ABSTRACT NCP5008 NCP5009 high efficiency boost converters operating cu

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ABSTRACT NCP5008 NCP5009 high efficiency boost converters operating cu

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AND8126/D NCP5008/NCP5009 Compensation Methods
ABSTRACT NCP5008 NCP5009 high efficiency boost converters operating current loop control mode drive Light Emitting Diode (LED). These products dedicated supply LEDs portable application. digital programming current level allows user different levels brightness intensity. photo sense input innovative function: adapts brightness backlight function ambient light order save battery lifetime duration. devices designed operate open-loop configuration enable digital photo-sense brightness control. This application note describes alternate application schematics increase output current accuracy. INTRODUCTION This manual describes several possible applications built around NCP5008 NCP5009. thank choosing Semiconductor part, hope this guide will help benefit from these devices' advanced features. Please NCP5009 data sheet reference source. up-to-date document, refer NCP5009 data sheet which available page: http://www.onsemi.com/home. Among electrical parameters, power supply must capable yield minimum continuous current operate system properly. Peak current developed when driver operates full power. GENERAL DESCRIPTION opposed classical Boost DC-DC converter, which operates closed loop mode, NCP5009 operating open-loop mode avoid need sense resistor series with white LEDs. This feature significantly simplifies implementation when network driver located separate boards because only connection required. second advantage that current flowing through photo sensor easily subtracted from reference current. external reference current generated connecting resistor between Iref ground. internally modified digital programming word current flowing through photo sensor. modified current used control inductor's current peak value. consequence open-loop operation, that current actually flowing through indirectly controlled, leading lower accuracy opposed closed loop operation. main cause variation power supply, because amount energy stored inductor proportional This issue when device supplied from nonconstant source, such battery.
Semiconductor Components Industries, LLC, 2003
August, 2003 Rev.
Publication Order Number: AND8126/D
AND8126/D
Vbat BandGap Vbat Vbat
Serial Parallel Latches
VBIAS
Iout Reference Vref Selection
Isense
Vbat
Vbat
A=10
LOCAL
Vref
Vbat
Vbat
CONTROLLER
Iref
PHOTO (See Note) Vbat Vbat Vbat_OK BANDGAP REFERENCE
BandGap
Figure
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AND8126/D
This behavior explained detail reviewing Boost converter operation. NCP5008 been modelized following schematic ideal switch.
Vbat1 MBR030
VOFF
Figure Typical Boost Topology Converter
NCP5008/9 constantly monitors current flowing through inductor. order achieve maximum efficiency, NCP5008/9 particularly controls inductor peak current, constantly switches inductor charge discharge between boundaries Ipeak Ivalley.
3.75 VMIDDLE 1.25 0.09 V(VMIDDLE) 3.9750 Phase I(L2) VOUT Inductor Phase
3.9625
3.9500 2.6328
2.6332
3.6336
2.63490 TIME
2.6348
2.6348 2.6351
Figure Typical Current/Voltage Diagrams (Spice Simulation)
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AND8126/D
basic operation boost converter divided phases: Phase Switch (see Figure Therefore, input voltage applied across inductor Inductor current linearly ramps from original value Imin (say peak value (when switch turns off). During this period, energy stored core material following equation:
Energy (Ipk-Imin)2
(eq. (eq.
flies back. Rectifier diode turns energy from inductor transferred output reservoir capacitor inductor voltage flies back above input voltage, output voltage then higher than input voltage such
(Vout-Vbat)
(eq.
From equations obvious that overall output power (and thus average output current) directly linked battery voltage Vbat. Consequently, battery voltage variation will noticeable output current. Compensate Battery Variation NCP5008/9 derives peak current (which determines time inductor) from external resistor Rref (between ground). Following diagram depicts internal blocks.
Vbat
Vbat
Phase Switch (see Figure Since inductor current must discontinuous, inductor voltage
Vbat
Ipeak Iref
Inductor Controller
U20A Iref
Bandgap 1.24
Iref
Rref Vref
Figure
Since
Ipeak Iref Iref 1.24 Rref Rref
(eq. (eq.
appears that when battery voltage varies, peak current threshold remains same. Hence, output
current will vary along with battery voltage: more Vbat high, more current will have output. Compensating battery variation means that when Vbat increasing, need decrease Iref (thus decreasing Ipeak) conversely, when Vbat decreasing, need increase Iref.
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AND8126/D
Vbat Vbat
Ipeak Iref
Inductor Controller
U20A Vbat Rcomp Icomp Iref
Bandgap 1.24
Iref
Rref Vref
Figure
With such compensation circuitry, have:
Vref Iref Rcomp Rref
(eq.
little calculation find:
DVbat DIref Rcomp
(eq.
Negative sign means that when Vbat decreasing, compensated current positive: that's exactly what want. This compensation method been applied NCP5009 several output current values.
Vbat Iref
Experimental Results Open Loop Operation following curves based experimental results ambient temperature. resistive divider been used reference current. been connected between Vbat input prevent current flow when circuit disabled. open-loop operation necessary allow Photo-transistor dimming (NCP5009 only) digital control.
NCP5009
Figure Compensation Schematic
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AND8126/D
CURRENT
CURRENT
53.6
BATTERY VOLTAGE BATTERY VOLTAGE
Figure Output Current with Four White LEDs Function Battery Voltage
Figure Output Current Function Battery Voltage with Three LEDs
ratio between modified reduce variation given battery voltage range. chosen values corresponding range.
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Closed Loop Operation When high accuracy requested, compensation method previously described, sufficient reduce current variations. Iref true current input. This
Vbat
means equivalent voltage generator having zero output resistor. additional transistor been used increase open-loop gain, reduce error voltage.
NCP5009
Iref 1.24 BC557C Rsense
Figure Closed Loop Compensation
10.8 10.6 CURRENT 10.4 10.2 BATTERY VOLTAGE
Figure
current adjust quite straightforward. only imposed sense resistor value. Sixty-eight resistor been used above curve. resistor prevents high current flow from Iref ground
circuit start-up. This would results very high peak current over-heating. BC557C transistor recommended because exhibits high Beta collector current values.
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AND8126/D
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