CN0509

The CN0509 converts a wide array of dc power sources from 5 V to 100 V into a regulated 5 V, 2 A supply capable of charging USB charged devices through a dual USB, Type A receptacle.

Two output ports are provided, one that leaves the USB D+ and D− signals open for general-purpose charging and one that includes a USB DCP controller that enables the high current charging modes for devices from most manufacturers. Both ports can be used simultaneously. However, the maximum total load current is 2 A.

This design uses a combination of a high efficiency, step-down (buck) dc to dc converter (LTC7103), capable of passthrough operation, and an isolated flyback converter (LT8302). This configuration combines the high power efficiency and wide operating range of a buck converter with the isolation and excellent regulation of a flyback converter.

Input Voltage Protection and LED Driver

The CN0509 power entry stage is shown in Figure 2. A high voltage Schottky diode and fuse protect the circuit from reverse supply connection and overcurrent conditions.

Two LEDs indicate the input power, green indicates correct polarity and red indicates a reversed connection. If the red LED appears, the input connections must be reversed for the circuit to operate. While a bridge rectifier could be used to allow operation in either polarity, the additional 400 mV drop increases the minimum operating voltage, which can be an issue when charging from lower voltage sources, such as solar panels or alkaline battery cells.

An active, constant current driver circuit maintains LED current over the entire operating range of the circuit, with minimal variation in brightness. Two circuits are connected in series, however, with opposite polarity (see Figure 3).

When power is applied, current flows through R1, which turns on Transistor Q3. Current then flows through the emitter of Q3, the LED, and R2. As the current increases through R2, the voltage across R2 increases as well. Once the voltage drop across R2 reaches the base to emitter voltage (V_BE) of transistor Q1 (approximately 0.7 V), Q1 begins to turn on. The resulting current through R1 reduces the base drive to Q1, effectively limiting current to the LED. This feedback loop maintains the green LED and the red LED current at approximately 2.41 mA and 2.432 mA, respectively, over a voltage range of 11 V to 100 V.

LTC7103 Buck Converter

The power entry circuit is followed by an LTC7103 synchronous buck converter. A buck converter, or step-down switch mode power supply, efficiently reduces a higher dc voltage to a lower voltage with low power dissipation and high power density in a small package relative to a linear regulator of similar current capability.

The dc input is filtered and bypassed by a total of 4.8 μF before going into the input of the buck converter. The LTC7103 then efficiently steps down the wide input voltage range of 12 V to 105 V to a regulated output voltage (V_OUT) of 12 V while delivering up to 2.3 A of output current at a 300 kHz switching frequency (f_SW).

The LTC7103 includes a selectable precision internal feedback divider, eliminating the need for external precision resistors. The digital state of the V_PRG1 and V_PRG2 pins sets the output voltage to one of the nine fixed options between 1.0 V and 15 V. Note that the CN0509 sets the output to 12 V by tying the V_PRG1 pin to the INTV_CC pin and leaving the V_PRG2 pin open.

The CN0509 takes advantage of the ability of the LTC7103 to operate in passthrough mode when the input voltage is between 4.4 V and 12 V (see Figure 5). The isolated flyback stage following the LTC7103 is optimized for 12 V but continues to operate down to voltages as low as 5 V with a reduced output current capability. Passthrough operation allows the circuit to continue to operate as long as possible, even as the voltages of the emergency power source begins to drop.

The architecture of the LTC7103 provides inherent protection against short-circuit conditions without the need for folding back the output current or the oscillator frequency. This protection is made possible because the pulse-width modulation (PWM) comparator is continuously receiving inductor current information from the average current amplifier. This results in automatic cycle skipping under short-circuit conditions if the minimum on time of the top switch is too long to maintain control of the inductor current at the full switching frequency. Typical current-limit operation is shown in Figure 6.

LT8302 Flyback Converter

The buck stage is followed by an LT8302 micropower, no opto, isolated flyback converter. The LT8302 maintains regulation by indirectly sensing the output voltage by sampling the primary side flyback waveform, eliminating the need for optocouplers or a third sense winding on the coupled inductor.

Isolation is essential in this application because the polarity and ground connections of a makeshift power source may not be known or may inherently be incorrect for a charging application by design. A −48 V telecommunications supply is a common example (see Figure 7).

Telecommunication supplies are negative with respect to Earth ground to prevent galvanic corrosion of wires. Therefore, a charger based on a nonisolated buck converter ties the outer case of a cell phone to −48 V, posing a hazard if the case comes into contact with a grounded object. Similar situations can occur with improperly wired solar panels or generators.

In addition to providing isolation, the LT8302 further steps down the 12 V output from the buck converter to 5 V. V_OUT is programmed using two external resistors and a third optional temperature compensation resistor as follows:

where:
R_FB is the LT8302 feedback resistor.
R_REF is the LT8302 reference resistor.
N_PS is the transformer effective primary to secondary turns ratio.
V_OUT is the output voltage.
V_F(T0) is the output diode forward voltage at 25°C = ~0.3 V.
V_REF is the LT8302 internal voltage reference.

Figure 8 shows the typical maximum output power at 5 V V_OUT for common winding ratio values at various input voltages. The CN0509 transformer has a 3:1 turns ratio for a maximum V_OUT of approximately 10 W.

Fast Charging Up to 2 A

USBs have become a defacto standard for device charging, and typical chargers can deliver more than the minimum 500 mA USB 2.0 specification. Chargers must provide a special voltage signature on USB data lines to let the device recognize itself and to determine the maximum charging current it can draw from the power source, which can be higher than the 500 mA minimum standard.

U3 is a DCP controller that monitors the USB data line voltages (D+ and D−) and provides signatures for enabling fast charging modes for several common device manufacturers (see Figure 9). While both ports can be used simultaneously up to a maximum total load current of 2 A, when using the DCP port, leaving the other port disconnected is recommended.

Note that the quality of USB cables varies significantly. Long, small gauge cables can result in significant voltage drop at the load.

System Performance

CN0509 operation is nearly constant for any input voltage between 12 V and 100 V because the input to the isolated converter is at a constant 12 V. Lower input voltage reduces the available charging current according to Figure 10.

Reverse Input Survival

The CN0509 is able to survive reverse input connections up to 100 V. Figure 11 shows the reverse input to the CN0509 vs. the LTC7103V_IN.

Load Regulation

Figure 12 shows that the load regulation of the CN0509 across input voltages above 12 V is within 65 mV because the load increases from 0.1 A to 2 A, corresponding to an output impedance of approximately 32.3 mΩ.

Load Connection Transient

Figure 13 and Figure 14 show the CN0509 turn on transient curves for the current and USB data lines for a cell phone (Phone A) and USB power bank (Power Bank B), respectively.

Thermal Performance

Figure 15 displays the thermal response of CN0509 while charging a load at 5.62 V, 2 A for one hour with the board positioned horizontally above a workbench in still air at 25°C ambient. The maximum temperature on the EVAL-CN0509-EBZ board occurs on the D3 diode at 83.8°C, well below the maximum operating temperature of 150°C.