8/10/22, 5:59 PM
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`How to Generate Auxiliary Supplies from a Positive Buck DC-DC Converter - Maxim/Dallas
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`
`
`SDALLAS AVLALKLAVL [ttt(ty
`
`Maxim/Dallas > Designer's Library > App Notes > POWER-SUPPLY CIRCUITS
`Keywords: step-down, buck, transformer, flyback, SEPIC
`Related Parts
`
`APP 3740: Jan 10, 2006
`
`i.
`if=Download, PDF Format (219kB)
`
`APPLICATION NOTE 3740
`How to Generate Auxiliary Supplies from a Positive Buck DC-
`DC Converter
`
`Abstract: Many applications require a low-power supply in addition to the main supply. For reasons ofcost, inventory
`management, or electromagnetic compatibility (EMC), a separate converter may not be appropriate. Consequently,
`another meansof providing extra powerrails from the main supply is needed. This application note shows how to use a
`step-down IC converter’s switching action to derive one or more outputs, isolated or non-isolated, quasi-regulated or
`unregulated.
`
`Introduction
`
`Many applications require a low-power supply in addition to the main supply. A typical example is when an analog front-
`end amplifier needs +5V, while the main digital circuitry requires +5V only. For reasons of cost, inventory management,
`or EMC, a separate -5V converter may not be appropriate. Consequently, another means of providing extra power rails
`from the main supply is needed.
`
`As a solution to this problem, a step-down IC converter's switching action can be used to derive one or more outputs,
`isolated or non-isolated, quasi-regulated or unregulated. Auxiliary output currents of 10% to 30%of the main output are
`perfectly possible. This application note will illustrate this technique using the MAX5035 DC-DC converter.
`
`Step-Down Waveforms
`
`A review of the waveforms found in a working step-down converter will identify the voltage and currents that can be used
`to generate additional outputs. See Figure 1 below and Example 1 waveformsat the end ofthis article.
`
`|»Figure 1. The MAX5035 schematic illustrates step-down converter operation.
`Figure 1. The MAX5035 schematic illustrates step-down converter operation.
`
`There is a switching voltage waveform of amplitude at the LX pin:
`
`Vix = [Vin (max) to -V(diode)] < Vix < Vin(min) -V(diode)]
`
`The voltage across the main inductor during the power cycle (LX connected to Vyy) is:
`
`VinD = [Vy (max) - Vout] < VinD < [Vin(min) - Vout]
`
`Continuous Inductor Current Operation
`
`When the power switchis off, the voltage at the LX connection flies negative, turning on the diode, D1, to ensure that the
`inductor current continues to circulate. Operation is said to be continuous when the power cycle begins before the
`circulating current in D1 falls to zero (Figure 2).
`
`|.Figure 2. Continuous inductor current waveforms. TS = switching period; D = dutycycle.
`Figure 2. Continuous inductor current waveforms. TS = switching period; D = duty cycle.
`
`Knowing the various RMS currents and voltages associated with the key components, power dissipation can be calculated
`as follows:
`
`iz
`
`Definitions
`
`RON_SW-—Datasheet on-resistance of the internal power switch (Vj, to LX)
`RLOAD—Effective resistance connected at the power-supply output.
`IQUIESCENT—Quiescent current of the control IC with no switching action.
`IDIODE_RMS—Schottky diode (D1) forward RMS current.
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`How to Generate Auxiliary Supplies from a Positive Buck DC-DC Converter - Maxim/Dallas
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`VFORWARD—Forward voltage drop across Schottky diode, D1, at rated current.
`ILOAD_RMS—RMS load current.
`
`Auxiliary Outputs
`
`Auxiliary outputs can be added to the main step-down by an additional winding on its inductor. The additional output
`relies on flyback action in the main inductor during the time that the ‘catch’ Schottky diode (D1 in Fig1) is conducting.
`Because the diode voltage drop is relatively constant (300mV to 500myV,typically, depending on current), and because the
`controller regulates the output voltage, the inductor's voltage dropis also relatively constant during the OFF time of the
`power switch. For the voltage drop to remain consistent, the main inductor should be in continuous conduction throughout
`the main step-down load range.
