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New Technology Solutions: Combining
The Best of LDOs and Switchers
T
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
devices.
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
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VOLUME
5
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
plot.
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.
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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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