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LM21215AMHE-1 Datasheet(PDF) 22 Page - Texas Instruments

Part No. LM21215AMHE-1
Description  15-A Ultra High-Efficiency Synchronous Buck Converter
Download  38 Pages
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM21215AMHE-1 Datasheet(HTML) 22 Page - Texas Instruments

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Z2
C
mid
P1
P2
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20
40
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-360
-320
-280
-240
-200
-160
-120
-80
-40
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FREQUENCY (HZ)
GAIN
PHASE
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OUT
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LM21215A
SNOSB87C – MARCH 2011 – REVISED JANUARY 2016
www.ti.com
There are three transfer functions that are taken into consideration when obtaining the total open-loop transfer
function; COMP-to-duty cycle (modulator), duty cycle-to-VOUT (power stage), and VOUT-to-COMP (compensator).
If
ΔVRAMP is the peak-to-peak ramp voltage (nominally 0.8 V), the COMP-to-duty cycle transfer function is simply
the PWM modulator gain given by Equation 10.
(10)
The duty cycle-to-output transfer function includes the filter inductor, output capacitor, and output load resistance.
The inductor and capacitor create a pair of complex poles at the LC tank frequency expressed by Equation 11.
(11)
In addition to two complex poles, a left half plane zero is created by the output capacitor ESR located at a
frequency described by Equation 12.
(12)
A Bode plot showing the –40dB/decade power stage response is shown in Figure 33
Figure 33. Power Stage Bode Plot
The complex poles created by the filter inductor and output capacitor cause a 180° phase lag. The phase is
boosted back up to –90° by the output capacitor ESR zero. The compensator must provide sufficient phase
boost to stabilize the loop response. The type-III compensation network shown around the error amplifier in
Figure 32 creates two poles, two zeros and a pole at the origin. Placing these poles and zeros at the correct
frequencies stabilizes the closed loop response. The compensator transfer function is given by Equation 13.
where
Kmid is the mid-band gain, RC1/RFB1
(13)
22
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