SI-8511NVS Application Note Surface Molding synchronized rectification Chopper Regulator Control IC SI-8511NVS 0.03 Edition January 2005 SANKEN ELECTRIC CO., LTD. 1 SI-8511NVS －－－ Contents －－－ 1. General Description 1-1 Features －－－－－－－－－－ 3 1-2 Applications －－－－－－－－－－ 4 1-3 Type －－－－－－－－－－ 4 2. Specification 2-1 Package Information －－－－－－－－－－ 5 2-2 Ratings －－－－－－－－－－ 6 2-3 Circuit Diagram －－－－－－－－－－ 9 3. Terminal Description 3-1 Terminal List －－－－－－－－－－ 11 3-2 Functional Description of Terminal －－－－－－－－－－ 12 4. Operational Description 4-1 ON Time Fixed PWM Output Voltage Control －－－－－－－－－－ 15 4-2 Overcurrent Protection / Thermal Shutdown －－－－－－－－－－ 15 5. Cautions 5-1 External Components －－－－－－－－－－ 16 5-2 Pattern Design Notes －－－－－－－－－－ 20 5-3 Operation Waveform Check －－－－－－－－－－ 21 5-4 Thermal Design －－－－－－－－－－ 22 6. Applications 6-1 Soft Start －－－－－－－－－－ 24 6-2 Output ON / OFF Control －－－－－－－－－－ 24 6-3 Controllable Output Voltage －－－－－－－－－－ 24 6-4 Overcurrent Setting －－－－－－－－－－ 24 6-5 Spike Noise Reduction －－－－－－－－－－ 25 6-6 Variable Operating Frequency －－－－－－－－－－ 25 6-7 Overvoltage Setting －－－－－－－－－－ 25 6-8 Power Good Output －－－－－－－－－－ 26 7. Typical Characteristics －－－－－－－－－－ 27 8. Terminology －－－－－－－－－－ 29 2 SI-8511NVS 1. General Description SI-8511NV series are a synchronized rectification chopper type switching regulator IC which is provided with various functions required for the buck switching regulator and protection functions. The synchronized rectification type switching regulator with high precision and high efficiency can be composed by connecting external logic Nch-MOS FET, diode etc. ● 1-1 Features - Surface mounting type and compact size VSOP24 package which is compact and thin - High efficiency 93% (VIN = 5V, Vo = 2.5V, Io = 1A) Since the synchronized rectification type buck regulator can be composed, so high efficiency will be realized. - Reference voltage accuracy ±1.25% Internal reference voltage is so accurate as 1.1V ±1.25%. - Output voltage can be adjusted by an external resistor. Two resistors are used and the output voltage is made variable within the range of 1.1 – 6V. - Operating frequency variable The operating frequency may be made variable in 400 KHz – 100KHz. - Overcurrent protection Overcurrent protection (automatically restoration type) is built in and the threshold value can be set by an external resistor. - Output overvoltage protection The output overvoltage protection function is built in. In the case that the output voltage exceeds the set voltage due to the destruction of high side MOSFET etc., the low side MOS is compulsorily turned on to blow a fuse on the input side.The operating voltage may also be changed by an external resistor. - Soft start function (capable of ON/OFF output) By adding an external capacitor, it is possible to delay the rise speed of the output voltage. ON/OFF control of the output is also possible. - Output ON/OFF function The ON/OFF control of output is made at Lo/Hi level. Control can be made by an external CPU etc. - Power Good output The state of output voltage (Vo) is outputted. 3 SI-8511NVS ● 1-2 Applications For power supply for laptop PC, power supplies for OA equipment, stabilization of secondary output voltage of regulators, power supply for communication equipment and power supply for amusement/leisure instruments. ● 1-3 Type - Type: Semiconductor integrated circuits (monolithic IC) - Structure: Resin molding type (transfer molding) 4 SI-8511NVS 2. Specification ● 2-1 Package Information 5 SI-8511NVS ● 2-2 Ratings Absolute Maximum Rating Characteristic Symbol Ratings Unit DC input voltage (VCC pin) VCC 7 V DC input voltage (VIN pin) VIN 25 V DC input voltage for Boost Block VH 30 V EN pin input voltage Ven VCC V Vpwrgd 7 V Junction Temperature Tj 150 °C Storage Temperature Tstg -40 - 150 °C PWRGD supply voltage *1: Absolute Maximum Ratings shows the destruction limits. It is required to take care so that even one item does not exceed the specified value for a moment during instantaneous or normal operation. Recommended Conditions Rating Characteristic Symbol Unit MIN MAX DC input voltage range 1 VCC 4.5 5.5 V DC input voltage range 2 VIN 3 18 V Output voltage range 2 Vo 1.1 6 V Operation temperature range Top -20 85 °C *2: Recommended Conditions shows the operation conditions required for maintaining normal circuit functions. It is required to meet the conditions in actual operations. 