FA7764 FUJI Power Supply Control IC DC/DC Power Supply control IC FA7764 Application Note June-2010 Fuji Electric Systems Co.,Ltd Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 1 http://www.fujielectric.co.jp/fdt/scd/ FA7764 WARNING 1. This Data Book contains the product specifications, characteristics, data, materials, and structures as of June 2010. The contents are subject to change without notice for specification changes or other reasons. When using a product listed in this Data Book, be sure to obtain the latest specifications. 2. All applications described in this Data Book exemplify the use of Fuji's products for your reference only. No right or license, either express or implied, under any patent, copyright, trade secret or other intellectual property right owned by Fuji Electric Co., Ltd. is (or shall be deemed) granted. Fuji makes no representation or warranty, whether express or implied, relating to the infringement or alleged infringement of other's intellectual property rights, which may arise from the use of the applications, described herein. 3. Although Fuji Electric is enhancing product quality and reliability, a small percentage of semiconductor products may become faulty. When using Fuji Electric semiconductor products in your equipment, you are requested to take adequate safety measures to prevent the equipment from causing a physical injury, fire, or other problem if any of the products become faulty. It is recommended to make your design fail-safe, flame retardant, and free of malfunction. 4. The products introduced in this Data Book are intended for use in the following electronic and electrical equipment, which has normal reliability requirements. • Computers • OA equipment • Communications equipment (Pin devices) • Measurement equipment • Machine tools • audiovisual equipment • electrical home appliances • Personal equipment • Industrial robots etc. 5. If you need to use a product in this Data Book for equipment requiring higher reliability than normal, such as for the equipment listed below, it is imperative to contact Fuji Electric to obtain prior approval. When using these products for such equipment, take adequate measures such as a backup system to prevent the equipment from malfunctioning even if a Fuji's product incorporated in the equipment becomes faulty. • Transportation equipment (mounted on cars and ships) • Trunk communications equipment • Traffic-signal control equipment • Gas leakage detectors with an auto-shut-off feature • Emergency equipment for responding to disasters and anti-burglary devices • Safety devices 6. Do not use products in this Data Book for the equipment requiring strict reliability such as (without limitation) • Space equipment • Aeronautic equipment • Atomic control equipment • Submarine repeater equipment • Medical equipment 7. Copyright © 1995 by Fuji Electric Co., Ltd. All rights reserved. No part of this Data Book may be reproduced in any form or by any means without the express permission of Fuji Electric. 8. If you have any question about any portion in this Data Book, ask Fuji Electric or its sales agents before using the product. Neither Fuji nor its agents shall be liable for any injury caused by any use of the products not in accordance with instructions set forth herein. Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 2 http://www.fujielectric.co.jp/fdt/scd/ FA7764 Contents 1. Overview ························································································· 4 2. Features ·························································································· 4 3. Outline····························································································· 4 4. Block diagram ················································································· 5 5. Description of the terminal functions ··············································· 5 6. Rating and characteristics ······························································· 6 to 8 7. Characteristic curve········································································· 9 to 11 8. Operation description of each block ················································ 12 to 14 9. Design tips······················································································· 15 to 16 10. Example of an applied circuit ························································ 17 to 18 Notices: - The specifications in this document are subject to change without notice. - Parts tolerance and characteristics are not defined in all application described in this Data book. When design an actual circuit for a product, you must determine parts tolerances and characteristics for safe and stable operation. Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 3 http://www.fujielectric.co.jp/fdt/scd/ FA7764 1. Overview The FA7764N is a single output step-down DC-DC converter with built-in power MOSFET. Featuring a high voltage process, it is suitable for the DC-DC converter with input voltages of up to 45 V and output currents of up to 1.5 A. It operates with the current mode control and also it can reduce external parts. Furthermore it achieves a fast response. The circuit protections are built-in to achieve safety power supply circuit. 2. Features - Single output with the built-in power MOSFET. - High efficiency 85% at 24V input and 5 V/1.2 A output with 200kHz operation. - Stable operation with current mode control. - Switching frequency selectable from 30 kHz to 400 kHz. - ON/OFF function: Current consumption 0.1mA(max.) at OFF mode. - Current consumption of the VCC terminal during operation is 0.5mA(typ.). - Protection functions: Over current limitation for the power MOSFET Soft start (8 ms) Short-circuit protection with the timer and latch (90ms delay time) Thermal shut down Protection for opened rectifier diode - SOP-8pin package with the exposed pad. 3. Outline ・FA7764AN(SOP-8 E-Pad) ・FA7764P(DIP-8) 8 1 D 09 5 1 2 6.5± 0.3 6.00 ±0.20 3.90 ±0.20 7 764 A 5 4 1 9.4±0.3 3.3± 0.2 5 .35 Ma x. 3.4 1.7 MAX 0.18 MAX 5.0±0.25 0.40 ±0.1 1.27 (3.10) 0.25 ±0.1 0.5±0.1 2.54 7.6 (2.41) 2.54×3=7.62 Units : mm Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 4 http://www.fujielectric.co.jp/fdt/scd/ FA7764 4. Block diagram 5. Description of the terminal functions FA7764AN(SOP-8 E-Pad) 1 CREG VCC 8 2 ENB OUT 7 3 RT VBIAS 6 4 GND IN FA7764P(DIP-8) 1 CREG VCC 8 2 ENB OUT 7 3 RT VBIAS 6 IN 5 5 4 GND Exposed PAD on backside Terminal No. 1 Terminal code CREG 2 ENB 3 4 5 6 RT GND IN VBIAS Terminal function Internal power supply output ON/OFF control (L, operation; H or open; standby mode) Timing resister for the oscillator Ground Inverted input of error amplifier Voltage input for internal power supply Remarks Connect stabilization capacitor Supply from output voltage VBIAS = up to 3.1 V; CREG is powered by VCC. VBIAS: 3.1 V or more; CREG is powered by output voltage. 7 8 OUT VCC Switching output Power supply input Connect bypass capacitor Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 5 http://www.fujielectric.co.jp/fdt/scd/ FA7764 6. Ratings and characteristics (1)Absolute maximum rating Following data are subject to change without notice. When using this IC, be sure to obtain the latest specifications. *The items except operation junction temperature, operating ambient temperature, and storage temperature. Item Symbol Condition Rating Units Power Supply voltage (VCC) VCC 50 V IN input voltage VVIN -0.3 to 4.0 V CREG input voltage VCREG -0.3 to 4.0 V VRT -0.3 to 4.0 V VBIAS input voltage VVBIAS -0.3 to 6.0 V ENB input voltage VENB -0.3 to 4.0 V RT input voltage Power dissipation *1 SP-8(E-Pad) Pd 3.9 Ta≦25°C DIP-8 1.39 Operation junction temperature TJ +150 Operation ambient temperature TOPR -40 to +85 Storage temperature TSTG -50 to +150 W °C °C °C The characteristic of power dissipation Loper dissipation Pd [W] 5 4 SOP-8(E-Pad) 3 2 DIP-8 1 0 -40 -20 0 20 40 60 80 100 120 140 160 Ambient temperature Ta[℃] *1 This IC should be used less than the total power dissipation (Pd). This Pd is based on the condition that this IC is mounted on 4-multi-layer PCB(the size is 50mm×40mm) and the Exposed pad (E-pad, in the reverse side of the IC package) is connected to the ground at Ta=25 degrees. Over 25 degrees, delating the Pd as follows: Thermal resistance θj-a(Junction to Ambient） SOP-8(E-Pad):32 degrees/W, DIP-8:95 degree/W θj-c(Junction to Case） SOP-8(E-Pad):16.5 degrees/W, DIP-8:55 degree/W θEPAD-c(E-pad to Case） SOP-8(E-Pad):6 degrees/W Note) This IC should be used on the condition that the junction temperature is less than 150 degrees, based on the total power dissipation. Also this IC should be examined that the junction temperature is actually less than 150degrees by measuring the surface temperature of this package. Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 6 http://www.fujielectric.co.jp/fdt/scd/ FA7764 (2) Recommended operating conditions Item Power Supply voltage (VCC) VBIAS input voltage Oscillation frequency CREG pin capacitance VBIAS pin capacitance Symbol Condition MN. TYP. MAX. Units VCC 9 45 V VVBIAS 3.1 5.5 V fOSC 30 400 kHz CREG 0.1 - - μF CVBIAS - 0.1 - μF (3) Electrical Characteristic The characteristics is based on the condition of VCC=42V,CREG=0.1uF,RT=39kΩ,Ta=+25°C,unless otherwise specified. Line regulation is the difference from 9V to 45V. ・ ・ (1)Output feedback voltage (IN,VBIAS pin) Item Symbol Feedback reference voltage Variation with temperature Input pin current VIN VＩＮｄＴ IIN Condition Ta=-40 to +85°C VIN=0 to 2V (2)Regulated voltage for internal blocks (CREG pin) Item Symbol Condition Regulated voltage *2 VＲＥＧ *2 Oscillation frequency Line regulation Variation with temperature (4)Soft start section Item Soft start time fOSC fdV fdT Condition RT=39kΩ VCC=9V to 45V MAX. Units 0.990 1.000 1.010 ±1 0.15 V % μA MIN. TYP. MAX. Units 2.6 3.0 -0.15 V Symbol tS MIN. TYP. MAX. Units 108 120 ±3 ±3 132 ±5 ±5 kHz % % MIN. TYP. MAX. Units 4 8 12 ms MIN. TYP. MAX. Units 45 90 180 ms 0.70 0.75 0.80 V Ta=-40 to +85°C Condition *4 ts is the time of output voltage change from 10% to 90% (5)Timer and latch for short circuit protection (CP pin) Item Symbol Condition Delay time of timer latch tprot IN pin on threshold voltage VTHINON IN pin off threshold voltage IN pin Hysteresis voltage VTHINOFF VTHINHYS *5-1 *5-2 *5-3 TYP. CREG pin occurs over shoot voltage until about 4V in case of starting by ENB pin. (3)Oscillator section (RT pin) Item Symbol *4 MIN. VCC=10V VCC=10V *5-1 *5-2,5-3 *5-3 0.85 0.10 V V The counter of timer latch starts when IN pin voltage is lower than VTHINON, The counter of timer latch stops when IN pin voltage is higher than VTHOFF. Design value. Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 7 http://www.fujielectric.co.jp/fdt/scd/ FA7764 (6)Under voltage lockout circuit section (VCC pin) Item Symbol Condition ON threshold voltage OFF threshold voltage Hysteresis voltage VUVLOON VUVLOOFF VUVLOHYS IC is enabled. IC is disabled. (7)ON/OFF control section (ENB pin) Item Symbol Source current of ENB TYP. MAX. Units 6.9 5.9 1.0 8.2 4.9 0.4 V V V MIN. TYP. MAX. Units 10 15 1 μA V V MAX. Units VENB=0V IC is enabled 2 IC is disabled *7 External voltage input is not necessary to the ENB pin because it clamps to the self-limited voltage near 5V. ON threshold voltage OFF threshold voltage *7 IENB Condition MIN. VENBON VENBOFF (8)Thermal shutdown section Item Symbol Thermal shutdown enable Thermal shutdown disable TOHPON TOHPON (9)Output section (OUT pin) Item Symbol On resistance of built-in P-channel MOSFET Current limit Short circuit detect current Open diode detect voltage Maximum duty limit RONP IOLMT12 IOLMT42 IOSH VDOP DMAX (10)Overall section (VCC,VBIAS pin) Item Symbol Supply current (VCC) ICCVCC ICCSTBVCC Supply current (VBIAS) ICCVBIAS Condition Output is disabled Output is enabled Condition VCC=10V，IDS=1.0A Ta=85°C VCC=10V，IDS=1.0A VCC=12V VCC=42V VCC=42V Condition Switching at non-load (at VBIAS<3.1V) Switching at non-load (at VBIAS>3.1V) Stand by (ENB=open) Switching at non-load (at VBIAS<3.1V) Switching at non-load (at VBIAS>3.1V) ICCSTBVBIAS Stand by (ENB=open) MIN. TYP. 125 135 115 MIN. TYP. MAX. Units 0.8 1.0 Ω 1.2 1.5 Ω 2.3 2.5 4.0 -10 3.0 3.5 6.0 -9 95 3.7 5.0 8.5 -8 A A A V % MIN. TYP. MAX. Units 0.85 1.5 °C °C mA 0.5 1.0 50 100 μA 0 20 μA 0.5 0.8 mA 0 20 μA Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 8 http://www.fujielectric.co.jp/fdt/scd/ FA7764 7. Characteristic curves Timing resistor vs. Oscillation frequency Oscillation frequency vs.ambient temperature 130 400 Oscillation frequency fosc[ｋHz] Oscillation frequency fosc［kHz］ 450 350 300 250 200 150 100 50 RT=39k Ω 125 120 115 110 0 0 20 40 60 80 100 120 Timing resistor RT [kΩ] 140 -40 160 Oscillation frequency