New Technology Solutions Combining The Best of LDOs and Switchers by ghostface


									 New Technology Solutions: Combining
 The Best of LDOs and Switchers

                                                     wo popular solutions that are
                                                     commonly used in power
                                                     electronics are linear regulators
                                                     and switching regulators. This
                                          article discusses current solutions on the
                                          market and then presents new technology
                                          alternatives to typical LDO and switching

                                          Linear Regulators

                                          A linear regulator is a dissipative step-down
                                          power regulator. This initial description
                                          sounds wasteful because a linear regulator
                                          will literally convert a higher input voltage
                                          to a lower output voltage by dissipating
                                          power through an active component. This
                                          component is usually a bipolar junction
                                          transistor (BJT). The output voltage is set
                                          by the feedback resistor network (R1 and
                                          R2), which tells the error amplifier (EA) how
                                          much power to dissipate in order to get the
                                          desired output voltage.

                                          Linear regulators are simple, offer good
                                          transient performance, have very low output
                                          noise and ripple, and can be inexpensive.
                                          However, one huge drawback inherent to
                                          linear regulators is efficiency. The efficiency
                                          of a linear regulator depends largely upon
                                          the input to output voltage ratio. If the input
                                          voltage is much higher than the output
                                          voltage, then more voltage needs to be
                                          dropped across the BJT. The BJT acts
                                          as a variable resistor to actively manage
                                          the voltage drop (Vdrop). Since current
                                          from input to output goes through the BJT
                                          continuously, the input current is about
                                          equal to the output current, neglecting
                                          some losses in the control circuitry. For
                    By Brian Huang,       simplicity’s sake, assume the input current
                  Product Marketing       is equal to the output current. To calculate
                         Micrel, Inc.     the efficiency of a linear regulator, divide the
                                          output power by the input power:

                  Linear Regulator Efficiency   = Output Power / Input Power
                                                = Vout * Iout / Vin * Iin (Assume Iin = Iout)
                                                =Vout / Vin

Switching Regulators                             Hyper Light Load ModeTM

A switching regulator converts power             The Micrel MIC23050/MIC23051 Buck
much more efficiently than a linear              Regulator is one of the most advanced
regulator because it utilizes the electrical     switching regulators for portable
properties of inductance and capacitance         applications using the trademark switching
to store and transfer energy. There are          scheme known as Hyper Light LoadTM.
three main types of switching regulator          The “Hyper” refers to the ultra-fast load
configurations — the buck (step-down),           transient response. The “Light Load”
the boost (step-up) and the fly-back (buck       means that the devices are very efficient
and boost) regulator. In this article we         at light loads. Hyper Light LoadTM was
will mainly focus on the buck switching          developed to fill a need in the portable
regulator.                                       electronics market where efficiency and            Figure 3. MIC23050 Load Transient.
                                                 fast transient performance is a must.
The buck regulator converts a higher                                                           The MIC23050/51 load transient response
input voltage to a lower output voltage          “Hyper”                                       is ultra-fast because any change in the
just like a linear regulator. The difference                                                   feedback is immediately compared and
is that it uses a pair of transistors (BJTs or   The MIC23050/51 uses an error                 outputted to the control circuitry. The
MOSFETs) and an inductor to alternately          comparator that compares the feedback         main difference between MIC23050/51
deliver energy to the output.                    voltage ripple with an internal band gap      and other switching regulators is that it

A typical constant frequency, pulse-
width-modulated (PWM), buck regulator
can convert a 3.6V input voltage to a
1.8V output voltage at over 90 percent
efficiency under “optimum output current
conditions.” Unfortunately, typical PWM
buck regulators are not 90 percent
efficient throughout the entire output
current range. At light loads, the PWM
buck regulator will continue to switch no
matter what the output current is. Due
to losses in non-ideal switches (BJT
and MOSFET), the efficiency of a typical
PWM buck regulator suffers at light loads.
Figure 1 shows an efficiency comparison

                                                                             Figure 2. MIC23051 Hyper Light LoadTM
                                                                                         Block Diagram

                                                 voltage. By only regulating the off-time,
                                                 a single error comparator can control the     does not have an Error Amplifier before
                                                 output. See Figure 2.                         the comparator and saves time needed to
                                                                                               charge the compensation capacitor often
                                                 As shown in Figure 2, the feedback            at the output of the Error Amplifier. Not
                                                 voltage ripple is compared to the band        using the Error Amplifier removes an extra
                                                 gap reference voltage by the Error            block in the control loop and reduces
                                                 Comparator. The regulation of the device      the amount of time it takes to respond to
                                                 depends upon how long Q2 stays on or          change. As a result, the load transient
     Figure 1. Efficiency Comparison.            off.                                          response of MIC23050/51 is unmatched,
                                                                                               as shown by Figure 3.

                                          Figure 4. Efficiency Comparison with Hyper Light LoadTM

 “Light Load”                                   by the load. After the on-time, everything          ILOAD < (Vin-Vout) * D / 2Lf
                                                is turned off in the control loop except
 The Hyper Light LoadTM has two modes           the band gap and the comparator. This               At higher output currents the MIC23050/51
 of operation. At low output currents           saves power during off-time. As the                 switches at around 4MHz and maintains
 (discontinuous mode) it is governed by         output voltage slowly decreases, it is              high efficiency like most switching
 pulse frequency modulation (PFM). At           being compared to the band gap voltage.             regulators (except it controls the off-time
 higher output currents (continuous mode)       Once it is below the band gap voltage, the          instead of the duty cycle). Refer to Figure 4
 it is governed by a constant-on-time,          comparator immediately tells the control            for the updated performance comparison.
 controlled off-time, control scheme. The       loop to turn the Q1 transistor on again.
 combined control method is what allows         This control method uses PFM mode to                Conclusion
 the MIC23050/51 to be efficient under all      vary the switching frequency depending
 load conditions.                               on the output current. If the output current        There will always be innovation to create
                                                decreases, the frequency decreases and if           the best power converter in power
 Typical constant frequency PWM buck            the output current increases, the frequency         electronics. As the demand for smaller,
 regulators have been shown to be less          increases.                                          faster, more efficient, less noisy, easier to
 efficient at light loads due to switching       This reduces excessive switching and               use and cheaper regulators grow, there
 losses. In order to improve light load         reduces power loss. The reduced                     will be innovators to fill the need. The
 efficiency, at low output currents the Hyper   switching and the power saved from                  MIC23050/51 is a modern buck regulator
 Light LoadTM becomes pulse frequency           turning off most of the device saves power.         designed specifically to fit into today’s
 modulated (PFM). Since the output current      This makes the MIC23050/51 efficient,               demand. Micrel’s Hyper Light LoadTM
 is low, the output capacitor can maintain      even at light loads. The formula to calculate       switching scheme created a new standard
 the voltage longer during the off cycle.       when PFM mode takes place is:                       for other switching regulators to follow.
 During the on-time, the output voltage
 increases, but is slowly being pulled down
                                                                                                     Note: Hyper Light Load is a trademark of Micrel, Inc.


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