6 SI-8511NVS Electrical Characteristics (Unless otherwise note Ta = 25°C) Ratings Characteristic Symbol Unit MIN TYP MAX -1.2% 1.1 +1.2% Remark Loop Characteristic Output voltage Output voltage temperature coefficient Vo ΔVo/ΔT ±0.03 V mV/°C VIN=5V,VCC=5V, VSNS shorted to Vo, Io=0 VIN=5V, VCC=5V, Vo=1.1V, Io=0, Ta=0-85°C Circuit Current Current (VCC pin) Iop 6 mA VCC=5V, EN=H, FADJ:open Cureent (VIN pin) Iop 1 mA VIN=5V, EN=H Standby Current (VCC pin) Istd1 100 μA VCC=5V,EN＝L Standby Current (VIN pin) Istd2 50 μA VIN=5V,EN=L UVLO Block UVLO starting voltage1 (VCC) UVLO starting voltage2 (VIN) Vuvlo1 3.7 4.4 V VIN=5V Vuvlo2 2.5 2.9 V VCC=5V ON Time Control Block On time Ton μS 1.27 VCC=5V, VIN=5V, Vo=2.5V Minimum off time Toff FSET pin output voltage Vref FSET pin source current Iref μS VCC=5V 1.3 V VCC=5V 100 μA VCC=5V 0.7 1.1 1.2 High Side Drive Block ON Resistance (Pull-Up) Ron 5.5 Ω VH-VLIN=5V ON Resistance (Pull-Down) Ron 5.5 Ω VH-VLIN=5V ON Resistance (Pull-Up) Ron 5.5 Ω VCC=5V ON Resistance (Pull-Down) Ron 5.5 Ω VCC=5V Low Side Drive Block 7 SI-8511NVS Electrical Characteristics (Unless otherwise note Ta = 25°C) Ratings Characteristic Symbol Unit Remark MIN TYP MAX VH-VLIN 4.5 5 5.5 V Reference Current Ilim 90 100 110 μA VCC=5V, VIN=5V Soft Start pin current Iss μA VCC=5V Bootstrap Block Bootstrap Voltage Protection Block ±20 EN Input Low Voltage Vcelo 0 0.8 V VCC=5V EN Input High Voltage Vcehi 2.4 VCC V VCC=5V 5 μA VCC=5V, EN=5V EN Bias Current ICE OVP operation Voltage Vsns 1.32 V VCC=5V PERGD Trip Threshold Vsns 0.88 V VCC=5V PERGD Low level Vlotage Vpwrgd 0.4 V VCC=5V, Ipergd=120μA PWRGD pin current Ipwrgd 120 μA VCC=5V, Vpwrgd=0.4V PERGD leakage Current Ipwrgd 5 μA Vpwrgd=5V 8 SI-8511NVS ● 2-3 Circuit Diagram Block Diagram Vin +5V VCC1 VIN ISEN ILIM VCC2 OCP PRE_REG EN H:ON L:OFF EN Vpreg VH Level Shift Gate Driver OFF Clamp UVLO Latch Buff DRVH Vo Synchronous Cont. (Logic) POWER_GOOD PWRGD H:GOOD L:NG LIN Logic Power Good Buff DRVL PGND Switching VO Constant On Time Cont. VSNS COM P OSC SS GND OVP_SL FADJ FSET 14 SS 12 SKIP Open:Change Frequecy Short:400KHz Operation Open:Skip Mode L:No Skip Mode Typical Connection Diagram C9 0.022μF VIN R2 C1 10μF VCC 5V D2 SFPL52 C7 0.1μF R1 5mΩ R5 10Ω 1 2 3 4 0.01 C6 1μF 5 6 7 8 VCC R7 47kΩ PWRGD R10 2.2kΩ R9 9 10 11 NC NC DRVH LIN VH DRVL VIN PGND ISEN VCC2 ILIM OVP_SL SI-8511NVS GND VCC1 VSNS SS Vo EN PWRGD SKIP FSET FADJ 12 NC R8 200kΩ Q1 24 L1 5.6μH 23 22 21 20 Vo Q2 R6 10Ω D1 SFPJ73 C2 330μF C5 4.7μF R13 19 18 R12 C4 3.3μF 17 C8 220pF 16 15 EN SKIP VCC R4 47kΩ C3 0.1μF 14 NC 13 R11 100kΩ 9 SI-8511NVS Major Component List Component Description Maker Q1 N-ch MOS FET IRF7807 IR Q2 N-ch MOS FET IRF7807 IR L1 Inductor 10μH SLF12575-5R6N6R3 TDK D1 First Recovery Diode 1A SFPL-52 SANKEN D2 Schottky Diode 3A SFPJ-73 SANKEN C1 Ceramic Capacitor 10μF/25V GRM43-2X7R106K25 MURATA C2 Functional Polymer Tantalum Capacitor 330μF/6.3V PSLD0J337M NEC Component Layout Example D2 VCC C9 R2 VIN R1 R10 C6 R5 Q2 R6 D1 C5 C4 EN R4 C1 Q1 C7 C3 C8 R7 R11 R8 R9 D2 GND C2 SW L1 Vo 10 SI-8511NVS 3. Terminal Description ● 3-1 Terminal List No. Terminal Description 1 NC Non-connected terminal 2 DRVH High side drive terminal 3 VH Boot Strap capacitor connected terminal 4 VIN Input voltage terminal (power) 5 ISEN Overcurrent detection resistor connected terminal 6 ILIM Overcurrent limiting resistor connected terminal 7 GND Ground terminal for analogue 8 VSNS Output voltage sensing terminal 9 VO 10 PWRGD 11 FSET 12 NC Non-connected terminal 13 NC Non-connected terminal 14 FADJ Operating frequency adjustment terminal 15 SKIP SKIP mode select terminal 16 EN Circuit enable terminal 17 SS Soft start terminal 18 VCC1 Input voltage terminal (control) 19 OVP_SL Overvoltage detection terminal 20 VCC2 Input voltage terminal (Low side drive) 21 PGND Ground terminal for power 22 DRVL Low side drive terminal 23 LIN Switching terminal 24 NC Non-connected terminal Output voltage input terminal Power Good terminal Operating frequency setting terminal 11 SI-8511NVS ● 3-2 Functional Description of Terminal 1. VCC1 and VCC2 terminals These are +5V input terminals. VCC1 is a power supply terminal for internal power supplies and control circuits of SI-8511NVS. VCC2 is a power supply terminal for the low side drive circuit. Since much switching noise is imposed on VCC2, care should be taken not to affect the VCC1. 2. VIN terminal This is an input terminal for the external main voltage (3 - 18V). The VIN terminal is used for the frequency setting and overcurrent protection circuit. Please take care of switching noise not to cause adverse effect. 