vs. Supply voltage VCC 10 VCC terminal ON/OFF threshold ［Ｖ］ Oscillation frequency fosc[ｋHz] 0 20 40 60 80 100 120 Ambient temperature Ta[℃] 140 160 Under voltage lockout vs.Ambient temperature 135 125 115 105 ON threshold voltage 9 OFF threshold voltage 8 7 6 5 4 3 2 1 0 0 10 20 30 Supply voltage Vcc[V] 40 50 -40 -20 0 20 40 60 80 100 Ambient temperature Ta［℃］ 120 140 160 Delay time of timer latch vs.Ambient temperature Soft start time vs.Ambient temperature 10 100 VCC=10V Delay time of timer latch ［ms］ 9 8 Soft start time［ms］ -20 7 6 5 4 3 2 1 0 90 80 70 60 -40 -20 0 20 40 60 80 100 Ambient temperature Ta［℃］ 120 140 160 -40 -20 0 20 40 60 80 100 Ambient temperature Ta［℃］ 120 140 160 Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 9 http://www.fujielectric.co.jp/fdt/scd/ FA7764 Current limit vs.Ambient temperature Supply current vs.Ambient temperature VCC=20V 4.0 1.0 Supply current ［mＡ］ 3.5 Current limit ［Ａ］ VBIAS=0V 0.9 3.0 2.5 2.0 1.5 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1.0 -40 -20 0 20 40 60 80 100 Ambient temperature Ta［℃］ 120 Current limit vs.Ambient temperature 140 -40 160 0.8 20 40 60 80 100 Ambient temperature Ta［℃］ 120 140 160 140 160 140 160 VBIAS=5V 0.7 Supply current ［mＡ］ 4.5 Current limit ［Ａ］ 0 Supply current vs.Ambient temperature VCC=40V 5.0 4.0 3.5 3.0 2.5 0.6 0.5 0.4 0.3 0.2 0.1 0.0 2.0 -40 -20 0 20 40 60 80 100 Ambient temperature Ta［℃］ 120 140 -40 160 Supply current vs.Ambient temperature 80 -20 0 20 40 60 80 100 Ambient temperature Ta［℃］ 120 VBIAS terminal current vs.Ambient temperature 0.8 Stand by VBIAS terminal current ［mＡ］ 70 Supply current ［uＡ］ -20 60 50 40 30 20 10 0 VBIAS=5V 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -40 -20 0 20 40 60 80 100 Ambient temperature Ta［℃］ 120 140 160 -40 -20 0 20 40 60 80 100 Ambient temperature Ta［℃］ 120 Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 10 http://www.fujielectric.co.jp/fdt/scd/ FA7764 High side on resistance vs.Ambient temperature High side on resistance ［Ω］ 1.6 VCC=10V IDS=1A 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -40 -20 0 20 40 60 80 100 Ambient temperature Ta［℃］ 120 140 160 ON threshold voltage of ENB vs.Ambient temperature ON threshold voltage of ENB [V] 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -40 -20 0 20 40 60 80 100 Ambient temperature Ta[℃] 120 140 160 Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 11 http://www.fujielectric.co.jp/fdt/scd/ FA7764 8. Operation description of each block (4) PWM comparator The ON state of output (at the OUT pin) starts with the ON trigger signal of the oscillator. It turns off when the inductor current reaches to the specified level by the error amplifier (OTA) output. (1) Reference voltage circuit The reference voltage circuit generates the feed back voltage reference (VIN) of 1.00V±1% which compensated in temperature from VCC voltage, and the regulated voltage (CREG) of 3.0V The feedback voltage (VIN) is connected to the non-inverting input of error amplifier (OTA) as a reference voltage of the error amplifier. The regulated voltage (CREG) is provided as a regulated power supply for IC’s internal blocks, and it has to connect capacitors in order to stabilize voltages. The CREG voltage is designed specifically for the power supply for IC’s internal blocks. Therefore it cannot be used as external stabilized power supplies. To determine capacitance of CREG pin, refer to recommended operating conditions. (5) Soft start circuit In order to prevent the abnormal start up of DC-DC converter such as rush current, the soft start circuit is built-in. The soft start is performed by raising the feedback reference voltage of the error amplifier (OTA) step by step. Therefore the output voltage of DC-DC converter rises slowly. The soft start time is fixed to 8msec (typ.) internally. The soft start stars after the input voltage reaches the ON threshold (6.9V typ.) or more of under voltage lock out (UVLO). (Fig. 3). In power supplied condition, the soft start is controlled by the ENB signal. (2) Oscillator The oscillator generates a triangular waveform by charging and discharging the capacitor. Oscillation frequency can be set by the value of resistor connected to the RT pin 8 7 6 5 (Fig.1) (Operating frequency decreases when RT