A MHz Step-up Converter for Display Bias Supply

Transcription

1 Package EK: MLP/TDFN, 3 3 mm 0.7 mm Nominal Height Approximate Scale : FEATURES Output voltage up to 3 V. to 0 V input 0.9 to 8 V input with separate bias supply Delivers V at ma with.7 V input Constant. MHz switching frequency provides low noise 30 ma switch current limit μa shutdown current Low-noise PWM/analog dimming. MHz Step-up Converter for Display Bias Supply The is a current mode step-up dc-to-dc converter, available in a -pin 3 mm 3 mm thermally enhanced MLP/TDFN lead (Pb) free package. Smaller external components and integrated 36 V switch reduce component count and footprint for a variety of applications. APPLICATIONS bias supply / WLED backlight Portable battery-powered equipment Cellular phones PDAs (Personal Digital Assistant) Camcorders, personal stereos, MP3 players, cameras Mobile GPS systems. V to. V 80 kω. V to. V 73 kω D I OUT + I OUT = ma + V V C4 Figure. Bias Supply. Use for cell phones, MP3 players, DSCs, and PDAs. V 768 kω for = 3 V 604 kω for = V kω 3 V, 8 ma V, ma / 0 V Figure. Small LCD Bias Supply. Li+ battery to ± V. V Connect either V Z or V BIAS V Z 7. V V BIAS V 0. µh D 768 kω for = 3 V 604 kω for = V kω 3 V, 40 ma V, 80 ma / 0 V Figure 3. LCD TV, Plasma TV Bias, FED Display, Varactor Diode Bias. Using V input. Figure 4. LCD TV, Plasma TV Bias, FED Display, Varactor Diode Bias. Using V input.. V to. V 0.0 kω 47 Ω 4 kω 0. µf Analog Voltage or PWM. V to. V 4 60 kω GND FB 3 60 kω nf Figure. WLED Backlighting. Analog Voltage or PWM 0 khz for PWM dimming only Figure 6. Bias Supply. Use for low-noise PWM dimming, or with analog voltage through the FB pin. -DS, Rev.

2 . MHz Step-up Converter for Display Bias Supply FB Functional Block Diagram V REF. V 6 mv A R C C C A R S Q Driver Σ Ramp Generator. MHz Oscillator GND Terminal List Table Device Pin-out Diagram Pin Name Function Internal power FET GND Ground 3 FB Feedback input 4 input Input supply GND FB 3 4 Ab so lute Max i mum Rat ings Pin to 36 V Remaining Pins to 0 V Operating Ambient Temperature, T A C to 8 C Junction Temperature, T J(max)... 0 C Storage Temperature, T S... C to 0 C Package Thermal Characteristics R θja = 0 C/W, on a 4-layer board. Additional information is available on the Allegro Web site. Northeast Cutoff, Box 036 Worcester, Massachusetts (08) DS, Rev.

3 . MHz Step-up Converter for Display Bias Supply ELECTRICAL CHARACTERISTICS at T A = C, V IN = 3 V (unless otherwise noted) Characteristics Symbol Test Conditions Min. Typ. Max. Units Input Voltage Range V IN. 0 V Active: I OUT = ma, = V. 3. ma Supply Current I SUP Shutdown ( = 0 V) 0. μa Feedback Reference Voltage V FB mv Feedback Input Current I FB V FB = 0.6 V 0 00 na Switch Current Limit I LIM 30 ma Switch Frequency F MHz Switch Maximum Duty Cycle D 8 90 % Switch Saturation voltage E(SAT) I = 0. A 30 mv Switch Leakage Current I SL V = 36 V μa Input Input Threshold Low V IL 0.4 V Input Threshold High V IH. V Input Leakage Leakage I IL μa Operating Characteristics (V IN = 3 V) 90 Efficiency versus Input Voltage = V; Toko 098AS-00M Inductor Feedback Bias Current versus Temperature V FB = 0.6 V Efficiency (%) I OUT (ma) I FB (na) V IN (V) Temperature ( C) Switching Frequency versus Temperature Quiescent Supply Current versus Temperature F (MHz)..0 ISUPQ (ma) Temperature ( C) Temperature ( C) 3 Northeast Cutoff, Box 036 Worcester, Massachusetts (08) DS, Rev.

