1/5 1 - introduction the tsm108 is a step down controller that features constant voltage and constant current regulation. it can drive either a p-channel mosfet or a pnp bipolar transistor. the principle of operation of the tsm108 is widely described in the application note attached to the datasheet. the goal of this document is to describe the tsm108 evaluation board. the evaluation board was designed to allow different converter configurations. several options for power semiconductor footprints are available on the pcb in order to make it adaptable to a wide range of converter output power. in a first step, this application note will describe the typical application. then the various options available on the evaluation board pcb will be explained. 2 - tsm108 typical application 2.1 description the tsm108 is especially suited for cigarette lighter accessories. the typical application described in this note is a battery charger with the following characteristics: input voltage: vin = 12v output voltage: vout = 6v maximum output current: iout = 800ma a picture of the evaluation board in the typical application configuration is shown on figure 1. the related schematic is described on figure 2. figure 1 : tsm108 evaluation board AN1695 application note tsm108 evaluation board 12v to 6v dc/dc converter january 2003 by anthony boimond
AN1695 - application note 2/5 figure 2 : typical application schematic the list of components used in the typical application board is shown in the bill of material here below. all other components are not mounted. passive components like capacitors and resistors used in this evaluation board are through-hole components to be more easily changed by the user. of course, in real-life application, surface-mounted devices will be used and the whole application can fit into a very small pcb area. table 3 : bill of material reference value description manufacturer u1 tsm108id pwm controller stmicroelectronics q4 sts3dpfs30 mosfet + diode stmicroelectronics l1 p0250.154t 150h - buck main inductor pulse l short circuit filtering inductor c1 220pf oscillator capacitor c2-c3 22nf compensation capacitors c4-c7 47f-25v buck input and output capacitors rubycon zl c10 100nf voltage reference bias capacitor r8-r17-r18-r19 1 w shunt resistor r6-r7 22k w -10% compensation resistors r9 15k w -1% divider bridge upper resistor r10 11k w -1% divider bridge lower resistor r16-r22 short circuit 13 12 9 11 10 8 14 7 2 4 5 1 3 vs ictrl icomp vcomp vref vctrl 200mv 2,52v gnd 6 g vcc !stby uv ov uv/ov/!stby osc gd tsm108 vin vout l1 u1 q4-b q4-a r7 r6 c3 c2 c4 c7 r9 r10 c10 c1 r17 r8 r18 r19
AN1695 - application note 3/5 2.2 performances as explained in the datasheet, the step-down converter using tsm108 can feature an accurate constant-voltage and constant-current regulation. the v/i output characteristics of the dc/dc converter is shown in figure 4. input voltage was set at 12v during this test. as explained in introduction, voltage regulation is set to 6v and current regulation is set to 800ma. at output voltage lower than 1v, the foldback can be observed in the current regulation. figure 4 : output voltage versus output current characteristics the efficiency of the dc/dc converter was measured in different conditions, as shown in figure 5. on the left chart, the influence of output power (pout) over efficiency is studied. it should be noted that during this test, the converter was operating in constant-voltage mode, therefore with vout=6v and iout<800ma. input voltage vin was equal to 12v during the test. the efficiency appears to be always between 84% and 90%. on the right chart, the influence of input voltage (vin) over efficiency is studied. the converter was operating in constant-voltage mode and the output power was constant equal to 4w during the test. with the input voltage varying from 7v up to 25v, the converter efficiency appears to be between 84% and 90%. figure 5 : efficiency of the dc/dc converter versus output power and input voltage 0 1 2 3 4 5 6 7 0 100 200 300 400 500 600 700 800 900 iout (ma) vout (v) efficiency vs. output power @ vout=6v vin=12v 82% 84% 86% 88% 90% 92% 94% 0 1000 2000 3000 4000 pout (mw) efficiency vs. input voltage @ vout=6v pout=4,0w 82% 84% 86% 88% 90% 92% 94% 510152025 vin (v)
AN1695 - application note 4/5 3 - evaluation board description the tsm108 evaluation board is easily adaptable to a wide range of output power. several options are available on the board for the power semiconductor packages. figure 4 shows the electrical connections of all the components available on the evaluation board printed circuit board. figure 6 : evaluation board complete schematic figure 7 : evaluation board top layer silk screen and bottom layer overview 13 12 9 11 10 8 14 7 2 4 5 1 3 vs ictr l icomp vcomp vref vctrl gnd 6 g vcc !stby uv ov osc gd tsm108 vin vout l1 u1 q4-b q4-a r7 r6 c3 c2 c4 c14 r8 r9 r10 c10 c1 c5 r1 r2 r3 r4 c16 c17 c9 c8 r22 r20 r16 r21 r12 r17 r18 r19 r23 c11 c13 l q1 q3 d1 q2 r13 c12 r15 r11 vres1 c15 r14 c6 c7
AN1695 - application note 5/5 information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result f rom its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specificati ons mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectronics. ? the st logo is a registered trademark of stmicroelectronics ? 2003 stmicroelectronics - printed in italy - all rights reserved stmicroelectronics group of companies australia - brazil - china - finland - france - germany - hong kong - india - italy - japan - malaysia - malta - morocco singapore - spain - sweden - switzerland - united kingdom ? http://www.st.com the typical application described previously uses a power mosfet and a schottky diode integrated in the same so8 package, but several options for power semiconductor footprints are available on the pcb in order to make it adaptable to a wide range of converter output power. the power switch can be replaced by a mosfet in to220 or d2 pak package, but also by a bipolar in to92 pack age. the diode can be replaced by an axial or to220 package. the various options available on the evaluation board are presented in table 8 here below. table 8 : power semiconductors options available on the evaluation board component designator package pinout (topview) power switch (p-mosfet) q1 to220 q3 d2pak q4-b half so8 power switch (pnp bipolar) q2 to92 power diode d1 to220 or axial q4-a half so8 d gs d g s 1 2 3 4 5 6 7 8 b ce k a 1 2 3 4 5 6 7 8
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