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ADP3334 Datasheet(PDF) 7 Page - Analog Devices |
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ADP3334 Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page REV. B ADP3334 –7– To have the lowest possible sensitivity of the output voltage to temperature variations, it is important that the value of the parallel resistance of R1 and R2 be kept as close as possible to 50 k W. RR RR k 12 12 50 ¥ + =W (1) Also, for the best accuracy over temperature, the feedback volt- age should be set for 1.178 V: VV R RR FB OUT =¥ + Ê ËÁ ˆ ¯˜ 2 12 (2) where VOUT is the desired output voltage and VFB is the virtual band gap voltage. Note that VFB does not actually appear at the FB pin due to loading by the internal PTAT current. Combining the above equations and solving for R1 and R2 gives the following formulas: Rk V V OUT FB 150 =¥ Ê ËÁ ˆ ¯˜ W (3) R k V V FB OUT 2 50 1 = - Ê ËÁ ˆ ¯˜ W (4) Table I. Feedback Resistor Selection VOUT (V ) R1 (1% Resistor) (k ) R2 (1% Resistor) (k ) 1.5 63.4 232.0 1.8 76.8 147.0 2.2 93.1 107.0 2.7 115.0 88.7 3.3 140.0 78.7 5.0 210.0 64.9 10.0 422.0 56.2 Using standard 1% values, as shown in Table I, will sacrifice some output voltage accuracy. To estimate the overall output voltage accuracy, it is necessary to take into account all sources of error. The accuracy given in the specifications table does not take into account the error introduced by the feedback resistor divider ratio or the error introduced by the parallel combination of the feedback resistors. The error in the parallel combination of the feedback resistors causes the reference to have a wider variation over temperature. To estimate the variation, calculate the worst-case error from 50 k W, and then use the graph in Figure 4 to estimate the additional change in the output voltage over the operating temperature range. For example: VIN = 5 V VOUT = 3.3 V R1 = 140 k W, 1% R2 = 78.7 k W, 1% Rp ERROR – % 3.0 2.5 2.0 1.5 1.0 0.5 0 02 3 456 Figure 4. Output Voltage Error vs. Parallel Resistance Error The actual output voltage can be calculated using the following equation. V. V R R V. V OUT OUT =¥ + Ê ËÁ ˆ ¯˜ = 1 178 1 2 1 3 274 (5) So worst-case error will occur when R1 has a –1% tolerance and R2 has a +1% tolerance. Recalculating the output voltage, the parallel resistance and error are: V. V . . V. V Resistor Divider Error . . OUT OUT =¥ + Ê ËÁ ˆ ¯˜ = =- Ê ËÁ ˆ ¯˜ ¥= - 1 178 138 6 79 5 1 3 232 3 232 33 1 100 2 1 %. % (6) R RR RR .. .. . k R Error . PARALLEL PARALLEL = ¥ + = ¥ + = =- Ê ËÁ ˆ ¯˜ ¥= 12 12 138 6 79 5 138 6 79 5 50 51 50 51 50 1 100 1 02 W %. % (7) So, from the graph in Figure 4, the output voltage error is estimated to be an additional 0.25%. The error budget is 1.8% (the initial output voltage accuracy over temperature), plus 2.1% (resistor divider error), plus 0.25% (parallel resis- tance error) for a worst-case total of 4.15%. Thermal Overload Protection The ADP3334 is protected against damage from excessive power dissipation by its thermal overload protection circuit, which limits the die temperature to a maximum of 165°C. Under extreme conditions (i.e., high ambient temperature and power dissipation) where die temperature starts to rise above 165°C, the output current is reduced until the die temperature has dropped to a safe level. The output current is restored when the die temperature is reduced. |
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