MA MAILM MAX759 LCD Power Supply Evaluation Kit General Description Features The MAX759 LCD power supply evaluation kit (EV kit) is @ Fully Assembled Surface-Mount EV Kit a special application circuit that uses the MAX759 invert- @ -6V to -24V Adjustable Output ing DC-DC converter to generate the variable negative @ 80% C. ion Effici voltage needed by liquid crystal displays (LCDs). The kit Conversion Etticiency is fully assembled using surface-mounted components. # 40mA Output Current Apotentiometer mounted on the board allows adjustment of the output voltage over a -6V to -24V range. . . P g 9g Ordering Information PART TEMP. RANGE BOARD TYPE MAX759LCDKIT-SO 0C to +70 Surface-Mount Component List DESIGNATION QTY DESCRIPTION MANUFACTURER Ut 1 MAX759CPD None 1 2.0" x 3.0" printed circuit board None 1 MAX759 data sheet C1 1 150yF low-ESR tantalum capacitor Sprague 595D157X0016R7 C2, C4 2 0.1pF chip capacitor C3, C5 2 0.01pF chip capacitor C6, C7 2 33yF low-ESR tantalum capacitor Sprague 595D336X0035R7 D1 1 1N5819 diode, 40V NIEC EC10QS04 u1 1 50uH SMT transformer Coiltronics CTX50-1 R1 1 10kQ. 5% resistor R2 1 6.81kQ 1% resistor R3 1 100kQ single-turn potentiometer Bourns 3352T-104-ND R4 1 33.2kQ 1% resistor Bourns: Phone (714) 781-5500, FAX (714) 781-5273 Coiltronics: Phone (305) 781-8900 NIEC (Tokyo): Phone 81-3-3494-7411, FAX 81-3-3494-7414 Sprague: Phone (516) 746-1385 MAXIM 1 Call toll free 1-800-998-8800 for free samples or literature. uM AZ Aiddns agd1 6SZXVNMAX759 LCD Supply EV Kit MAX759 LCD Power Supply Evaluation Kit Detailed Description Input Supply Requirements The MAX759 LCD power supply EV kit has a 4.5V to 6V input range. The lower input limit is due to the minimum operating voltage of the MAX759. The maximum input voltage is limited by the absolute maximum rating of the differential voltage between the V+ and LX pins with some margin for leakage inductance and turns ratio error. You can determine the maximum input current require- ment by calculating the maximum output power levels. Formula 1 shows the relationship between output power and input power with 80% efficiency. Formula 2 is the result of expanding the first equation and solving for IIN. Assuming a 5V input and a 24V, 40mA load, the input current can then be calculated. (1) Pout / PIN = 80% (2) WN = (lout x Vout) / (0.80 x Vin) For VouT = -24V, lout = 40mA, and Vin = 5.0V: II = (40mA x 24V) / (0.80 x 5.0V) = 240mA Note that the current is an average value. Peak currents are much higher during the inductor charging cycle. The capacitor between V+ and ground should be a low-ESR type, located as close to the MAX759 as possible. Use the EV kit layout as a guide. Output Circuit The output circuit employs an autotransformer to limit the voltage swing on the MAX759s LX pin while producing the -24V output voltage. The voltage at the center tap of the transformer is one-half of the full output voltage. This keeps the input/output differential vollage below the 22V limit given in the MAX759 data sheet Absolute Maximum Ratings. The Schottky diode must have a breakdown voltage greater than 30V. At the maximum output voltage, the diode will be reversed biased by the combination of the -24V output and the +5V input supply voltage. The output voltage is determined by the resistors R2, R3, and R4, and the internal VREF voltage. Use Formula 3 to calculate the output voltage. R2 should be a fixed resistor in the 5kQ to 15kQ range. R4 sets the minimum output voltage when the potentiometer is adjusted to 0Q. The maximum output voltage is achieved when R@ is adjusted for maximum resistance. (3) Vout = -(VREF/R2) x (R3 + R4) When R3 is adjusted to zero: VouT = -(1.23V / 6.81kQ) (0Q + 33.2kQ) = -6.0V @ MAX759 LCD DRIVER EVKit @ I", . MAXIM AXTS9 ele cre E Cas 3 ee Rew = C2 SON CRE ' Rm [mee C6 eben [ . ; GND c7 e ten mere mek A e =m Figure 1. MAX759 LCD Power Supply EV Kit Component Placement Guide Figure 2. MAX759 LCD Power Supply EV Kit Component-Side Layout figure 3. MAX759 LCD Power Supply EV Kit Solder-Side ayout - MAAXLMAMAX759 LCD Power Supply Evaluation Kit When R3 is adjusted to maximum: Vout = (1.23V / 6.81kQ) (1O0KQ +33.2k2) = -24.1V Note that there are two 33uF output filter capacitors, which are necessary because of the lack of large-value capacitors with 35V breakdown in surface-mount pack- ages. Radial leaded low-ESR capacitors are preferred for applications built with through-hole component mounting. Shutdown Pin A 10kQ resistor pulls the shutdown (SHDN) pin to VIN for normal operation. The SHDN pin is connected to a pad near the edge of the card so the user may turn off the output voltage with an external signal. Some current will flow through R1 when SHDN is pulled to ground. The resistor is not needed in applications that strap SHDN to Vin or drive it with CMOS fogic. Vin % * 4.5V to OV pth te. in T [fea SS 10k \ Ve V+ 15 SHDN SHON Ve 2 3 MAAXIAN 14 1,3 u vREF mfay75g = |X CTX50-1 4 8 TING. ix 4 5 12 I NO. Mw 8 " Dt ostuk a NC. GND # in5e19 co }--+] 5 DRV- = 8 9 = v cc our 0s 0.01 pF R3 RA , 100k 93.2k sot ony q 6. [oa R2 co c7 | 30mA O1uF 6Bik SauF ST Sane Figure 4. MAX759 LCD EV Kit Schematic MAAKIMA 3M AZ Alddns G91 6SZXUVW