`
`The LX pin can also be used to provide a switching input to a discrete charge-pump circuit. For this to remain consistent,
`the LX pin must be active wheneverthe additional output is required. You can keep the LX pin active by ensuring that the
`main step-down output supports a minimum load.
`
`Inductor Selection
`
`Three functions are needed to set the value of the main inductor: the voltage across the inductor, the operating
`frequency, and the inductor's current ripple. Together, these functions will ensure that adequate energy is stored in the
`inductor. The inductor's minimum value is determined by the maximum duty cycle and minimum input voltage, and is
`given by:
`FSal
`
`Ripple current is a percentage of output current, and defined as 30%for the MAX5035. Note that the ripple current sets
`the minimum load current before the onset of discontinuous operation. Because an auxiliary supply increases the peak-
`current requirements of the power switch, care must betaken to limit the auxiliary power drawn.
`
`For many applications, the Evaluation (EV) kit's standard setup of 100uUH and 68uF outputfilter values will be suitable.
`These values are retained for the additional supplies. The MAX5035 featuresfixed, internal type-3 compensation which
`imposes limitations on the choice of output capacitor. Chose the ESR so that the zero frequency occurs between 20kHz
`and 40kHz. See the application section of the MAX5035 data sheet for more information.
`
`Auxiliary Output Derived from the Main Inductor's Transformer
`
`The inductor's voltage drop is relatively constant during the power switch's OFF time, because the primary Schottky diode
`voltage dropis relatively constant (300mV to 500mv\V,typically, depending on current), and the controller regulates the
`output voltage. Connecting the secondaryrectifier and capacitor so that conduction occurs during the flyback period
`(diode ON), allows some energy to be tapped off the main inductor. Figures 3a and 3b show two versionsof this
`arrangement. Isolating the auxiliary winding from the main step-down allows flexible connection arrangements. Figure 3a
`showsthe auxiliary output referred to zero volts, and Figure 3b showsthe auxiliary output referred to the main positive
`output. See also waveforms in Examples 2a and 2b.
`|.Figure 3a. Transformer serves as the main inductor (auxiliary output referenced to zero volts. T1 = Cooper Bussmann
`DRQ125-101. (Note the DOT convention for the start of windings.)
`Figure 3a. Transformer serves as the main inductor (auxiliary output referenced to zero volts. T1 = Cooper Bussmann
`DRQ125-101. (Note the DOT convention for the start of windings.)
`
`|».Figure 3b. Transformer as main inductor (+ve auxiliary output referenced to main output). T1 = Cooper Bussmann
`DRQ125-101. (Note the DOT convention for the start of windings. )
`Figure 3b. Transformer as main inductor (+ve auxiliary output referenced to main output). T1 = Cooper Bussmann
`DRQ125-101. (Note the DOT convention for the start of windings.)
`
`Auxiliary output voltage is given by:
`
`Vaux = N2/N1 (Vout + Vorope1) - Vor1opE2
`
`N1 = primary turns and N2 = secondaryturns.
`
`This output in Figure 3 is independent of input-voltage changes, as D2 is ON when the internal LX power switch is OFF.
`Capacitor C7 should be chosen to support the output during the maximum on-time of the power switch. The secondary
`output suffers a 2% to 3%output variation as the forward voltage drop of D1 varies with temperature and load current.
`Since N1 and N2 of the transformer are DC-isolated from each another, the extra output may be referenced to any DC
`voltage.
`
`For a given inductor value, secondary power at the auxiliary output is limited by the onset of discontinuous currentin the
`main primary loop. Restated simply, Di must remain in conduction at the end of the flyback period. At the onset of
`discontinuous operation, conduction through D1 becomes zero, and the voltage at LX will show the characteristic decaying
`‘ring' at a frequency determined by the output inductance and thetotal stray capacitance at the LX node.
