a voltage sufficient The peak inductor current, at which ICMP resets the SR to keep a low forward voltage drop from drain to source. latch, is controlled by the voltage on ITH. ITH is in turn If the voltage between DCIN and CLP drops to less than controlled by several loops, depending upon the situation 25mV, the input FET is turned off slowly. If the voltage at hand. The average current control loop converts the between DCIN and CLP is ever less than –25mV, then voltage between CSP and BAT to a representative cur- the input FET is turned off in less than 10µs to prevent rent. Error amp CA2 compares this current against the significant reverse current from flowing in the input FET. desired current programmed by RPROG at the PROG pin In this condition, the ACP/SHDN pin is driven low and the and adjusts ITH until: charger is disabled. VREF V = CSP – VBAT + 11.67μA • 3.01kΩ R 3.01kΩ
Input FET (LTC4008-1)
PROG therefore, The input FET circuit is disabled for the LTC4008-1. There is no low current shutdown mode when DCIN falls below ⎛ V ⎞ REF 3.01kΩ the CLP pin. The ACP/SHDN pin functions only to shut ICHARGE(MAX) = ⎜ – 11.67μA⎟ • ⎝R ⎠ R down the charger. PROG SENSE The voltage at BATMON is divided down by an external
Battery Charger Controller
resistor divider and is used by error amp EA to decrease The LTC4008 charger controller uses a constant off-time, ITH if the divider voltage is above the 1.19V reference. current mode step-down architecture. During normal When the charging current begins to decrease, the voltage operation, the top MOSFET is turned on each cycle when at PROG will decrease in direct proportion. The voltage at the oscillator sets the SR latch and turned off when the PROG is then given by: main current comparator ICMP resets the SR latch. While RPROG the top MOSFET is off, the bottom MOSFET is turned on VPROG = I(CHARGE •RSENSE +11.67μA •3.01kΩ) • 3.01kΩ until either the inductor current trips the current compar- ator I The accuracy of VPROG will range from 0% to ITOL. REV or the beginning of the next cycle. The oscillator uses the equation: VPROG is plotted in Figure 2. V The amplifier CL1 monitors and limits the input current, t DCIN – VBAT OFF = normally from the AC adapter to a preset level (100mV/ VDCIN • fOSC RCL). At input current limit, CL1 will decrease the ITH volt- to set the bottom MOSFET on time. This activity is dia- age, thereby reducing charging current. The ICL indicator grammed in Figure 1. output will go low when this condition is detected and the FLAG indicator will be inhibited if it is not already low. Rev C 10 For more information www.analog.com Document Outline Description Typical Application Absolute Maximum Ratings Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Test Circuit Operation Applications Information Package Description Revision History Related Parts