increases, Operating 1 2 3 4 frequency increases when RT decreases) R Set the oscillation frequency between 30 kHz and 400 kHz. The RT pin outputs DC voltage of 1 V. Fig.1 VCC OUT VBIAS IN CREG ENB RT GND Start up by the power supply with the low level ENB signal T Startup by the ENB signal VCC (3) Error amplifier (OTA) The IN pin (5 pin) is the inverting input of the error amplifier (OTA). The non-inverting input is internally connected to the feedback reference voltage (VIN) with 1.0 V ± 1%. The output of OTA has no external pin and the phase compensations are built-in. The feedback voltage of the DC-DC converter’s output is connected to the IN pin by the divider resister. The output voltage Vout of the DC-DC converter can be calculated as follows; Vout = R1 + R 2 × VIN R2 If it operates without stability, connect the capacitor C1 in order to adjust the phase compensation. C1 ENB terminal low ENB Fig. 3 8 7 6 5 OUT VBIAS IN CREG ENB RT GND 1 2 3 4 8msec typ. (6) Timer and latch for short circuit protection When the output voltage of the DC-DC converter drops continues in constant time, the timer and latch short circuit protection stops switching operation in order to protect the DC-DC converter. If the output voltage of DC-DC converter drops due to output short-circuit or over load, the output voltage feedback IN pin of the error amplifier will also decreases. When the IN pin voltage drops down to 0.75V (typ.), the timer latch starts its counter. If the drops continues and the counter operates beyond delay time (90msec typ.), the protection circuit regards the case as abnormal. Therefore it stops switching, and protect the circuit (Fig. 4). Vout R1 VCC Vout R2 Fig.2 Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 12 http://www.fujielectric.co.jp/fdt/scd/ FA7764 The delay time of the timer and latch is fixed internally and cannot be configured from outside. (9) ON/OFF controller This IC can be on/off controlled by the input of external signals to the ENB pin. To turn the output on, set the ENB pin voltage less than 1.0V. Then the output voltage gradually increases by the soft start (Fig. 3). The output can be turned off by either opening the ENB pin or input an external voltage of at least 2V. When the ENB pin is open, the ENB pin voltage will clamps to the self-limited voltage near 5V. In this case, the IC keeps standby mode where the regulated voltage CREG (3.0V typ.) is shut down, and also the current consumption of the IC is controlled less than 100 μA(typ.). (10) Under voltage lockout circuit (UVLO) This IC has a UVLO function to prevent circuit malfunction in case of a lower input power supply. When increasing the power supply voltage Vcc from 0V, the IC will be start operation over 6.9V (typ). When decreasing the power supply voltage Vcc from operation state, the IC will be shut down under 5.9V (typ). Fig. 4 In order to resetting from latch stoppage, resetting by using the ENB pin or setting the input voltage down to the UVLO threshold voltage are required. (7) Overcurrent limitation circuit This IC has a pulse-by-pulse overcurrent limitation function that detects and limits each peak current of the built-in MOSFET. The current of the built-in MOSFET is transferred to the voltage signal at current detect circuit, and the voltage signal is input into the overcurrent limitation comparator. If the current transferred voltage signal is higher than the reference voltage, the built-in MOSFET is turned off and the current is limited. The current limitation will be reset in the next switching period, and it will be repeated in each period along with overcurrent limitation. Furthermore, when the pulse-by-pulse overcurrent limitation continues for five times, the IC latch stops the switching in order to avoid inductor saturation and the destruction of the MOSFET. If the output voltage of the DC-DC converter drops to a level lower than 75% (typ) due to line impedance or some other factor, the IC will be shut down by the same way as timer latch short-circuit protection described in (6). (11) Output circuit The output circuit consists of P-channel MOSFET, with ON resistance of 0.8 Ω (typically). (12)Opened rectifier diode protection Generally when a rectifier diode in the buck converter is removed