3. EN terminal This is an ON/OFF control terminal of the SI-8511NVS. When VCC is 5V, the output is turned ON by setting the EN terminal at H (2.4V or higher) and it is turned OFF by setting it at L (0.8V or lower). Since this is CMOS logic input, it should be set either at H or L instead of floating. 4. PWRGD terminal The PWRGD terminal is in open drain output. The sink ability is 120μA (MAX). It should be pulled up to the power supply externally. The PWRGD signal also shows the state of output voltage Vo (VSNS voltage). When the VSNS voltage is 0.88V or lower or 0.88V or higher, L or H is outputted respectively. However, the voltage of OVP_SL terminal is 1.32V or higher, L is outputted to maintain the operating condition as it is. 5. FADJ terminal This is a terminal for the selection of operating frequency. When the FADJ terminal is connected to GND by a resistor of 100kΩ or so, this is a 400kHz fixed mode of the maximum frequency. If the FADJ terminal is made OPEN, this is the variable operation frequency mode. In this case, the operation frequency can be changed subject to the resistance value connected to the FSET terminal. When 200kΩ is connected to the FSET terminal, the operating frequency of 250kHz or so is set. 6. FSET terminal This is a 1.2V DC output terminal (source only). When the SI-8511NVS is in the operating frequency variable mode, the operating frequency can be changed subject to the resistor connected to the FSET terminal. 7. SS terminal The soft start time can be set by varying the frequency of a dedicated oscillator (OSC) from a capacitor connected to the SS terminal. 12 SI-8511NVS 8. ISEN terminal This is an overcurrent value detection terminal. One side of the current detection resistor is connected. 9. ILIM terminal This is an overcurrent value setting terminal. Current flowing into ILIM is set to be 100μA. The voltage drop caused by the resistor connecting this current to Vin is a reference of threshold of the overcurrent value. 10. VH terminal This is an input voltage terminal for boost strap. An external capacitor and diode should be connected in accordance with the standard connecting circuit. 11. DRVH terminal This is a terminal for driving a high side MOS. 12. DRVL terminal This is a terminal for driving a low side MOS. 13. LIN terminal This is a switching node terminal. The switching side of external inductor should be connected. 14. GND terminal This is an analog ground terminal. 15. PGND terminal This is a power ground terminal. 16. Vo terminal This is an output voltage detection terminal. This is used for setting the operating frequency. 17. VSNS terminal This is a terminal for setting the output voltage. A resistor for the detection of output voltage should be connected. 18. SKIP terminal This is a terminal for skip mode selection. If the SKIP terminal is connected to GND, the oscillating frequency can be stabilized to some extent even at light load. And if it is in OPEN, the operation is in skip mode (frequency varies.). 13 SI-8511NVS 19. OVP SL terminal This is a terminal for setting the operation voltage value of output overvoltage protection. When the voltage of the terminal exceeds the threshold value (1.32 V), the high side MOS turns off immediately and the low side MOS turns on. If it is connected to GND, the output overvoltage protection function does not work anymore. 14 SI-8511NVS 4. Operational Description ● 4-1 ON time fixed PWM output voltage control The SI-8511NVS is not provided with an internal oscillator for operating frequency. The SI-8511NVS adopts a PWM control system with constant ON time. ON time is controlled by the relation between voltage of VIN and Vo for 400 MHz operation. By using this system, rapid response to the steep variation of load is made possible. ● 4-2 Overcurrent Protection The SI-8511NVS uses two external resistors for the detection of overcurrent. R1 is used for the purpose of detection of current flowing on the high side of MOS, while R2 for the purpose of determining the detection point. The reference current of 100μA constantly flows across the R2. When the potential difference produced in the current which flows across R1 exceeds the potential difference produced in the reference current and R2, the overcurrent protection operation starts. 