4 . MHz Step-up Converter for Display Bias Supply Functional Description Pin Functions. Supply to the control circuit. A bypass capacitor,, must be connected from close to this pin to GND. GND. Ground reference connected directly to the ground plane. The feedback resistor should have a separate connection directly to this point.. Voltage lower than 0.4 V disables the and puts the control circuit into the low-power sleep mode. Voltage greater than. V fully enables the.. Low-side switch connection between the inductor,, and ground. Because rapid changes of current occur at this pin, the traces on the PCB that are connected to this pin should be minimized. In addition, and the diode, D, should be connected as close to this pin as possible. FB. Feedback pin for voltage control or WLED current control. The reference voltage is 6 mv. Connect the feedback resistor close to this pin to minimize noise. Device Operation The uses a. MHz constant switching frequency currentmode control scheme to regulate the output voltage or current through the load. A typical bias supply is shown in figure 7. For driving s, output voltage is sensed by the FB pin through a voltage divider network. Output voltage, (V), is set according to the following equation: + R = R 0.6. () A typical WLED backlight supply is shown in figure 8. The load current, I LOAD, is set by the selecting the external sense resistor,, to produce 6 mv at the desired load, for example: = 0.6 I LOAD. () Voltage sensed across the FB pin is compared with the internal 6 mv reference to produce an error signal. The switch current is sensed by the internal sense resistor and compared to the error signal for current mode PWM control. As the error signal increases, I LOAD, increases to increase either output voltage,, or current, I OUT, such that the FB pin voltage follows the internal 6 mv reference voltage. As I LOAD is reduced, the energy required in the inductor,, also reduces, resulting in the inductor current dropping to zero for low load current levels. This is known as Discontinuous mode operation and results in some low-frequency ripple. The average load current will, however, remain regulated down to zero. In Discontinuous mode, when I LOAD drops to zero, the voltage at the pin rings, due to the resonant LC circuit formed by and the switch and diode D capacitance. This ringing is lowfrequency and is not harmful. It can be damped with a resistor across the inductor but this will reduce efficiency and is not recommended.. V to. V 80 kω. V to. V 0.0 kω 47 Ω 4 kω 0. µf Analog Voltage or PWM Figure 7. Bias Supply. Use for cell phones, MP3 players, DSCs, and PDAs. (Circuit also shown in figure.) Figure 8. WLED Backlighting. (Circuit also shown in figure.) 4 Northeast Cutoff, Box 036 Worcester, Massachusetts (08) DS, Rev.

5 . MHz Step-up Converter for Display Bias Supply Applications Information Dimming Control Display dimming can be achieved by controlling the output voltage,, using the FB pin. The circuitry behind the FB pin is flexible, to accommodate a variety of schemes used for dimming: Adjust the duty cycle of the pulse train applied to the FB pin through resistor, as shown in figure. Capacitor is used to generate the average voltage level. Variation of output voltage with PWM duty cycle is shown in figure 9. Apply a constant dc voltage to the FB pin through resistor, as shown in figure. Variation of output voltage with dc voltage are shown in figure 0. Selection of the resistor connected between the FB pin and GND, as shown in figure for applications, and in figure 3 for WLED applications. The voltage drop across the transistor should be negligible compared to the FB sense voltage. VOUT (V) Soft Start Output voltage rise time at power-on can be extended by using a soft start circuit, such as the one shown in figure 4 for an application, and in figure 6 for a WLED application. At poweron, the device duty cycle begins initially at a high level, drawing a large current from the input supply,. The soft-start circuits shown can reduce the level of current flow by controlling the FB pin. When a signal is applied to the pin, capacitor discharges, pulling the FB pin high, and reducing to a minimum. When the signal is removed, recharges and as it does, the voltage drop across reduces, allowing the device duty cycle to. V to. V 4 60 kω GND FB 3 Analog Voltage or PWM 0 khz 60 kω nf for PWM dimming only Figure. Bias Supply. Use for low-noise PWM dimming, or with analog voltage through the FB pin Duty Cycle (%) Figure 9. PWM Dimming Control of Duty Cycle. Performance of V input circuit shown in figure 3.. V to. V 0 µh D Q 60 kω nf 0 µf VOUT (V) (V) Figure. Dimming Control. Single-bit resolution using external transistor.. V to. V 0 µh D 80 Ω 80 Ω Q 0. µf Figure 0. DC Voltage Dimming Control of Duty Cycle. Performance of circuit shown in figure. Figure 3. WLED Dimming Control. Single-bit resolution using external transistor. Northeast Cutoff, Box 036 Worcester, Massachusetts (08) DS, Rev.

