relationship, inductor value, switching frequency. result, system desi

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AND8117/D Understanding Output Current Capability DC-DC Buck Converters
relationship, inductor value, switching frequency. result, system designers need understand specifications switching regulator should interpreted apply their specific operating conditions. some cases, published output current ratings reflect true capability part given application, whereas other cases safe operating limits inadvertently exceeded. Buck Converter Topology "semi-ideal" synchronous buck converter illustrated Figure high-side power switch duty-cycle will depend step-down ratio. When high-side power switch turned current drawn from input begins flow through inductor. When high-side switch turned off, low-side (synchronous rectifier) switch turned current circulates through lower NMOS switch shown, since inductor current cannot instantaneously stop. During steady-state operation, "on" "off" times switch balanced maintain desired output voltage.
INTRODUCTION widespread availability highly integrated DC-DC switchmode converter devices, system design engineers longer have much effort into design low-power converters many applications. little analysis, however, allow system designer make sure that switching regulator being utilized full capability. Whether DC-DC converter circuit uses internal external power switch, critical parameter that circuit designer must determine load current capability. This value leads sizing power switch. peak switch current rating (the level current above which power device break down overheat) proportional load current. course, larger power device will able deliver more output current, given switch current rating, system designer does have some ability affect output current capability based external component values operating conditions well. addition peak switch current limit rating, effective output current capability overall power supply circuit also depends input-output voltage
IPMOS Current Flow during IPMOS INMOS Vout
DC-DC Controller Circuit DC-DC Regulator with Internal Switches
INMOS Current Flow during Toff
Cout System (Load)
Output Voltage Feedback
Figure "Semi-Ideal'' Synchronous Buck Regulator
Semiconductor Components Industries, LLC, 2003
April, 2003 Rev.
Publication Order Number: AND8117/D
AND8117/D
When operating continuous conduction mode, PMOS (high-side) switch duty cycle proportional step-down ratio,
Vout
Thus, dependent both switching frequency step-down ratio. Inductor Current Waveforms Equations Since objective this discussion characterize full-load operation DC-DC converter, assume that converter will operating continuous conduction mode (CCM). operation, inductor current stays above zero shown Figure
switching frequency on-time PMOS switch will
Imax Peak Inductor Current
Iout Average Inductor Current
Imin Minimum Inductor Current
TOFF
TOFF
TOFF
Figure Inductor Current Continuous Conduction Mode
average inductor current equal output (load) current. given constant load level, inductor current will ramping above below this level power switch turned off. Thus peak inductor current, therefore peak high-side power switch current, will higher than output current. output current average value inductor current, which varies between Imin Imax:
Iout max) (for only)
since know Iout (Imin Imax) seen Figure substitute Imin terms Iout determine upper limit inductor ramp current
Iout (Vin Vout) (Ton) (2L)
determine actual peak switch current value given load current, begin with familiar equation inductor voltage/current relationship:
case, differential voltage across inductor, (Vin Vout) when high-side power switch turned change inductor current "di" from initial turn-on PMOS switch until turned (Imax Imin). Finally, "dt" value switch on-time, defined earlier. Thus inductor voltage-current relationship defined
(Vin Vout) min) (Ton)
Rearranging terms results
min) (Vin Vout) (Ton)
{(Vin-V out)*Ton/(2L)} term above represents half peak-to-peak ripple current. Because value Imax limited high-side power switch current rating, reducing ripple current (the difference between peak inductor current average load current) allows effective output current circuit approach switch current rating. equation above indicates following general trends buck converter circuit: Higher inductance allows higher load current fixed frequency (larger reduced ripple current) Higher frequency allows higher load current fixed inductance level (smaller reduced ripple current) Vin/Vout levels affect output current opposing ways: Higher step-down ratio (Vout/Vin) results shorter switch on-time (Ton), hence lower peak switch current Lower step down ratio (Vin closer Vout) results lower differential inductor voltage, slope inductor current during ramp-up period reduced
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AND8117/D
NCP1501 Synchronous Buck Regulator From system designer's point view, NCP1501 device Figure appear similar "semi-ideal" buck converter. only external components required input capacitor, output inductor, output capacitor. additional features NCP1501 regulator allow external frequency input, shutdown mode, output voltage selection, high efficiency both high load currents. These features discussed device data sheet will covered this note.
