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MIC2245 Datasheet(PDF) 10 Page - Micrel Semiconductor |
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MIC2245 Datasheet(HTML) 10 Page - Micrel Semiconductor |
10 / 14 page Micrel, Inc. MIC2245 January 2006 10 M9999-012406 www.micrel.com Functional Description VIN VIN provides power to the MOSFETs for the switch mode regulator section, along with the current limiting sensing. Due to the high switching speeds, a 1µF capacitor is recommended close to VIN and the power ground (PGND) pin for bypassing. Please refer to layout recommendations. AVIN Analog VIN (AVIN) provides power to the LDO section. AVIN and VIN must be tied together. Careful layout should be considered to ensure high frequency switching noise caused by VIN is reduced before reaching AVIN. LDO The LDO pin is the output of the linear regulator and should be connected to the output. In LOWQ mode (LOWQ<1.5V), the LDO provides the output voltage. In PWM mode (LOWQ>1.5V), the LDO pin is high impedance. EN The enable pin provides a logic level control of the output. In the off state, supply current of the device is greatly reduced (typically <1µA). Also, in the off state, the output drive is placed in a "tri-stated" condition, where both the high side P-channel Mosfet and the low-side N-channel are in an “off” or non-conducting state. Do not drive the enable pin above the supply voltage. LOWQ The LOWQ pin provides a logic level control between the internal PWM mode and the low noise linear regulator mode. With LOWQ pulled low (<0.5V), quiescent current of the device is greatly reduced by switching to a low noise linear regulator mode that has a typical IQ of 20µA. In linear (LDO) mode, the output can deliver 60mA of current to the output. By placing LOWQ high (>1.5V), this transitions the device into a constant frequency PWM buck regulator mode. This allows the device the ability to efficiently deliver up to 500mA of output current at the same output voltage. BIAS The BIAS pin supplies the power to the internal power to the control and reference circuitry. The bias is powered from the input voltage through an RC lowpass filter. The RC lowpass filter frequency must be ≥ ()( ) 100nF 20.5Ω 2π 1 . FB The feedback pin (FB) provides the control path to control the output. For adjustable versions, a resistor divider connecting the feedback to the output is used to adjust the desired output voltage. The output voltage is calculated as follows: VOUT = VREF × R1 R2 +1 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ where VREF is equal to 1.0V. A feedforward capacitor is recommended for most designs using the adjustable output voltage option. To reduce battery current draw, a 100K feedback resistor is recommended from the output to the FB pin (R1). Also, a feedforward capacitor should be connected between the output and feedback (across R1). The large resistor value and the parasitic capacitance of the FB pin can cause a high frequency pole that can reduce the overall system phase margin. By placing a feedforward capacitor, these effects can be significantly reduced. Typically an 82pF small ceramic capacitor is recommended. SW The switch (SW) pin connects directly to the inductor and provides the switching current necessary to operate in PWM mode. Due to the high speed switching on this pin, the switch node should be routed away from sensitive nodes. PGND Power ground (PGND) is the ground path for the high current PWM mode. The current loop for the power ground should be as small as possible and separate from the Analog ground (AGND) loop. Refer to the layout considerations for more details. AGND Signal ground (AGND) is the ground path for the biasing and control circuitry. The current loop for the signal ground should be separate from the Power ground (PGND) loop. Refer to the layout considerations for more details. |
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