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AOZ1214 Datasheet(PDF) 11 Page - Alpha & Omega Semiconductors |
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AOZ1214 Datasheet(HTML) 11 Page - Alpha & Omega Semiconductors |
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11 / 16 page  AOZ1214 Rev. 1.0 December 2010 www.aosmd.com Page 11 of 16 The strategy for choosing RC and CC is to set the cross over frequency with RC and set the compensator zero with CC. Using selected crossover frequency, fC, to calculate RC: where;o fC is desired crossover frequency, VFB is 0.8V, GEA is the error amplifier transconductance, which is 200x10-6 A/V, and GCS is the current sense circuit transconductance, which is 5.64 A/V. The compensation capacitor CC and resistor RC together make a zero. This zero is put somewhere close to the dominate pole fp1 but lower than 1/5 of selected crossover frequency. CC can is selected by: Equation above can also be simplified to: Table 2. Recommended Parameters An easy-to-use application software which helps to design and simulate the compensation loop can be found at www.aosmd.com. Linear Regulator Design Adjustable Output Voltage The output voltage can be set by feeding back the output to FB2 pin with a resistor divider network. In the application circuit shown in Figure 1. The linear regulator output voltage can be obtained using the following equation: R4 should be less than 10 kohm to avoid bias current errors. External NPN Pass Transistor or MOSFET Both Transistor and MOSFET can be used as an external follower. Some cautions must be noticed: 1. The transistor and MOSFET should be able to supply maximum operating current for the linear regulator supply. 2. DC current gain hFE must be large enough so that the pass transistor and supply the maximum load current with 30 mA with base current. However, too big hFE may cause the LDO sensitive to current noise, and a compromised DC current gain transistor should be selected. 3. The total power dissipation should not be higher than the rated value. 4. Comparing with transistor, MOSFET has lower dropout voltage which is RDS(ON) times the output current. But the minimum input voltage will increase to VO plus VGS while transistor is VO plus VBE. Linear Regulator Output Capacitor The linear regulator requires using an output capacitor as part of the frequency compensation network, which affects the stability and high frequency response. The regulator has a finite band width. For high frequency transient loads, recovery from transient is determined by both output capacitor and the bandwidth of the regulator. A minimum output capacitor of 4.7 μF is recommended to prevent oscillations and provide good transient response. The ESR value should be maintained in the range that determines the loop stability. When small signal ringing Vin VO 1.2V 3.3V 5V 12V L = 1.5 μH–3.3μH RC = 10KΩ CC = 6.8nF L = 4.7 μH–10μH RC = 30KΩ CC = 2.2nF L = 6.8 μH–15μH RC = 30KΩ CC = 3.3nF 20V L = 1.5 μH RC = 10KΩ CC = 6.8nF L = 4.7 μH–10μH RC = 30KΩ CC = 2.2nF L = 6.8uH–15 μH RC = 30KΩ CC = 3.3nF R C f C V O V FB ---------- 2 π C O × G EA G CS × ------------------------------ × × = C C 1.5 2 π R C f p 1 × × ----------------------------------- = C C C O R L × R C --------------------- = V OUT 2 0.8 1 R 3 R 4 -------- + ⎝⎠ ⎛⎞ × = |
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