`
`Secondary loading causes a change of primary current at the point of transition when the internal LX switches from on to
`off. This current step shown in Figure 4 is given by:
`
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`How to Generate Auxiliary Supplies from a Positive Buck DC-DC Converter - Maxim/Dallas
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`Ixtra = Psec (D x Vix)
`D = duty cycle
`Poec = secondary power
`Vix = peak voltage excursion at LX
`
`In principle, there is much flexibility in the choice of turns ratio. However, in practice, the availability of standard 1:1
`transformers with suitable inductance and peak-current values makes this the most popular choice of turns ratio.
`
`|»Figure 4. Primary inductor current due to secondary loading.
`Figure 4. Primary inductor current due to secondary loading.
`
`Note how the additional loading produces changed primary ripple current. Bold lines identify simplified changes to the
`main-inductor current shape with active auxiliary output.
`
`Relative Advantages of this Approach
`
`1. Positive or negative auxiliary output
`2. Quasiregulated auxiliary output
`3. Isolated; can be referenced to ground or main positive output
`4. Inductance value set by main step-down
`5. Off-the-shelf magnetics (1:1 transformer ratio)
`
`Relative Disadvantages of this Approach
`
`1. Increased primary ripple current increases onset of discontinuous current
`2. Minimum load required on aux output
`3. Minimum load required on main positive output to maintain switching action at LX
`
`Negative Auxiliary Output Derived from a Charge Pump
`
`The LX terminal voltage excursion can be used as a source for a charge pump to generate an unregulated auxiliary
`negative output. The additional output is unregulated because the voltage at LX is not isolated from changes of Vyyj. The
`additional charge-pump components are shown in Figure 5. See also waveforms in Example 3.
`
`When the power switch closes at the start of the power cycle, current flows into C7 through D2 and R6 and begins to
`ramp in the inductor, L1. On the flyback cycle when D1 conducts, the charge on C7 is transferred to C8 and the load. R6 is
`an important addition, as it limits the peak current into C7. Without R6, the current limit of the power switch will be
`exceeded, causing premature termination of the power cycle and even shutdownon protected step-down converterslike
`the MAX5035. See Figure 6.
`|»Figure 5. Schematic for an auxiliary negative output derived from a charge pump.
`Figure 5. Schematic for an auxiliary negative output derived from a charge pump.
`
`|».Figure 6. Current waveform from an inductor and charge pump.
`Figure 6. Current waveform from an inductor and charge pump.
`
`The source impedance of the unregulated charge pump dueto R6 and C7is given by:
`FSraid
`
`Identifying the source impedance of the unregulated charge pump allows the designer to estimate the charge-pump
`output voltage under variable load conditions.
`
`The open-circuit, charge-pump auxiliary output voltage is given approximately by:
`Fsal
`
`The loaded charge-pump auxiliary output voltage is given by:
`FSail
`With capacitor values in the 1yHF to 10UF range, Ri will dominate the source impedance. Output ripple is due almost
`entirely to ESR of C8 (output capacitor in Figure 4). As the charge pump is unregulated, a linear regulator can be
`connected at the output to provide a regulated negative output.
`
`Relative Advantages of this Approach
`
`1. Small components
`2. Lower cost than 1:1 transformer architecture
`
`Relative Disadvantagesof this Approach
`
`1. Unregulated output; an additional regulator may be needed at output if the input voltage has a wide range.
`2. High peak currents for modest auxiliary load currents (approx 4 x Iqut_ave)
`
`
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`How to Generate Auxiliary Supplies from a Positive Buck DC-DC Converter - Maxim/Dallas
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`3. Negative auxiliary output only; the output can be referenced to ground or the main regulated output, provided
`that enough voltage difference is available to charge the pump capacitor (C7 in Figure 5).
