• Transmitter Power Supply
• Laptop PC Battery Charger
• Portable PC Printer (HP DJ400) Power Supply
• Camcorder Battery charger
• Power supply for any other mobile device requiring more than 12 Volts

CIRCUIT DESCRIPTION
The presented switching regulator is a step-up or "boost" regulator. The boost circuit stores energy in the inductor and then delivers this stored energy along with the energy from the input to the load. When the switch is closed (MOSFET conducting), current flows trough the inductor and the switch, charging the inductor but delivering no current to the load. When the switch is open, the voltage across the load equals the DC input voltage plus the charge stored in the inductor. The inductor discharges, delivering current to the load. A TL497 regulator was chosen for this step-up converter. It's primary feature is design simplicity. The controller contains all active elements required for constructing a single-ended DC to DC converter. Basic configuration uses only six external components; three resistors, two capacitors and one inductor. Fig.1

The TL497 is a fixed ON-Time, variable-frequency controller. The ON-Time is controlled by an external (C1) capacitor connected to frequency control pin 3 and ground. Two series resistors (R2, R3) control the ouput voltage in parallel with the supply output. The resistance ratios are calculated to supply 1.2 V to pin 1 (comparator input) at the desired output voltage. The current limit is adjustable and is set by a single sense resistor between pins 13 and 14. The current limit threshold 0.5 V is developed across R1 resistor. For a Ni-Cad charger, calculate R1 for maximum charging current. Switching an additional parallel resistor with a switch will provide two different current limits.

DESIGN CONSIDERATIONS
High frequency circuit layout is necessary. Keeps lead as short as possible and use a single ground. Resistors R2 and R3 should be as close as possible to pin 1 to eliminate noise pick-up in the feedback loop. All high current loops should be kept to a minimum length using adequate lead size. Capacitor C3 passes high frequency noise to ground. Due to the broad frequency range of operation, high "Q" inductors are not desired. A toroid inductor is preferred. Without an external power switch (transistor, MosFet), the basic configuration is limited to Ipk=500mA (peak switching current), which translates to about 120mA load current. This project will use the extended power configuration with an external switching transistor and a diode shown in Fig.2
T2 transistor, D2 diode and R5, R6 resistors represent the MosFet driver section.

DESIGN CALCULATION EXAMPLE
Voltage Input Vin= 12V
Desired voltage output Vout= 16V
Maximum output current Ild= 0.8A
Maximum ripple voltage Vrp= 0.05V
Assume on-time Ton= 20uS
Calculations:

Peak switching current Ipk=2*0.8*(1+(16/12))=3.73
Inductor inductance l=(12/3.73)*20=64uH

Current limiting resistor R1=0.5V/Ipk=0.5/3.73=0.13 Ohm 2Watt
Voltage output resistor R2=(Vout-1.2)=16-1.2=14.8 kOhm

C1=12*20=240pF
C2=(3.73-0.8)^2)/(0.05*2*3.73)*((20*12)/16)=345uF round-up to 470uF/25V
Download a Components Calculation Spreadsheet (MS Excel) file, which will assist in all calculations.

PARTS LIST
R1= calculate current limit resistor
R2= calculate voltage output resistor
R3= 1.2kOhm 0.25W
R4= 10 Ohm 0.25W
R5,R6= 1kOhm 0.25W

Switching frequency capacitor C1= calculated value
Output filter capacitor C2= calculated value
C3= 0.01uF/50V ceramic
L= calculated value (in our example choose 68uH Digi-Key PN:M9798-ND)
D1= 1N5821 Schottky diode (choose 1.5 x Vout and Iout)
D2= 1N4148
T1= MosFet ie. IRF531, BUZ71A (choose low Rds)
T2= 2N3904, PN2222

THESE PROJECTS ARE PROVIDED 'AS-IS', WITH NO WARRANTY OF ANY KIND. USE OF THESE CIRCUITS SHALL BE ENTIRELY AT THE USER'S OWN RISK.