15 SI-8511NVS 5. Cautions ● 5-1 External Components 5-1-1 Choke coil L1 The choke coil L supplies current to the load side when the switching transistor is OFF. The coil is one of the most important components in the chopper type switching regulator. In order to maintain the stable operation of the regulator, such dangerous state of operation as saturation state and operation at high temperature due to heat generation must be avoided. The following points should be taken into consideration for the selection of the choke coil. a) The choke coil should be fit for the switching regulator. The coil for a noise filter should not be used because of large loss and generated heat. b) The inductance value should be appropriate. The larger is the inductance of the choke coil, the less is the ripple current flowing across the choke coil, and the output ripple voltage drops and as a result, the overall size of the coil becomes larger. On the other hand, if the inductance is small, the peak current flowing across the switching transistor and diode is increased to make the ripple voltage higher and this operation state is not favorable for maintaining the stable operation. Large Inductance Small Inductance Large Ripple Voltage/ Current Small Ripple Voltage/ Current The larger is the inductance, the smaller will be the ripple current/voltage. But the outer size of the coil becomes larger. The smaller is the inductance, the larger will be the ripple curtent/voltage. Although the outer size of the coil is smaller, the operation is likely to be unstable. The inductance value shown in typical connection diagram should be considered as a reference value for the stable operation and the appropriate inductance value can be calculated by the following equation. ΔIL shows the ripple current value of the choke coil and the lower limit of inductance is set as described in the following. - In the case that the output current is about 3A: output current × 0.2 - 0.6 - In the case that the output current is over 5A: output current × 0.2 – 0.3 L (Vin Vout ) Vout －－－(1) IL Vin f For example, where VIN = 10V, VOut = 2.5V, ΔIL = 0.9A, frequency = 400KHz, 16 SI-8511NVS L (10 2.5) 2.5 ≒ 5.2uH 0.9 10 400 103 As shown above, the coil of about 220μH may be selected. c) The rated current shall be met. The rated current of the choke coil must be higher than the maximum load current to be used. When the load current exceeds the rated current of the coil, the inductance is sharply decreased to the extent that it causes saturation state at last. Please note that overcurrent may flow since the high frequency impedance becomes low. d) Noise shall be low. In the open magnetic circuit core, since magnetic flux passes outside the coil, the peripheral circuit may be damaged by noise. Especially, when noise is imposed on the Vo detection line, it is likely to cause mal function. It is recommended to use a closed magnetic circuit type core as much as possible. 5-1-2 Input Capacitor C1 The input capacitor is operated as a bypass capacitor of the input circuit to supply steep current to the regulator during switching and to compensate the voltage drop of the input side. Therefore, the input capacitor should be connected as close as to the regulator IC. In addition, in the case that the smoothing capacitor of the AC rectifier circuit is located in the input circuit, the input capacitor may be also used as a smoothing capacitor, but similar attention should be paid. In order to reduce ringing noise, it will be effective to connect a ceramic capacitor of 0.1 – 0.47μF in parallel with C1. The selection of C1 shall be made in consideration of the following points: a) The requirement of withstand voltage shall be met. b) The requirement of the allowable ripple voltage shall be met. Current Flow of C1 Current Waveform of C1 C1電流波形 Iin Vin 0 Ripple Current リップル電流 Iv Ton T D= Ip Ton T The ripple current of the input capacitor is 入力ｺﾝﾃﾞﾝｻのﾘｯﾌﾟﾙ電流は、負荷 increased in accordance with the increase of 電流の増加に伴って増大する。 the load current. If the withstanding voltages or allowable ripple voltages are exceeded or used without derating, it is in danger of causing not only the decreasing the capacitor lifetime (burst, capacitance decrease, equivalent impedance increase, etc) but also the abnormal oscillations of regulator. Therefore, the selection with sufficient margin is needed. 17 SI-8511NVS The effective value of ripple current flowing across the input capacitor can be calculated by the following equation: Irms 1.2 Vo Iout Vin －－(2) For instance, where Io = 3A, VIN = 10V, Vo = 2.5V, Irms 1.2 2.5 3 0.9 A 10 Therefore, it is necessary to select the capacitor with the allowable ripple current of 0.9A or higher. 