DN06022/D Design Note – DN06022/D Tracking boost Regulator Device Application Input Voltage Output Power Topology I/O Isolation CS5171 General Up to 30V Circuit dependant Modified Boost no Other Specifications Output Voltage Output 1 Output 2 Output 3 Output 4 Vin +10V N/A N/A N/A Minimum Efficiency 70% Operating Temp. Range -40C to +125 °C (Vout − Vin − Vbe(Q1A)) = R2 1mA Circuit Description Adding a current mirror circuit to a typical boost circuit allows the user to select the amount of boost voltage and ensure a constant difference between input and output voltage. This is useful for high side drive applications where a simple voltage doubling circuit is not practical, either because of the voltage range of the components involved or where the input voltage can vary widely. This circuit can also be used at the front end of a power supply to ensure that the PWM controller has enough voltage to start correctly in low input voltage conditions. The schematic shown will maintain a 10V difference between Vin and Vout and is easily changed to provide other voltages. The PWM in the design example is the CS5171. However, this circuit can be used with any boost controller or regulator. The current mirror circuit, comprising the dual PNP transistor Q1 and the associated resistors, establishes a current that depends on the voltage difference between Vin and Vout. The dual PNP transistor NST30010MXV6 has a Vceo of 30 V so it is used in this example. If higher output voltages are required a device such as the BC856BDW1T1G has a Vceo of 65 V. In this example Vin =14V nominal, so Vout is 10V greater, i.e., 24V nominal. We begin by selecting a reference current in Q1A. We can assign an arbitrary value of 1 mA. Next calculate a value for R2 to establish the reference current. With a reference current of 1mA we get: May 2007, Rev. 2 ( 24V − 14V − 0.6V ) = 9 .4 K Ω 1mA Since the output voltage is not critical we use a 10K Ohm resistor. This current gets mirrored in Q1B and sets up the feedback voltage to the PWM. The CS5171 has an internal voltage of 1.27V typical, so we choose R3 to give the correct feedback voltage when the current flowing through it is 1mA. In this case, R3 is 1.27K. This gives an output voltage of 24V with Vin =14V. As Vin varies, Vout will track it while maintaining a 10V difference between the input and output. R4 is added to help reduce the power dissipation in Q1B. As the input voltage varies, Vout will remain a constant 10V above the input voltage. The only limitation is the breakdown voltage of the regulator and transistors used. In this case, since the CS5171 has a Vsw (switch input voltage) maximum of 40V, the VRRM of the Schottky rectifier used is 40V, and the Vceo of the transistor pair is 30 V the input voltage should be kept below 20V. Key Features • Simple, low cost circuit to ensure a constant boost voltage • Easily adaptable for any boost circuit. Easy to modify on demo board • Uses off the shelf components. www.onsemi.com 1 DN06022/D Schematic Q1A May 2007, Rev. 2 www.onsemi.com Q1B 2 DN06022/D 1 ID C1 C2 R1 R2 R3 R4 Q1A &Q1B L1 U1 D1 Bill of Material Part Number Description Capacitor Capacitor Resistor Resistor Resistor Resistor Dual PNP Inductor Boost Controller Schottky Rectifier NST30010MXV6 TP4_220 CS5171 MBRS140T3 Value 22uF 22uF 1k 10k 1.27k 8.2k 30V Transistor 22uH 40V 1A Vendor ON Semiconductor Coiltronics ON Semiconductor ON Semiconductor 1 © 2007 ON Semiconductor. Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its products at any time, without notice. Design note created by Kieran O’Malley, e-mail: [email protected] May 2007, Rev. 2 www.onsemi.com 3