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LM21215AMHX-1/NOPB Datasheet(PDF) 21 Page - Texas Instruments

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Part No. LM21215AMHX-1/NOPB
Description  15-A Ultra High-Efficiency Synchronous Buck Converter
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM21215AMHX-1/NOPB Datasheet(HTML) 21 Page - Texas Instruments

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VIN
LF
COUT
RFB1
RFB2
CC2
CC1
RC1
VOUT
CC3
RC2
FB
+
Å
COMP
Driver
EA
+
Å
PWM
Error Amplifier and Compensation
RO
PWM Modulator
SW
RESR
Power Stage
0.6V
RDCR
LM21215A
www.ti.com
SNOSB87C – MARCH 2011 – REVISED JANUARY 2016
The highest input capacitor RMS current occurs at 50% duty cycle, at which point the RMS ripple current rating
should be greater than half the output current. Place low ESR ceramic capacitors in parallel with higher value
bulk capacitance to provide optimized input filtering for the regulator and damping to mitigate the effects of input
parasitic inductance resonating with high-Q ceramics. One bulk capacitor of sufficiently high current rating and
one or two 22-
μF 10-V X7R ceramic decoupling capacitors are usually sufficient. Select the input bulk capacitor
based on its ripple current rating and operating temperature.
When operating at low input voltages (3.3 V or lower), additional capacitance may be necessary to avoid
triggering an undervoltage condition during an output current transient. This depends on the impedance between
the input voltage supply and the LM21215A, as well as the magnitude and slew rate of the load transient.
The AVIN pin requires a 1-µF ceramic capacitor to AGND and a 1-
Ω resistor to PVIN. This RC network filters
inherent noise on PVIN from the sensitive analog circuitry connected to AVIN.
8.2.1.2.6
Control Loop Compensation
This section walks through the various steps in obtaining the open-loop transfer function. There are three main
blocks of a voltage-mode buck converter that the power supply designer must consider when designing the
control system: the power stage, the PWM modulator, and the compensated error amplifier. The control loop
architecture of a voltage-mode buck converter is provided in Figure 32.
Figure 32. Voltage-mode Buck Converter Architecture
The power stage consists of the filter inductor (LF) with DCR (DC resistance RDCR), output capacitor (COUT) with
ESR (effective series resistance RESR), and load resistance (RO). The LM21215A incorporates a high-bandwidth
error amplifier between the FB and COMP pins to achieve high loop bandwidth. The error amplifier (EA)
constantly regulates FB to 0.6 V. The compensation component network around the error amplifier establish
system stability. The modulator creates the duty cycle command by comparing the error amplifier output with an
internally-generated PWM ramp set at the switching frequency.
Copyright © 2011–2016, Texas Instruments Incorporated
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