6 . MHz Step-up Converter for Display Bias Supply increase gradually. When the voltage drop across is reduced to less than about 0.8 V, the feedback from the sense resistor,, takes over full control of the output voltage. The length of the soft start delay depends on the combined effect of,, R4, and the amplitude of the signal. The delay can be adjusted by the selection of these values. Component Selection The component values shown in figure are sufficient for most applications. To reduce output ripple, the value of the output inductor,, may be increased, but in most cases this results in excessive board area and additional cost. Inductor Selection (). With an internal PWM frequency of. MHz, the optimum inductor value for most cases would be 0 μh. The inductor should have low winding resistance, typically < Ω, and the core should have low losses at. MHz. For worst-case conditions of high output voltage and current, and low input voltage, the inductor should be rated at the switch current limit of 30 ma. If high temperature operation is required, derating should be considered. In some cases where lower inductor currents are expected, the current rating can be decreased. Several inductor manufacturers, including Coilcraft, Murata, Panasonic, Sumida, Taiyo Yuden, and TDK, have and are developing suitably small-size inductors. Two recommended inductors are: TDK: NLCV3T-00K-PF, 0 μh Toko: 098AS-00M, 0 μh Diode Selection (D). The diode should have a low forward voltage to reduce conduction losses, and a low capacitance to reduce switching losses. Schottky diodes can provide both these features if carefully selected. The forward voltage drop is a natural advantage for Schottky diodes, and it reduces as the current rating of the component increases. However, as the current rating increases, the diode capacitance also increases, so the optimum selection is usually the lowest current rating above the circuit maximum. In this application, an average current rating of 00 to 00 ma is usually sufficient. Capacitor Selection. Because the values recommended for the capacitors are low, ceramic capacitors are the best choice for this application. To reduce performance variation over temperature changes, low drift types such as X7R and XR should be used. 0 µh Figure 4. Soft Start Circuit I IN. V to. V µh R4 R4 0 nf. A.0 μf capacitor on the pin is suitable for most applications. In cases where large inductor currents are switched, a larger capacitor may be needed.. The output capacitor can be as small as 0. μf for most applications and most V IN / combinations. Increasing this capacitor value aids in reducing ripple and increasing efficiency in low input voltage / high output voltage conditions. Suitable capacitors are available from: TDK, Taiyo Yuden, Murata, Kemet, and AVX. D D D 4 kω kω 4 kω 80 kω Figure. Soft Start Circuit. Performance of circuit shown in figure 4.. V to. V Figure 6. WLED Soft Start Circuit 0.0 D 4.7 Ω 0. µf 6 Northeast Cutoff, Box 036 Worcester, Massachusetts (08) DS, Rev.

7 . MHz Step-up Converter for Display Bias Supply. V to. V 73 kω D I OUT + I OUT = ma + V V C4 (V) Line Regulation VOUT..0.8 VOUT (V) Figure 7. Line Regulation Perfomance for Small LCD Bias Supply, I OUT = I OUT = 7. ma (circuit also shown in figure ) V 768 kω for VOUT = 3 V 604 kω for = V kω 3 V, 8 ma V, ma / 0 V (V) Load Regulation 34 3 VOUT = 3 V VOUT = V IOUT (ma) Figure 8.Load Regulation for LCD TV, Plasma TV Bias, FED Display, Varactor Diode Bias; = V (circuit also shown in figure 3) V Connect either V Z or V BIAS V Z 7. V V BIAS V 0. µh D 768 kω for = 3 V 604 kω for = V kω 3 V, 40 ma V, 80 ma / 0 V (V) Load Regulation 34 3 VOUT = 3 V VOUT = V IOUT (ma) Figure 9. Load Regulation for LCD TV, Plasma TV Bias, FED Display, Varactor Diode Bias; = V (circuit also shown in figure 4). The can operate with a from 0.9 to 8 V with a separate bias supply to operate the in the normal V IN range. The bias voltage can be supplied by an external power supply, such as 3.3 or V, or by using a suitable Zener diode, V Z, for > 0 V. 7 Northeast Cutoff, Box 036 Worcester, Massachusetts (08) DS, Rev.

8 . MHz Step-up Converter for Display Bias Supply Use the following complete part number when ordering: Part Number Packing Description EEKTR-T 7-in. reel, 00 pieces/reel Surface Mount Leadframe plating 00% matte-tin. Solder pad layout compatible with SOT3- (SOT9P80-). Package EK, MLP/TDFN A B C D Preliminary dimensions, for reference only (reference JEDEC MO-9 WEEA) Dimensions in millimeters U.S. Customary dimensions (in.) in brackets, for reference only Dimensions exclusive of mold flash, gate burrs, and dambar protrusions Exact case and lead configuration at supplier discretion within limits shown Terminal # mark area Exposed thermal pad (reference dimensions only, terminal # identifier appearance at supplier discretion) Reference pad layout (reference IPC SON9P300X30-7WEEAN); adjust as necessary to meet application process requirements Reference pad layout with contact pads only; adjust as necessary to meet application process requirements X X 0.08 [.003] C 0.0 [.004] M C A B 0.0 [.00] M C A A 3..8 B REF SEATING PLANE C R0.0 REF.008 B.00 NOM NOM MAX MAX MAX 0.0 x 0.0 REF.008 x MAX D C Northeast Cutoff, Box 036 Worcester, Massachusetts (08) DS, Rev.

9 . MHz Step-up Converter for Display Bias Supply The products described here are manufactured under one or more U.S. patents or U.S. patents pending. reserves the right to make, from time to time, such de par tures from the detail spec i fi ca tions as may be required to permit improvements in the per for mance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the information being relied upon is current. Allegro products are not authorized for use as critical components in life-support devices or sys tems without express written approval. The in for ma tion in clud ed herein is believed to be ac cu rate and reliable. How ev er, assumes no re spon si bil i ty for its use; nor for any in fringe ment of patents or other rights of third parties which may result from its use. Copyright 00 AllegroMicrosystems, Inc. 9 Northeast Cutoff, Box 036 Worcester, Massachusetts (08) DS, Rev.