DC/DC CONTROL Ilim Vout
Vbat
Sync
CONTROL
Cout
Figure NCP1501 Block Diagram Application Circuit
Calculation peak current equations applied NCP1501 device show that effective output current device indeed vary function external components operating conditions. peak switch current limit PFET shown Figure nominally
0.70 0.68 PEAK SWITCH CURRENT 0.66 0.64 0.62 0.60 0.58 0.56 0.54 0.52 0.50 0.40 0.45 Vout 0.50 0.55 0.60 AVERAGE LOAD CURRENT 0.65 PEAK SWITCH CURRENT Peak Current Limit
allow component tolerances, derating typical value gives which used minimum limit. Figures illustrate difference output current capability converter using NCP1501 adjusting frequency and/or inductor value.
0.70 0.68 0.66 0.64 0.62 0.60 0.58 0.56 0.54 0.52 0.50 0.40 0.45 Vout 0.50 0.55 0.60 AVERAGE LOAD CURRENT 0.65 Peak Current Limit
Figure Output Current,
Figure Output Current,
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operating frequency, with inductor, converter only deliver about before worst- case switch rating reached. However, switching frequency increased same inductor value, over output capability possible. increasing inductor load current pushed above NCP1501 allows user select switching frequency applying external clock signal.
0.600 0.595 0.590 OUTPUT CURRENT 0.585 0.580 0.575 0.570 0.565 0.560 0.555 0.550 INPUT VOLTAGE Vout Vout Vout Vout
Figure other hand, shows effect input voltage output current capability fixed operating frequency (1.0 MHz) inductor value (6.8 mH). this case, input voltage restricted, output current capability increased. However, full input voltage range required, system designer should aware that worst case current rating power switch will reached lower levels load current high input voltage conditions.
Figure NCP1501 Output Current Variation with Input Output Voltage
This phenomenon seem somewhat counterintuitive thinks high line input requiring less current from input power source however, higher input-output differential, inductor current ramps very quickly this condition ripple current higher.
2.00 1.95 1.90 OUTPUT VOLTAGE 1.85 1.80 1.75 1.70 1.65 1.60 1.55 1.50
actual line load regulation performance NCP1501 device, operating with inductor (TDK LLF40176R8), shown Figure
OUTPUT CURRENT (mA)
Figure NCP1501 Output Performance Input Range (PWM Mode)
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AND8117/D
Final Observations seem from above discussion that would simply with largest possible inductor highest possible frequency order maximize output current capability. reality, though, compromises must made other considerations. example, larger inductance values will typically require larger physical case dimension same saturation current capability. Component tolerances derating factors also need taken into account, magnetic components will start decrease effective inductance value current increases toward saturation limit. Larger inductances will slow down response time switching regulator when subjected line load transient conditions. Furthermore, since NCP1501 designed require minimum amount external components, control loop stability compensation circuit completely internal This also limits range values allowed output inductor capacitor. Thus, applications where space premium, system designer instead choose with lowest possible inductor value that will reliably provide "just enough" output current load. Furthermore, higher switching frequencies will result higher switching losses. particular case NCP1501, this translates into percentage points lower converter efficiency. This, course, results slightly reduced battery life portable device. Higher frequency operation also restrict choice inductors need magnetic core materials that maintain their performance characteristics high dV/dT conditions. minimum switch current rating NCP1501 device allows easy design converter with output current capability, using standard surface-mount components. While today's integrated switchmode regulators substantially easier system design than their predecessors were, proper external component selection still critical order achieve best performance. table possible component values listed below.
Inductor LLF4017-6R8 (6.8 LLF4017-100 Coilcraft DO1606T-682 (6.8 Coilcraft DO1606T-103 Coilcraft LPO6610-682 (6.8
Description 0.122 0.70 4.0x4.1x1.7 0.145 0.50 4.0x4.1x1.7 6.5x5.3x2.0 6.5x5.3x2.0 0.32 0.90
Capacitor C2012X5R0J106 mRata GRM21BR60J106
Description Irms MHz, 2.0x1.25x1.25 Irms MHz, 2.0x1.25x1.25
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AND8117/D
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