RC4190N

Electronics Semiconductor Division RC4190 Micropower Switching Regulator Features • • • • • High efficiency – 85% typical Low quiescent current – 215 µA Adjustable output – 1.3V to 30V High switch current – 200 mA Bandgap reference – 1.31V • • • • • Accurate oscillator frequency – ±10% Remote shutdown capability Low battery detection circuitry Low component count 8-lead packages including small outline (SO-8) Description The RC4190 monolithic IC is a low power switch mode regulator intended for miniature power supply applications. This DC-to-DC converter IC provides all of the active components needed to create supplies for micropower circuits (load power up to 400 mW, or up to 10W with an external power transistor). Contained internally are an oscillator, switch, reference, comparator, and logic, plus a discharged battery detection circuit. Application areas include on-card circuits where a non-standard voltage supply is needed, or in battery operated instruments where an RC4190 can be used to extend battery lifetime. These regulators can achieve up to 85% efficiency in most applications while operating over a wide supply voltage range, 2.2V to 30V, at a very low quiescent current drain of 215 µA. The standard application circuit requires just seven external components for step-up operation: an inductor, a steering diode, three resistors, a low value timing capacitor, and an electrolytic filter capacitor. The combination of simple application circuit, low supply current, and small package make the RC4190 adaptable to a wide range of miniature power supply applications. The RC4190 is most suited for single ended step-up (VOUT > VIN) circuits because the NPN internal switch transistor is referenced to ground. It is complemented by another Raytheon micropower switching regulator, the RC4391, which is dedicated to step-down (VOUT < VIN) and inverting VOUT = –VIN) applications. Between the two devices the ability to create all three basic switching regulator configurations is assured. Refer to the RC4391 data sheet for stepdown and inverting applications. With some optional external components the application circuit can be designed to signal a display when the battery has decayed below a predetermined level, or designed to signal a display at one level and then shut itself off after the battery decays to a second level. See the applications section for these and other unique circuits. The RC4190 micropower switching regulator series consists of three devices, each with slightly different specifications. The RM4190 has a 1.5% maximum output voltage tolerance, 0.2% maximum line regulation, and operation to 30V. The RC4190 has a 5.0% maximum output voltage tolerance, 0.5% maximum line regulation, and operation to 24V. Other specifications are identical. Each type is available in plastic and ceramic DIPs, or SO-8 packages. Block Diagram 4190 LBR LBD C2 Q2 CX OSC C1 +1.31V 1.31V REF VFB Gnd Q1 +1.2V LX BIAS IC +VS 65-3464-01 Rev. 1.0.0 PRODUCT SPECIFICATION RC4190 Pin Assignments LBR CX GND LX 1 2 3 4 8 7 6 5 65-3464-02 Pin Definitions LBD VFB IC +VS Pin Name LBR CX Gnd LX +VS IC VFB LBD Pin Number 1 2 3 4 5 6 7 8 Pin Function Description Low Battery (Set) Resistor Timing Capacitor Ground External Inductor Positive Supply Voltage Reference Set Current Feedback Voltage Low Battery Detector Output Absolute Maximum Ratings (beyond which the device may be damaged)1 Parameter Supply Voltage (Without External Transistor) PDTA < 50°C RM4190 RC4190 SOIC PDIP CerDIP Operating Temperature Storage Temperature Junction Temperature Switch Current For TA > 50°C Derate at SOIC, PDIP CerDIP Peak SOIC PDIP CerDIP Note: 1. Functional operation under any of these conditions is NOT implied. Min Typ Max 30 24 300 468 833 Units V V mW mW mW °C °C °C °C °C RM4190 RC4190 -55 0 -65 125 175 125 70 150 375 4.17 6.25 8.33 mA mW/°C mW/°C mW/°C Operating Conditions Parameter θJC θJA Thermal resistance Thermal resistance CerDIP SOIC PDIP CerDIP Min Typ 45 200 160 120 Max Units °C/W °C/W °C/W °C/W 2 RC4190 PRODUCT SPECIFICATION Electrical Characteristics (+VS = +6.0V, IC = 5.0 µA over