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LTC4088-2 Datasheet(PDF) 11 Page - Linear Technology |
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LTC4088-2 Datasheet(HTML) 11 Page - Linear Technology |
11 / 24 page LTC4088-1/LTC4088-2 11 40881fb capacitor are connected from CLPROG to GND, the voltage on CLPROG represents the average input current of the switching regulator. As the input current approaches the programmed limit, CLPROG reaches 1.188V and power delivered by the switching regulator is held constant. Several ratios of current are available which can be set to correspond to USB low and high power modes with a single programming resistor. The input current limit is programmed by various com- binations of the D0, D1 and D2 pins as shown in Table 1. The switching input regulator can also be deactivated (USB Suspend). The average input current will be limited by the CLPROG pro- gramming resistor according to the following expression: II V R h VBUS BUSQ CLPROG CLPROG CLPROG =+ + () •1 where IBUSQ is the quiescent current of the LTC4088- 1/LTC4088-2, VCLPROG is the CLPROG servo voltage in current limit, RCLPROG is the value of the programming resistor and hCLPROG is the ratio of the measured current at VBUS to the sample current delivered to CLPROG. Refer to the Electrical Characteristics table for values of hCLPROG, VCLPROG and IBUSQ. Given worst-case circuit tolerances, the USB specification for the average input current in 1x or 5x mode will not be violated, provided that RCLPROG is 2.94k or greater. Table 1 shows the available settings for the D0, D1 and D2 pins. Table 1. Controlled Input Current Limit D0 D1 4088-1 D2 4088-2 D2 CHARGER STATUS IBUS(LIM) 0 0 0 1 Off 100mA (1x) 0 0 1 0 On 100mA (1x) 0 1 0 1 Off 500mA (5x) 0 1 1 0 On 500mA (5x) 1 0 0 1 Off 1A (10x) 1 0 1 0 On 1A (10x) 1 1 0 1 Off 2.5mA (Susp High) 1 1 1 0 Off 500µA (Susp Low) Notice that when D0 is high and D1 is low, the switching regulator is set to a higher current limit for increased charging and power availability at VOUT. These modes will typically be used when there is line power available from a wall adapter. While not in current limit, the switching regulator’s Bat- Track feature will set VOUT to approximately 300mV above the voltage at BAT. However, if the voltage at BAT is below 3.3V, and the load requirement does not cause the switching regulator to exceed its current limit, VOUT will regulate at a fixed 3.6V as shown in Figure 2. This will allow a portable product to run immediately when power is applied without waiting for the battery to charge. If the load does exceed the current limit at VBUS, VOUT will range between the no-load voltage and slightly below the battery voltage, indicated by the shaded region of Figure 2. OPERATIO BAT (V) 2.4 4.5 4.2 3.9 3.6 3.3 3.0 2.7 2.4 3.3 3.9 408812 F02 2.7 3.0 3.6 4.2 NO LOAD 300mV Figure 2. VOUT vs BAT For very low-battery voltages, the battery charger acts like a load and, due to limited input power, its current will tend to pull VOUT below the 3.6V “Instant On” voltage. To prevent VOUT from falling below this level, an undervoltage circuit automatically detects that VOUT is falling and reduces the battery charge current as needed. This reduction ensures that load current and voltage are always prioritized and yet delivers as much battery charge current as possible. (See Over Programming the Battery Charger in the Applications Information section). The voltage regulation loop compensation is controlled by the capacitance on VOUT. An MLCC capacitor of 10µF is required for loop stability. Additional capacitance beyond this value will improve transient response. |
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Similar Description - LTC4088-2 |
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