5-1-3 Output Capacitor C2 The output capacitor C2 composes a LC low pass filter together with a choke coil L and functions as a rectifying capacitor of switching output. The current equivalent to the pulse current ΔIL of the choke coil current is charged and discharged in the output capacitor. Therefore, it is necessary to meet the requirements of withstand voltage and allowable ripple current with sufficient margin like the input capacitor. Additional points to be checked are DC equivalent series resistance (ESR) and capacitance. The following points should be taken into consideration. Current Flow of C2 Current Waveform of C2 Ripple Current The ripple current of the output capacitor is equal to the ripple current of the choke coil and does not vary even if the load current increases or decreases. - Allowable Ripple Current The ripple current effective value of the output capacitor is calculated by the equation. Irms IL 2 3 －－－(3) When ΔIL = 0.5A, Irms 0.5 ≒ 014 . A 2 3 Therefore a capacitor having the allowable ripple current of 0.14A or higher is required. - DC equivalent series resistance (ESR) It is necessary for the stable operation to select the ESR properly. When the ESR is too large or too small, 18 SI-8511NVS abnormal oscillation due to increase of ripple voltage or insufficient phase margin occurs respectively. The output ripple voltage is determined by a product of the pulse current ΔIL (=C2 discharge and charge current) of the choke coil current and the ESR, and the output ripple voltage which is 0.5 - 3% of the output voltage (for example, where 1% at Vout = 5V, 50mV) is good for the stable operation. Please refer to the equations (4) and (5) to obtain the output ripple voltage. Please take note that ESR of aluminum electrolytic capacitor varies subject to temperature and ESR falls especially at high temperature. Vrip Vin Vout Vout ESR L Vin f －－－(4) Vrip IL ESR －－－(5) When the ESR is too low (approx. 10Ω or lower), the phase delay becomes larger, resulting in abnormal oscillation. Therefore, it is not appropriate that a tantalum capacitor or a laminated ceramic capacitor is used for the output capacitor as an independent component. However, connecting a tantalum capacitor or a laminated ceramic capacitor in parallel with an electrolytic capacitor is effective in reducing the output ripple voltage only when it is used at low temperature (< 0°C). In addition, in order to further decrease the ripple voltage, as shown below, it is also effective to add one stage of the LC filter to form the π type filter. L1 L2 References SI-8511NVS Vsns L2: 5.6μH Co1 Co2 Co2: 47μF It should be noted that the operating stability is more influenced by the ESR than the capacitance as described above if the requirements of withstand voltage and allowable ripple current are met. With respect to the layout of the output capacitor, if it is located far from the IC, it will give same effect as the increase of ESR due to wiring resistance etc., therefore it is recommended to connect it near the IC. 5-1-4 Switching MOSFET The logic type Nch-MOS should be used at low RDSON. If the general purpose MOS is used, ON resistance is not reduced to be inefficient because of shortage of Vgs and significant heat generation may happen. Since the power MOS driving terminal (DRVH, DRVL) of the SI-8511NVS has driving ability of around 0.5A at 2.5V output, a MOSFET should be selected in accordance with this ability. 5-1-5 Overcurrent detection resistor Resistance values of several dozen mΩ to several mΩ should be used. Please do not use resistors with large L values. When Vin is about 3V, please try to limit the voltage drop produced at R2 within 100mV or so. 19 SI-8511NVS 5-1-6 Voltage detection resistor The internal reference voltage of the SI-8511NVS is set at 1.1V. The output voltage is set by connecting two resistors (R9 and R10) to the VSNS terminal. In addition, in order to maintain the stability of operation, please set a resistance value so as to permit the flow of current of 500μA – 2mA. ● 5-2 Pattern Design Notes 5-2-1 VCC Line Since the current of 1A or so flows in the VCC line at the moment of MOS switching, the pattern of appropriate thickness should be used. 5-2-2 Input/ Output Capacitor The input capacitor C1 and the output capacitor C2 should be connected to the IC as close as possible. If the rectifying capacitor for AC rectifier circuit is on the input side, it can be used as an input capacitor. However, if it is not close to the IC, the input capacitor should be connected in addition to the rectifying capacitor. Since high current is discharged and charged through the leads of input/output capacitor at high speed, the leads should be as short as possible. A similar care should be taken for the patterning of the capacitor. C1,C2 Improper Pattern Example C1,C2 Proper Pattern Example 5-2-3 Vo detection Line For the Vo detection line (see the typical connection diagram.), the shortest patter from the vicinity of the output capacitor C2 should be used. In the case of long pattern, please take note that abnormal oscillation may be caused because of degraded regulation and increase of switching ripples. Interference with the inductance L1 should be avoided. In addition, it is recommended to use the pattern of appropriate thickness in order to reduce the impedance. 