Abstract NCP5007 Basic Analysis Portable applications, such cellu

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AND8172/D Understanding Noise Issue Inductor Based DC-DC Converter
Abstract NCP5007 Basic Analysis
Portable applications, such cellular phones hand-held computers, have sensitive electronic functions, making electromagnetic noise real issue. Even power converter generate noise during transients since converters operate megahertz range. high frequency switching interact with parasitic circuit elements create uncontrolled noise. addition, associated voltage spikes might difficult filter out, detailed analysis converter operation being necessary properly design system. course, mechanical layout parameter minimize perturbation. main purpose this document clarify source such noise, assess risk system's pollution. NCP5007 device, together with based structure, will used vehicles perform associated engineering test real application.
NCP5007 dc-dc converter dedicated supply High Efficiency White LEDs used back light photo flash light source. Generally speaking, with typical drop during normal operation, these LEDs capable operate from standard battery pack. boost circuit must provided raise voltage bias called such diode. Moreover, these LEDs commonly connected series constant current through diodes, assuring smooth consistent light across panel. basic application, depicted Figure drives back light standard display, command being provided external other digital controller.
Vbat Pulse NCP5007 Vbat
Vout
MBR0530
LWT67C
LWT67C
LWT67C
LWT67C
Figure Typical Back Light Application
Semiconductor Components Industries, LLC, 2004
July, 2004 Rev.
Publication Order Number: AND8172/D
AND8172/D
With sense resistor, current through LEDs with "10% tolerance over temperature range. output voltage will typical) capacitor must sized accordingly. Using ceramic type mandatory both signal (Enable, useful power ON/OFF diodes, chip providing constant current LEDs when Vbat. other hand, signal also used control brightness back light means signal applied Basically, chip operates with cycles mode: Cycle energy stored into inductor Cycle energy dumped load course, from practical standpoint, inductor must sized cope with peak current present circuit avoid saturation core. that, ferrite material shall capable operate high frequency (>1.0 MHz) minimize Foucault's losses developed during cycles. waveforms captured NCP5007 Demo Board, V3.10, show voltage spikes VSPK#1 VSPK#2 across output voltage (Figure VSPK#1 signal synchronized with positive going slope output voltage, VSPK#2 signal being synchronized with negative slope same voltage. Both these signals come voltage ripple. Generally speaking, such spikes will named "noise'', know what energy content what could influence electrical environment. this point, consider switching operation only since there spikes outside transients demonstrated waveforms.
Figure NCP5007 Normal Operation
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silicon chip connected external world means bonding wires attached internal pads. bonding wires have finite inductance stray capacitance. addition, internal overvoltage protection circuits trace leads contribute such parasitic elements. simplified PSPICE model (Figure includes parasitic inductances associated with NCP5007 internal bonding, stray capacitance between output ground. load built with White LWT67C model (courtesy: OSRAM GmbH) with reservoir capacitor parallel. inductor includes stray capacitance developed wiring. printed circuit tracks wiring neglected first order importance, unless layout really poor.
Stray Capacitance NCP5007 Chip NMOS MBR030
Schottky Stray Capacitance
LW_5413-TYP
LW_5413-TYP
LW_5413-TYP
Parasitic Inductance Stray Capacitance
Figure PSPICE Output Circuit Model
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SPIKE VSPK#1
period derived from:
T+1+
(eq.
Assuming system operates steady state, consider noise VSPK#1. current flowing into NMOS abruptly switched voltage rises (according Lenz's law) until Schottky diode forward biased. inductor current will diverted load when output voltage will higher than Vout During positive going output voltage slope, stray capacitances parasitic inductances create network, yielding oscillation NMOS device switched off. According MBR0530 data sheet, intrinsic stray capacitance varies from (when when consider since Vout Vf*LED, then:
10-9 10-12
(eq.
This value well within signal given Figure captured during test performed with NCP5007 demo board. course, frequency varies along output voltage slope since semiconductor based capacitance varies voltage varies across junctions. Beside voltage effect, must consider level energy radiated such spike. this application, energy transferred from parasitic inductance stray capacitance, depicted waveforms (oscillations). Consequently, calculate amount energy stored into parasitic inductor:
10-9 0.062
(eq.
Such level energy extremely will pollute environment.
Note: dI/dt NCP5007 internal NMOS mA/ns, very fast transient.
Figure NCP5007 Output Voltage Noise NMOS Turn
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SPIKE VSPK#2
spike developed during negative going slope output voltage, when current transfer load completed NMOS switch turns (Figure this point, voltage across Schottky diode reverses same parasitic structures activated, yielding oscillation depicted trace (Figure energy transferred from charged stray capacitance parasitic inductor, yielding oscillations. Assuming total stray capacitance (NCP5007 pad, PCB, Schottky), get:
white total stray capacitance, first order only) 10-12 (eq.
