DC631B Analog Devices

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC631B USB POWER SOLUTION IN QFN 5X5 LTC3455 DESCRIPTION Demonstration circuit DC631 is a complete power solu- specification, two 1.5MHz synchronous step down con- tion for a USB powered device. Power is input from one verters and reset and power button status outputs for of three sources, and three outputs are provided, as well interface with a microprocessor. All this functionality as supervisory signals for a microprocessor. DC631 consumes only 225mm 2 of PCB area. provides the following functions 700mA CC/CV timer Design files for this circuit board are available. Call terminated charger suitable for Li-ion cells, USB power the LTC factory. manager that insures compliance with the USB power Table 1. Performance Summary PARAMETER CONDITION VALUE Wall Adapter Input Voltage Range 4.75V-5.5V USB Input Voltage Range 4.2V-5.5V Battery Charger Iout Vin = 5V, Vbat = 3.6V 700mA 15% Float voltage Vin = 5V 4.20V1% Trickle Charge threshold Vin = 5V (Battery voltage rising) 2.85V Recharge threshold Vin = 5V Vfloat-150mV USB Current Input Current 500mA mode Vin = 5V USB Active 500mA mode 500mA max Input Current 100mA mode Vin = 5V USB Active 100mA mode 100mA max Input Current suspend mode Vin = 5V USB suspend 100uA max Buck Converters Switcher 1 V batt 3.6V Iout 550mA 3.15V 4% Switcher 2 Vbat 3.6V Iout 400mA 1.8V 4% Hot Swap Output Vbat 3.6V 100mA max OPERATING PRINCIPLES Demo DC631 is based on the LTC3455. This chip man- tery and to generate two low voltage outputs via high ages the power supplies that would be typical for a USB efficiency step down converters. The intermediate volt- powered device. Power is input from either the USB age bus is preferentially powered from the adapter, then cable or an adapter to an intermediate voltage bus. This USB and finally if required from the battery via an ideal intermediate voltage is then used to charge a Li-ion bat- diode. The battery charger is a CC/CV timer terminated 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC631B USB POWER SOLUTION IN QFN 5X5 type capable of charge currents of up to 800mA and is vides for some of the housekeeping functions typically roughly equivalent to the LTC4053. The two step down associated with a small battery powered device. The converters are high efficiency synchronous buck types LTC3455 provides a reset signal for 200msec after similar to the LTC3405 and LTC3406.The outputs of the power up. The LTC3455 has a pin specifically designed buck converters are set at 1.8V and 3.15V, other settings to interface to a power ON/OFF button and reports this are possible by altering the feedback resistor ratios. The button status on the PBStat pin to allow the host micro- chip is normally ON if either USB or adapter is present. processor to shut down. Additionally there is an un- If the battery is the only source of power then the chip committed op-amp that can be used as it is on the demo must be turned on by pressing the ON button, the chip board as a low battery detector. Another possible use can be turned off by depressing the OFF button. The off for the uncommitted op amp would be as a LDO control- button will only function when in battery power mode. ler to generate an additional output or to augment the In addition to these basic functions the LTC3455 pro- peak current capability of one of the existing outputs. QUICK START PROCEDURE Demonstration circuit DC631 is easy to set up to evalu- 5. Place the USB Suspend jumper in the Suspend posi- ate the performance of the LTC3455. Refer to Figure 1 tion and note that the USB current falls to under for proper measurement equipment setup and follow the 100uA to comply with the suspend mode current. procedure below: Adapter Charging 1. Connect input power supply, meters and output load 6. Ramp up the wall adapter voltage. When the wall as shown in figure 1. adapter exceeds 4.7 volts the LTC3455 will cease USB Charging drawing current from the USB and switch over to the wall adapter. 2. Place the USB Suspend Jumper in the USB Active Position. Place the USB current jumper in the 500mA 7. Note that the charge current to the battery has in- position. Set the battery simulator voltage to zero, creased to 700mA. slowly raise the USB input voltage. When the supply 8. Increase the battery voltage to 4.2V and note that the voltage exceeds 4.1V the charger should activate and charge current falls to 0 this illustrates the constant the charge LED will illuminate. The battery will be in voltage portion of the charging characteristic. trickle charge mode. Slowly increase the battery volt- Battery powered Operation age. At about 2.85V the battery charge current will in- crease to 450mA. Note that the input current from 9. Set Both USB input voltage and the adapter input the USB does not exceed the USB spec of 500mA. voltage to zero. The battery will now support the op- eration of the demo board. 3. Now increase the loads on the outputs. Note that as the output load is increased the charging current is 10. In battery mode operation the OFF pushbutton is now decreased and the USB current remains within the active. Push the OFF button and note battery current 500mA limit. draw falls to a very low level, less than 10uA. 4. Place the USB current jumper in the 100mA position 11. Now restart the converter using the ON pushbutton. and note that USB input current falls to maintain Note that the Reset LED goes on for 200msec. This compliance with the USB input current spec. Also simulates the Reset signal that the LTC3455 provides note that as the external load is increased the battery to the processor upon initial power up. will start to discharge to power the Vmax intermediate 12. In battery mode the converter can be shifted into voltage bus, illustrating the ideal diode function of the burst mode, by use of the burst mode jumper. In LTC3455 2