20 SI-8511NVS 5-2-4 Overcurrent detection For the connection of terminals (ISEN and ILIM) which detect overcurrent, Kelvin wiring should be made at both ends of the current detection resistor. Wiring should be made in a manner that the line connected to the both ends is not affected by large current line. Please refer to the following diagram for wiring. Large current ⇒大電流 R1 R2 Overcurrent detection resistors R1 and R2 should be of 5 – 30mΩ. In the case that the output current exceeds 5A, they should be 5 – 10mΩ in consideration of decrease of loss and reduction of noise. It is recommended to mount a capacitor in parallel with R2 in order to reduce the effect of switching noise. The capacitance should be several thousand pF – 0.33μF. In the case that noise due to switching current is intense, the effect of noise may be alleviated, if a resistor (1 – 15Ω) is inserted between the VIN terminal (No.4 pin) and VIN line in series. ● 5-3 Operation Waveform Check It can be checked by the waveform between the pin 18 to 20 (SWOut waveform) of the SI-8511NVS whether the switching operation is normal or not. The examples of waveforms at normal operations are shown below: 1. Normal Operation (continuous area) 2. Normal Operation (discontinuous area) SKIP: Open 3. Normal Operation (discontinuous area) SKIP: GND 21 SI-8511NVS The continuous area is an area where the DC component of the triangular wave is superimposed on the current flowing across the choke coil and the discontinuous area is an area where the current flowing across the choke coil is intermittent (a period of zero current may happen.) because the current flowing across the choke coil is low. Therefore, when the load current is high, the area is a continuous area and when the same current is low, the area is a discontinuous area. In the continuous area, the switching waveform is formed in the normal rectangular waveform (waveform 1) and in the discontinuous area, damped oscillation in the switching waveform (waveform 2) or lower frequency (waveform 3) are formed, but this is a normal operation without any problem. When the IC is far from C1 and C2, the ON – OFF time of switching may be disturbed. Therefore, C1 and C2 should be connected close to the IC. ● 5-4 Thermal Design 5-4-1 Calculation of Heat Dissipation The relation among the power dissipation Pd of MOSFET, junction temperature Tj, case temperature Tc, heat sink temperature Tfin and ambient temperature Ta is as follows: Pd (Power dissipation) Tj: Junction Temperature (125°C max) Θjc: Thermal Resistance (Junction – Case) Chip Tc: Case temperature (internal frame temperature) Case Heat sink Θi: Thermal Resistance (Case - Heat sink) Tfin: Heat sink temperature Θfin: Heat sink Thermal resistance Ta: Ambient temperature Tj Tc －－(6) jc Tj Tfin －－(7) Pd jc i Tj Ta －－(8) Pd jc i fin Pd The TjMAX is an inherent value for each product, therefore it must be strictly observed. For this purpose, it is required to design the heat sink in compliance with PdMAX, TaMAX (determination of 22 SI-8511NVS θfin). The heat derating graphically describes this relation. The designing of the heat sink is carried out by the following procedure: 1) The maximum ambient temperature Ta MAX in the set is obtained. 2) The maximum power dissipation PdMAX is obtained. (The following calculation is approximate.) Vo 1.1 (high side) －－－(9-1) Vin Vin Vo －－－(9-2) Pd Rdson Io 2 1.1 (low side) Vin Pd Rdson Io 2 3) The pattern is determined from the intersection of the heat derating. The required thermal resistance of the heat sink can be also calculated. The thermal resistance required for the heat sink is obtained by the following equation: i fin Tj Ta jc Pd －－－(10) The thermal resistance data should be in accordance with the specifications of MOS to be used. Generally heat sink will be used in the case of derating is 10 - 20% or more. Actually, heat dissipation effect significantly changes depending on the difference in component mounting. Therefore, heat sink temperature or case temperature at mounted should be checked. 