by some kind of abnormality states, large voltage occurs on switching MOSFET by the energy of the inductor and it has a possibility of destruction the MOSFET with emitting smoke and ignition. Therefore, the IC will shut down the operation when the OUT pin voltage falls under -9V with synchronized switching. (8) Thermal shut down circuit The IC has an thermal shut down function that stops switching operation when the IC overheats due to overcurrent or other error. The IC stops switching when the chip temperature is heated up to 135 degrees (typ.), and the IC resumes switching when the chip temperature is cooled down to 115 degrees. The latching function does not work when thermal shut down is operated. Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 13 http://www.fujielectric.co.jp/fdt/scd/ FA7764 (13) VBIAS circuit This IC provides the VBIAS pin to increase the efficiency of the power supply circuit and reduce the IC power loss. The internal regulated power supply CREG is usually generated by the VCC voltage. If CREG is generated from the output voltage of the DC-DC converter by the VBIAS pin, it is able to reduce the power loss at CREG regulator and lower power consumption operation (Fig. 5). Fig. 5 When the VBIAS pin is not used, the operating current consumption of the VCC terminal is 0.75 mA (typ), on the other hand, when the VBIAS pin is used, the comparable consumption is 0.4 mA(typ). This operation switchover is performed at the VBIAS pin voltage (i.e., output voltage of the DC-DC converter) of 3.1V. Therefore, the VBIAS operation will be effective only at the input voltage more than 3.1V. Furthermore, a 5.5 V Zener diode for protection is connected to the VBIAS pin. Therefore, if more than 5.5V voltage is applied to the VBIAS pin, a leak current will be occurred in the Zener diode and also increases the power loss. Therefore, apply the voltage from 3.1V to 5.5V to the VBIAS pin when you use the VBIAS pin function. Connect a ceramic capacitor close to the VBIAS pin as shown in Fig. 5 to eliminate noise. The recommended value of the capacitance is about 0.1 μF. If you do not use the VBIAS pin and in case of set the DC-DC converter output voltage to less than 3.1V or more than 5.5V, connect the VBIAS pin to the GND (Fig. 6). Fig. 6 Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 14 http://www.fujielectric.co.jp/fdt/scd/ FA7764 (3) Restriction of external discrete components and recommended operating conditions To achieve a stable operation of the IC, the value of external discrete components and the voltage and the current applied to each pin should be within the recommended operating conditions. 9. Design tips (1) Setting an oscillation frequency The oscillation frequency can be adjusted by changing the value of the resistor connected to the RT pin as described under item (2) in "Operation description of each block." you can set any oscillation frequency between 30 kHz and 400 kHz. The oscillation frequency can be determined by the characteristic curve of "Timing resister vs. Oscillation frequency" or the RT value can be approximately calculated as follows: This IC incorporates a P-channel MOSFET between the OUT pin and the VCC pin. Since the P-channel MOSFET has a parasitic diode, so if the voltage of OUT pin becomes higher than the VCC pin voltage, the current flows from the OUT pin to the VCC pin. fosc = 3738 × RT −0.95 (4) Preventing the application of negative voltage If rather large negative voltage is applied to any pins of this IC, internal parasitic elements start operating, and they may cause malfunctions. Therefore the negative voltage which is applied to each terminal of the ICs must be kept above -0.3V. Where, fosc: Oscillation frequency [kHz] RT: Timing resistance [kΩ] This expression, however, can be used as rough calculation, the obtained value is not guaranteed. The operation frequency varies due to the conditions such as tolerance of the characteristics of the ICs, influence of noises, or external discrete components. When determining the values, examine the effectiveness of the values in an actual circuit. The timing resistor RT should be connected to the GND pin as shortly as possible because the RT pin is high impedance pin and is easy affected by