`4. Minimum load required on auxiliary output to prevent spike storage overvoltage
`5. Minimum load required on main positive output to maintain switching action at LX
`
`SEPIC Auxiliary Supply
`
`A negative output can be obtained from the LX pin by employing a second inductor, L2, which shares the same core and,
`therefore, the same value as the main step-down inductor. Figure 7 shows how C5, D2, C6, and L2 form a SEPIC
`topology. See also waveforms in Example 4. The switching signal at LX driving the positive-output step-down is also the
`same level for driving the negative output. During the switch ON period, the voltage across L1 is V_y - Voyz, and during
`the OFF period is Voyt + Vpiope_1)- By transformer action (1:1) this voltage is also impressed across L2 and generates-
`Vout with D2 and C5. Because of the less-than-perfect coupling of the two windings L1 and L2, C5 creates the SEPIC
`connection and improves regulation of what would be a normal flyback auxiliary output with very modest regulation.
`
`The coupling capacitor, C5, is chosen to produce a low-voltage ripple across it as a function of auxiliary load-current duty
`cycle and clock period.
`as
`Lie
`
`Relative Advantages of this Approach
`
`1. Quasiregulated output
`2. 'Clean' inductor current waveform; less noise generation
`3. Ripple reduction due to coupled inductors
`4. Single magnetic component(off-the-shelf 1:1 transformer)
`
`Relative Disadvantagesof this Approach
`
`1. -Vourt only available
`2. Ground referenced output
`
`|Figure 7. Coupled inductor SEPIC auxiliary supply. L1, L2 = Cooper Bussmann DRQ125-101. (Note the DOT convention
`for the start of windings.)
`Figure 7. Coupled inductor SEPIC auxiliary supply. L1, L2 = Cooper Bussmann DRQ125-101. (Note the DOT convention for
`the start of windings.)
`
`Conclusions
`
`A number of auxiliary output topologies can be added to an integrated, positive step-down converter. The MAX5035 was
`chosen for the examples, but the lower output MAX5033 can employ the same circuits, but at reduced outputs.
`
`Flyback Auxiliary
`For complete independence from auxiliary output reference, the flyback circuit adds a winding to the main step-down
`inductor, a Schottky diode, and a capacitor. This design is very appealing and comes with modest regulation. With a 1:1
`transformer (Cooper Bussmann DRQ125-101 for the MAX5035), the auxiliary output can be Voy with respect to ground
`or the main Voyr- Auxiliary output current can be up to 20%of the main output, although some distortion of the main
`inductor current is to be expected.
`
`Coupled Inductor SEPIC Auxiliary
`Not as versatile in grounding arrangements, the coupled inductor SEPIC topology provides a regulated -Vgyy referenced
`to ground only. Regulation is better than the flyback approach, and inductor current waveform distortion is small. Auxiliary
`output current can be up to 20%of the main output. The coupled inductor aids ripple reduction in the auxiliary output.
`
`Charge Pump Inverter
`The charge pump is the lowest cost option with no additional inductor winding. This design is suitable for low-power
`outputs only because of the high peak currents and voltages associated with the topology. Open-circuit output is
`approximately V;,, reducing as the loading is increased on the auxiliary output. Suggested maximum loading is 5%or
`less of the main positive output.
`
`With this approach the main positive output must remain active atall times, and the main step-downinductor current
`must remain continuous at all times. Extra peak current will be demanded by the auxiliary output, and this must be taken
`into account when minimum loading of the main output and maximum loading of the auxiliary output are considered.
