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MAX1729 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX1729 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page 2) Given the maximum output voltage (VMAX) and mini- mum output voltage (VMIN), calculate values for R3 and R4 as follows: 3) For first-order temperature compensation, calculate R5 as shown below. (If temperature compensation is not used, leave R5 open.) where Tempco is the negative temperature coefficient needed to compensate the ECB or LCD display for changes in temperature. 4) Solve for VCOMP. The duty cycle used here corre- sponds to the duty cycle that yields the maximum out- put voltage, not including first-order temperature compensation. where a 90% duty cycle corresponds to Duty Cycle = 0.9. 5) Use the results from the above calculations to solve for R2. (For applications not utilizing temperature com- pensation, use 1 / R5 = 0.) External Component Value Example The example application requires the output voltage to adjust between 5V and 10V, using the circuit shown in Figure 3. The device in our example needs a tempera- ture coefficient of 33mV/°C, which yields the following results. 1) VMAX = 10V and IFB = 29.24µA is within the limits and yields a reasonable resistor value, therefore: 2) VMAX = 10V and VMIN = 5V, therefore: with R3 = 36.7k Ω, then VMIN = 5.019V. Let R4 = R3 = 36.7k Ω. 3) Tempco = 33mV/°C, therefore: 4) If external circuitry limits the duty cycle to 90%, the following equation is true: 5) Solving for R2: With R2 = 56k Ω, a duty cycle of 87.4% generates a VOUT of 10V. Component Selection Inductors Use a 220µH inductor to maximize output current (2.5mA typical). Use an inductor with DC resistance less than 10 Ω and a saturation current exceeding 35mA. For lower peak inductor current, use a 470µH inductor with DC resistance less than 20 Ω and a satu- ration current over 18mA. This limits output current to typically less than 1mA. See Table 1 for a list of recom- mended inductors. The inductor should be connected from the battery to the LX pin, as close to the IC as pos- sible. Capacitors The equivalent series resistance (ESR) of output capac- itor C2 directly affects output ripple. To minimize output ripple, use a low-ESR capacitor. A physically smaller capacitor, such as a common ceramic capacitor, mini- mizes board space and cost while creating an output ripple that’s acceptable in most applications. Refer to Table 2 for recommended capacitor values. 1 R2 V R1 V R3 V R5 1 V 1 R1 1 R3 1 R5 1 56560 OUT COMP FB FB =+ + −+ + = V 1.228 1 0.9 2 0.6754V COMP =− = R5 300k 33mV/ C = ° °= Ω Ω / 16. mV k 5 150 C R3 1/2 300k 5V 1.228 36,840 = = Ω Ω R1 10V 1.228V 29.24 A = − µ = 300kΩ 1 R2 1 V V R1 V R3 V R5 1 R1 1 R3 1 R5 FB OUT COMP FB =+ + −+ + V =V 1 – Duty Cycle R4 R3 R4 COMP FB ⋅ + R5 = R1 Tempco 16.5mV/ C ° R3 = 1/2 R1 V – V V R4 = R3 MAX MIN FB ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation _______________________________________________________________________________________ 9 |
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