23 SI-8511NVS 6. Applications ● 6-1 Soft Start In the case that the capacitance of output capacitor is large, the overcurrent protection circuit may be operated due to the in-rush current of output capacitor at startup and as result, the output may not be activated easily. As a countermeasure, it is effective to suppress the in-rush current by utilizing soft start. When the capacitance of Css is increased, it will have an effect of reduction of load short circuit current. The soft start time can be set by varying the frequency of dedicated oscillator (OSC) from the capacitor connected to the SS terminal. In addition, a 32-bit counter is built in and when this counter counts 32 bits, the output voltage becomes a set voltage value. Since the current flowing into the SS terminal is set at ±20μA, the soft start time can be calculated by the following equation.: Tss 20310C-6 32 S Tss :soft start time, C: capacitance connected to CSS terminal For example, when a 220pF capacitor is connected to the SS terminal, Tss 3 220 10 -12 32 1ms 20 10 -6 Then, the soft start time can be 1mS. Since this soft start is used as an overcurrent limiting function, the soft start time should be limited within the range of 0.5 – 100mS. ● 6-2 Output ON / OFF Control By setting the EN terminal at H (2.4V or higher) or L (0.8V or lower), the output is turned ON or OFF respectively. As logic input is used for the EN terminal it should be fixed either at H or L. ● 6-3 Controllable Output Voltage The output voltage can be set by changing the values of R9 and R10. The current of 50μA – 2mA should flow across these resistors. The reference voltage of SI-8511NVS is set at 1.1V. For example, when the current of 500μA flows across R9, the result is expressed as R10 = 1.1V/500μA = 2.2KΩ. Then, please set the resistance of R9 by the following equation. R9 Vo / 1.1 1 R10 When Vo is 2.5V, then R9 = (2.5 / 1.1 -1 × 2. 2KΩ) ＝ 2.8KΩ. ● 6-4 Overcurrent Setting The SI-8511NVS compares the voltage produced at a resistor connected to ILIM (reference voltage) with the 24 SI-8511NVS voltage produced at a current detection resistor connected to ISEN to perform the overcurrent protection at a voltage value where the voltage drop of current detection resistor is equal to the voltage drop of a resistor connected to ILIM. Since the current which flows into ILIM is set to be 100μA, the overcurrent value is determined by the following equation. Is R2 A 100 10-6 R1 In this equation, R1: current detection resistor value, R2: resistance value connected to ILIM terminal When VIN is such low voltage as 3V, the voltage drop by R1 and R2 should be below 100mV to ensure the operation of an internal comparator. ● 6-5 Spike Noise Reduction In order to decrease spike noise, it is effective to compensate the output waveform of high side MOSFET and the recovery time of diodes by capacitors and resistors. It is effective for either high side MOS or diode side. Please set in reference to the following figure. Please take note, however, that in either case, the efficiency is slightly degraded. 5 ISEN SI-8511NVS PGND DRVH 20Ω 100 1000pF LIN DRVL 5 20Ω 100 2200pF ● 6-6 Variable Operating Frequency In order to avoid f trouble, the SI-8511NVS can change the internal ON time by using the FADJ terminal and FSET terminal. When 100kΩ is connected between the FADJ terminal and GND terminal, the maximum frequency is set at 400KHz. When the FADJ terminal is made in OPEN state and a resistor is connected between FSET terminal and GND terminal, the frequency can be variable. This resistance is set at 100kHz or so by a 100kΩ resistor, 250kHz or so by 200kΩ resistor, 300kHz or so by 400kΩ resistor. The resistor connected between FSET and GND should be set in the range of 100kΩ －1MΩ. ● 6-7 Overvoltage Setting The setting of overvoltage value can be made by using two external resistors. The threshold value is 1.32V. When the same value as the value of a resistor for setting the output voltage is connected, the overvoltage protection operation starts at the voltage of 20% higher than the output voltage. When the overvoltage 25 SI-8511NVS protection operation starts, the DRVH terminal is set at low voltage (same voltage as LIN terminal) and the DRVL terminal is set at high voltage (same voltage as VCC2 terminal) and this state is maintained. As the DRVL terminal is in high voltage, the low side MOS turns on to blow the fuse of input side. However, as the fuse may not be blown subject to the ON resistance of MOS, state of input voltage etc., the rated value of fuse should be determined after adequate reviewing. The overvoltage protection is reset, when the power supply is reconnected