`
`a
`
`Suggested Component Suppliers
`
`AVX Ceramic
`Coilcraft
`
`capacitors
`Power inductors
`
`Power inductors
`Coiltronics
`Diodes Incorporated Schottky diodes
`
`www.avxcorp.com
`www.coilcraft.com
`
`www.cooperET.com
`www.diodes.com
`
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`How to Generate Auxiliary Supplies from a Positive Buck DC-DC Converter - Maxim/Dallas
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`Panasonic
`Sanyo
`TDK
`Vishay
`On-Semiconductor
`
`www. panasonic.com
`Ceramic/Al capacitors
`www.sanyo.com
`Ceramic/Al capacitors
`www.component.tdk.com
`Ceramic capacitors
`Diodes, resistors, capacitors www.vishay.com
`Schottky diodes
`www.onsemi.com
`
`Example 1 Waveforms: Step-down converter, MAX5035 EV Kit No Auxiliary (Fig 1):
`
`Viy = +15V
`Vout = +5V
`lout = 465mA (Rroap = 1002 +5%)
`
`Waveforms:
`
`1. LX inductor current ramp (Yellow, 0.1A / sq)
`2. LX voltage (Green)
`3. Vout (Violet)
`
`fay
`em
`
`TLx_pPEAK = 550mA
`Vix_pEAK = 15V
`Period = 8us
`
`Example 2a Waveforms: Transformer as Main Inductor, Flyback Auxiliary Output (Fig 3):
`
`VIN = +15V
`Vout = +5V
`Ipyut = 465MA (Rioap = 10£2 5%)
`
`-Vout = 5.02V
`-TouT_aux = -152mA (Rioap = 3382)
`C3 = 100pF
`D2 = 1N5817MDICT
`
`Waveforms:
`
`1. LX inductor current ramp (Yellow, 0.1A / sq)
`2. LX voltage (Green)
`3. VouT_AUX (Violet)
`
`PS
`ILX_PEAK = 0.63A
`
`Note: The LX waveform distortion is caused by additional loading during the flyback (D1 ON) period.
`
`Example 2b Waveforms: Transformer as Main Inductor, Flyback Auxiliary Output (Fig 3):
`
`Vin = +15V
`Vout = +5V
`Ionut = 465mA (Rigap = 1022 5%)
`
`-VouT = 5.3V
`-Iout_aux = -104MA (Rioap = 5122)
`C3 = 100pF
`D2 = 1N5817MDICT
`
`Waveforms:
`
`1. LX inductor current ramp (Yellow, 0.1A / sq)
`2. LX voltage (Green)
`3. VouT_AUX (Violet)
`
`aU
`
`rs
`
`TLx_peak = 0.6A
`
`Note: the reduced LX waveform distortion is caused by reduced loading on flyback (D1 ON) period. Comparethis to
`Example 2a above.
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`How to Generate Auxiliary Supplies from a Positive Buck DC-DC Converter - Maxim/Dallas
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`Example 3 Waveforms: Charge Pump Negative Auxiliary Output (Figure 5).
`
`Viy = +15V
`Vout = +5V
`
`-Vout = -12.3V
`-Iput_aux = 82MA (Rioap = 15082
`D2, 3 = 1N5817MDICT
`C7 = ip
`C8 = 10yuF
`R6 = 5.602
`
`Waveforms:
`
`1. LX + Charge Pump current ramp (Yellow, 0.2A / sq)
`2. LX voltage (Green)
`3. -Vout_aux (violet, 500mvV/ sq), AC-coupled.
`
`la
`
`Lx current waveform = 750mA pk. Contrast with 550mA peak of the basic step down.
`
`Note: the dV/dT spikes at auxiliary output. Post filter with small LC. L may be formed from pc copper track.
`
`Example 4 Waveform: SEPIC Auxiliary Supply (Fig 7):
`
`L1 = L2 = 100uUH coupled (1:1) inductor
`VIN = +15V
`Vout = +5V
`Tout = 465mA (RLoap = 1002 y5%)
`
`-VouT = -5.02V
`-TouT_aux = 228mA (Rioap = 2262)
`C5 = 10pF
`C6 = 100yF
`D2 = 1N5817MDICT
`
`Waveforms:
`
`1. LX current ramp (Yellow, 0.2A / sq)
`2. LX voltage (Green)
`3. VouT_AUX (Violet)
`
`le
`
`Txpeak = 1-15A
`Vout_aux Ripple = 100mV pk-pk excluding narrow dV/dT pulses.
`
`AN3740, AN 3740, APP3740, Appnote3740, Appnote 3740
`
`Note: dV/dT spikes at auxiliary output. Post filter with small LC. L may be formed from pc coppertrack.
`
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`More Information
`MAX5035 1A, 76V, High-Efficiency MAXPower Step-Down DC-DC
`Converter
`
`APP 3740: Jan 10, 2006
`Full Data Sheet Free Samples
`(PDF, 360kB)
`
`ifs Download, PDF Format (219kB)
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