noises. (5) Improving the transient response characteristic The transient response characteristic of a DC-DC converter generally shows the overshoot in the output voltage when the power supply voltage is started, and the overshoot/undershoot in the output voltage when the load current changes abruptly (Fig. 7). This IC has few overshoot at startup due to the soft start function. However, the overshoot/undershoot may occur when the load changes abruptly, depending on the conditions. This overshoot/undershoot can be reduced by connecting a capacitor to the output voltage detection resistor as shown in Fig. 2. No universal constant can be proposed because it varies with the conditions. However, we believe that an appropriate effect can be produced when the constant is between hundreds of pico-Farads and tens of nano-Farads. (2) IC losses This IC incorporates a switching MOSFET. The loss generated in this MOSFET accounts for most of the IC loss, and the loss is determined by the input/output conditions of the DC-DC converter. Attention is needed to the permissible loss of the IC (SOP-8 E-Pad: 3.9 W at 25°C). The permissible loss in SOP-8 (E-Pad) specified in this document is under the condition of mounting a 4-layer board (50 mm x 40 mm) at Ta of 25°C when the exposed pad is connected by solder. Therefore the permissible loss will be smaller than the specified loss by using any other board (such as 2-layer board) or the exposed pad is unconnected. The recommended maximum load current for the DC-DC converter consisting of this IC is about 1.5A. This load current depend on the input voltage and operating ambient temperature. Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 15 http://www.fujielectric.co.jp/fdt/scd/ FA7764 (6) Mounting SOP-8 (E-Pad) Mounting the SOP-8 (E-Pad) package of this IC is by providing a 4-layer board on which to mount components for reducing the thermal resistance of the package, along with a GND pattern in the intermediate layer of the board, and connect the package E-Pad by using several thermal via. Fig. 8 shows a recommended foot pattern. 3×1.27=3.81 0.76 (1.2) (φ0.3) Thermal via 1.27 Note 2. 3.10 Notes: 1. The exposed pad pattern conforms to JEDEC JESD51-5. 2. The resist is the same as that of the exposed pad. Thermal via should also be placed in positions outside of the resist opening area. (7) Recommendation of continuous mode operation There is a possibility of jumping up the output voltage in a discontinuous mode. In order to prevent from this jumping up voltage, the continuous mode operation is recommended. Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 16 http://www.fujielectric.co.jp/fdt/scd/ FA7764 10. Example of applied circuits (1) Output voltage is between 3.1V and 5.5V. 1 CREG CIN RT 3 RT FA7764N 2 ENB VCC 8 4 GND L1 OUT 7 VBIAS 6 IN 5 CV CB IC CREG R3 Cout D1 R1 ENB C1 (2) Output voltage is less than 3.1V or more than 5.5V. 1 CREG CIN RT 3 RT FA7764N 2 ENB VCC 8 4 GND L1 OUT 7 VBIAS 6 IN 5 CV D1 Cout IC CREG R3 R1 ENB C1 When you determine values and external discrete components, examine under the actual circuit condition. Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 17 http://www.fujielectric.co.jp/fdt/scd/ FA7764 Example of Parts list Parts reference numbers are shown in the figures of applied circuits. (1)Input voltage：20～30V, Output voltage：5V Ref. Description Type name Maker Ref. Description Type name Maker IC IC FA7738N Fuji CV Ceramic Capacitor 0.1uF D1 Schottky Diode SD833-06 Fuji CB Ceramic Capacitor 0.1uF L1 Inductor CDRH104R-47uH SUMIDA C1 Ceramic Capacitor 220pF CIN Electrolytic Capacitor 220uF/35V Panasonic RT Resistor 22kΩ(fosc=200kHz) Cout Electrolytic Capacitor 220uF/6.3V Panasonic R1 Resistor 40kΩ CREG Ceramic Capacitor 0.1uF R3 Rsisitor 10kΩ (2)Input voltage：20～30V, Output voltage：3.3V Ref. Description Type name Maker Ref. Description Type name IC IC FA7738N Fuji CV Ceramic 0.1uF D1 Schottky Diode SD833-06 Fuji CB Ceramic 0.1uF L1 Inductor CDRH104R-47uH SUMIDA C1 Ceramic 220pF CIN Electrolytic 220uF/35V Panasonic RT Resistor 22kΩ(fosc=200kHz) Cout Electrolytic 220uF/6.3V Panasonic R1 Resistor 23kΩ CREG Ceramic 0.1uF R3 Resistor 10kΩ Maker Fuji Electric Systems Co., Ltd. AN-059E Rev.1.0 June.2010 18 http://www.fujielectric.co.jp/fdt/scd/