or the EN terminal is once on low side and then changed to high side. ● 6-8 Power Good Output The PWRGD terminal is in open drain output. The sink ability is 120μA (Max.). Please pull up the power supply externally. The PWRGD signal shows the state of output voltage (VSNS voltage if defined more accurately). When the VSNS is below 0.88V, L is outputted, while H is outputted, when it is above 0.88V. However, when OVP SL terminal is above 1.32V, L is outputted and this condition is maintained. This state is reset, when the power supply is reconnected or the EN terminal is once made low and then high. Timing chart ﾀｲﾐﾝｸﾞﾁｬｰﾄ OVP_SL VSNS 1.32V 0.88V PWRGD This is effective for forming the power supply sequence. After the PWRGD is made high, load can be applied. 26 SI-8511NVS 7. Typical Characteristics Enable Characteristics Vo Vin=5V Vo/Io=2.5V/0A Css=220pF EN Load Responsiveness Vin=12V Vo =1.9V 100mV Io＝0 - 4A SW_out(LIN) Vo 2A 5μS Io 27 SI-8511NVS Electrical Characteristics Efficiency(Vo=3.3V) Load Regulation(Vo=3.3V) SKIP:Connect to GND 80 Vin=5V 8V 10V 12V 15V 18V 60 40 20 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 Output Voltage(V) Efficiency(%) 100 Output Current(A) SKIP:Connect to GND 3.35 3.34 3.33 3.32 3.31 3.30 3.29 3.28 3.27 3.26 3.25 V in=5V 8V 10V 12V 15V 18V 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Output Current(A) Frequency(Vo=3.3V) Over Current Protection(Vin=12V) SKIP:Connect to GND 450 Output Voltage(V) Output Voltage(V) 500 400 350 Vin=5V 8V 10V 12V 15V 18V 300 250 200 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 Output Current(A) 4 3.5 3 2.5 2 1.5 1 0.5 0 0 1 2 3 4 5 6 Output Current(A) Rise Characteristic(VCC=5V) Output Voltage(V) SKIP:Connect to GND 4 3.5 3 2.5 2 1.5 1 0.5 0 0 2 4 6 8 10 Input Voltage(V) 28 5.5 SI-8511NVS 8. Terminology - Recommended Conditions It shows the operation conditions required for maintaining normal circuit functions. It is required to meet the conditions in actual operations. - Absolute Maximum Ratings It shows the destruction limits. It is required to take care so that even one item does not exceed the pacified value for a moment during instantaneous or normal operation. - Electrical Characteristics It is the specified characteristic value in the operation under the conditions shown in each item. If the operating conditions are different, it may be out of the specifications. - PWM (Pulse Width Modulation) It is a kind of pulse modulation systems. The modulation is achieved by changing the pulse width in accordance with the variation of modulation signal waveform (the output voltage for chopper type switching regulator). - ESR (Equivalent Series Resistance) It is the equivalent series resistance of a capacitor. It acts in a similar manner to the resistor series-connected to the capacitor. 29 SI-8511NVS Notice ・The contents of this description are subject to change without prior notice for improvement etc. Please make sure that any information to be used is the latest one. ・Any example of operation or circuitry described in this application note is only for reference, and we are not liable to any infringement of industrial property rights, intellectual property rights or any other rights owned by third parties resulting from such examples. ・In the event that you use any product described here in combination with other products, please review the feasibility of combination at your responsibility. ・Although we endeavor to improve the quality and reliability of our product, in the case of semi-conductor components, defects or failures which occur at a certain rate of probability are inevitable. The user should take into adequate consideration the safety design in the equipment or the system in order to prevent accidents causing death or injury, fires, social harms etc.. ・Products described here are designed to be used in the general-purpose electronic equipment (home appliances, office equipment, communication terminals, measuring equipment etc.). If used in the equipment or system requiring super-high reliability (transport machinery and its control equipment, traffic signal control equipment, disaster/crime prevention system, various safety apparatus etc.), please consult with our sales office. Please do not use our product for the equipment requiring ultrahigh reliability (aerospace equipment, atomic control, medical equipment for life support etc.) without our written consent. ・The products described here are not of radiation proof type. ・The contents of this brochure shall not be transcribed nor copied without our written consent. 30