This energy partially dissipated into ohmic resistance, partially into RDS(on) internal NMOS device (1.7 typical NCP5007), yielding 1000 mVpp spike depicted Figure Although visible, amount energy likely pollute environment power device. Moreover, energy radiated external inductance element, limited parasitic structures only, thus less prone trouble rest application.
Figure NCP5007 Output Voltage Noise NMOS Turn
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Discontinuous Mode Operation Analysis
When chip operates fully discontinuous mode, large peak current takes place inductor depicted Figure mode prone such discontinuous cycle. output voltage largely different, comparison with NCP5007, expects from discontinuous mode operation. main consequence large oscillations developed when transfer current between inductor load completed. course, internal circuit implemented avoid such oscillation (so-called ring killer), might have such features. oscillations longer only consequence Schottky diode, come from main inductor associated with stray capacitances. Assuming have stray capacitance (the Schottky
diode minimum parasitic capacitance since reverse voltage maximum), frequency MHz, yielding period. calculated values well within evaluation tests carried with based white driver used reference (Figure trace). this point, derive level energy radiated during this period:
level current during oscillation, first order only) 10-6 0.042 (eq.
Although such level low, generated main inductor this element prone radiate most this energy core, unless shielding magnetic core used.
Figure Typical Normal Operation
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detailed view based chip output voltage shows non-negligible spike when inductor current hits maximum peak value (Figure Basically, this same mechanism depicted NCP5007 application, similar voltage spike developed with mVpp amplitude. Leaving aside structure, main difference between mode NCP5007 from power device standpoint, RDS(on) internal NMOS. have compared chip. consequence twofold: With lower RDS(on), based chip probably larger Cdg, capacitances, thus slower dI/dt. Consequently, slower dI/dt minimizes effect parasitic inductance. Similarly, lower RDS(on) generates lower voltage spike same amount energy transferred from stray capacitance.
Figure Chip Output Voltage Noise NMOS Switched
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SPECTRUM ANALYSIS
waveforms captured oscilloscope very useful evaluate behavior given product, they don't give information about performance. this point, spectrum analyzer necessary characterize device over considered bandwidth. spectrum analysis been performed with same demo board, antenna being located above inductor. Although tests have been performed shielded cage, results provide good understanding situation terms EMI. waveforms provided Figures
clear that, although apparent voltage spikes higher NCP5007, result, terms real noise, much better NCP5007 comparison with circuit depicted records. order better analyze spikes coming from chips, noise return battery been measured recorded (Figure 10). spectral density, mathematical integration curves, yields 0.826 mVrms NCP5007, mVrms based chip, both over bandwidth. Once again, behavior NCP5007 better than based white driver, particularly frequency band.
Figure NCP5007 Spectrum Analysis (Unit: mV): Relative
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Figure Circuit Spectrum Analysis
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Figure Reflected Noise Battery
CONCLUSION
Since possible reduce parasitic inductance zero (using micro bump technique could improve reduce inductance zero nH), since fast dI/dt necessary lower switching losses into chip, spikes voltage might exist. However, mandatory consider behavior power device real environment, keeping into account parasitic effects. spectrum analysis, provided
Figures demonstrates excellent behavior NCP5007 with much lower noise, comparison with based white driver operation. same comment applies reflected noise battery. waveforms very useful fully characterize system, prime importance identify quantify risks, terms noise reliability, voltage spikes when detailed analysis requested.
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PSPICE SIMULATIONS
circuit been simulated prior analysis laboratory. same schematic (Figure been used evaluate configurations.
Using lower RDS(on) yields expected lower spike stray capacitance diode provides much lower noise during transients, expected.
Vbat
Vbat
MBR030 LW_5413-TYP
NMOS
LW_5413-TYP
LW_5413-TYP
Figure PSPICE Schematic Diagram
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parasitic elements based existing package silicon designs. value stray capacitance depends, besides internal structure, upon layout.
Test Conditions
Temperature Oscilloscope Voltage Probes Current Probe Input Power Supply Input Power Supply Bias Output Load Output Current Inductor Spectrum Analyzer Spectrum Analyzer probe probe
:Room, +20°C :Tektronix, TDA754/D, operating mode :Tektronix P6139A :Tektronix TCP202 :Tektronix PS2520G :3.60 White :NCP5007 based chip (Rsense :ROHDE SCHWARZ, FSIQ :HP4195A :HP11941A :HP41800A
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