High efficiency (Figure 2) even at multi-amp charge rates is critical not only to make optimal use of available input power, but also to control power dissipation inside the portable device. The combination of high power dissipation and marginal thermal performance in a tightly enclosed space can make a device with a less efficient charging solution too warm to hold comfortably. To help keep things cool, the LTC4155’s integrated power switches feature an on-resistance well under 100m?.
Figure 2: Switching regulator efficiency
While the LTC4155’s power switches are sized to handle higher currents than available from USB, the LTC4155 remains USB compatible for opportunistic charging. Input current is automatically measured internally and limited to any of sixteen I2
C user-selectable values. Of these settings, three correspond to guaranteed maximum limits of 100mA and 500mA for USB 2.0 and 900mA for USB 3.0. Automatic input current limiting can also be used with AC adapters or other sources by choosing any of the other current limit settings up to a maximum of 3A.
The LTC4155 supports a pin-programmable power-on default input current. For high power applications that do not require USB compatibility, a single resistor connected to the CLPROG1 pin programs a default power-on input current. This resistor is chosen to correspond to an initial current limit most appropriate to the particular application, intended power source capability, etc. After power-up, the input current limit can be modified under I2
C control to any of the sixteen other available settings up to 3A.
For USB applications, the CLPROG1 and CLPROG2 pins can be connected together to program the LTC4155 to enforce USB current limit rules. The input current limit will default to 100mA upon application of external power. After successful enumeration with the USB host controller, the input current limit setting can be increased under I2C control to 500mA or 900mA as appropriate. Figure 3
shows the available current to the system load and battery charger. Note that the switching regulator output current is higher than the USB-limited input current. If the system detects that the power source is an AC adapter, dedicated USB charger, or other non-USB source, the input current limit setting can be increased under I2
C control to any other setting up to 3A.
Figure 3: USB-compliant load current available before discharging battery