the full operating temperature range unless otherwise noted.) RM4190 Symbol Parameters +VS VREF ISY Supply Voltage Reference Voltage (Internal) Supply Current Line Regulation LI IC ICO ISO ILBD Load Regulation Reference Set Current Switch Leakage Current Supply Current (Disabled) Low Battery Output Current Oscillator Frequency Temperature Drift V4 = 24V (RC4190) 30V (RM4190) VC ≤ 200 mV V8 = 0.4V, V1 = 1.1V 500 1200 ± 200 Measure at Pin 5 I4 = 0 0.5 VOUT < VS < VOUT VS = 0.5 VOUT PL = 150 mW 1.0 Conditions Min 2.6 1.25 1.31 235 0.2 0.5 5.0 Typ Max 30 1.37 350 0.5 1.0 50 30 30 500 1200 ± 200 1.0 Min 2.6 1.20 1.31 235 0.5 0.5 5.0 RC4190 Typ Max 24 1.42 350 1.0 1.0 50 30 30 Units V V µA % VO % VO µA µA µA µA ppm/°C 3 PRODUCT SPECIFICATION RC4190 Electrical Characteristics (+VS = +6.0V, IC = 5.0 µA, and TA = +25°C unless otherwise noted.) RM4190 Symbol +VS VREF ISW ISY ef Parameters Supply Voltage Reference Voltage (Internal) Switch Current Supply Current Efficiency Line Regulation LI FO IC ICO Load Regulation Operating Frequency Range Reference Set Current Switch Leakage Current Supply Current (Disabled) Low Battery Bias Current Capacitor Charging Current Oscillator Frequency Tolerance +VTHX -VTHX IFB ILBD Capacitor Threshold Voltage + Capacitor Threshold Voltage – Feedback Input Current Low Battery Output Current V7 = 1.3V V8 = 0.4V, V1 = 1.1V 500 V4 = 24V (RC4190) 30V (RM4190, RC4190A) VC ≤ 200 mV V1 = 1.2V 0.5 VOUT < VS < VOUT VS = +0.5 VOUT PL = 150 mW 0.1 1.0 V4 = 400 mV Measure at Pin 5 I4 = 0 Conditions Min 2.2 1.29 100 1.31 200 215 85 0.04 0.2 25 5.0 0.01 0.2 0.5 75 50 5.0 0.1 1.0 300 Typ Max 30 1.33 Min 2.2 1.24 100 1.31 200 215 85 0.04 0.2 25 5.0 0.01 0.5 0.5 75 50 5.0 300 RC4190 Typ Max 24 1.38 Units V V mA µA % % VO % VO kHz µA µA ISO I1 ICX 0.1 0.7 8.6 ± 10 1.4 0.5 0.1 1500 5.0 0.1 0.7 8.6 ± 10 1.4 0.5 0.1 500 1500 5.0 µA µA µA % V V µA µA 4 RC4190 PRODUCT SPECIFICATION Typical Performance Characteristics 4.0 300 250 3.0 200 VS (V) 2.4V 2.0 2.0V 1.8V IQ (µA) 150 100 1.0 65-2670 230 215 195 VS = +6V -50 -25 0 +25 +50 TA (°C) 0 -75 -50 -25 0 +25 +50 TA (°C) +75 +100 +125 0 -75 +75 +100 +125 Figure 1. Minimum Supply Voltage vs. Temperature Figure 2. Quiescent Current vs. Temperature 1.33 FO (Normalized) (%) 1.32 VREF (V) 1.31 1.30 1.29 1.28 -75 +2.0 +1.5 +1.0 +0.5 0 -0.5 -1.0 -1.5 -2.0 -75 -50 -25 0 +25 +50 TA (°C) Figure 4. Oscillator Frequency vs. Temperature 65-2672 -50 -25 0 +25 +50 TA (°C) +75 +100 +125 65-1488 +75 +100 +125 Figure 3. Reference Voltage vs. Temperature +2 FO (Normalized) (%) +1 0 -1 -2 0 5 10 15 +VS (V) Figure 5. Minimum Supply Voltage vs. Temperature 20 25 65-2667 30 65-2671 50 5 PRODUCT SPECIFICATION RC4190 Principles of Operation Simple Step-Up Converter The most common application, the step-up regulator, is derived from a simple step-up (VOUT > VBAT) DC-to-EC Converter (Figure 6). L D (+) If the switch is opened and closed repeatedly, at a rate much greater than the time constant of the output RC, then a constant dc voltage will be produced at the output. An output voltage higher than the input voltage is possible because of the high voltage produced by a rapid change of current in the inductor. When the switch is opened, the inductor voltage will instantly rise high enough to forward bias the diode, to VOUT + VD. In the complete RC4190 regulator, a feedback control system adjusts the on time of the switch, controlling the level of inductor current, so that the average inductor discharge current equals the load current, thus regulating the output voltage. VBAT S C RL VOUT (–) 65-1646 Figure 6. Simple Set-Up When switch S is closed, the battery voltage is applied across the inductor L. Charging current flows through the inductor, building up a magnetic field, increasing as the switch is held closed. While the switch is closed, the diode D is reverse biased (open circuit) and current is supplied to the load by the capacitor C. Until the switch is opened, the inductor current will increase linearly to a maximum value determined by the battery voltage, inductor value, and the amount of time the switch is held closed (IMAX = VBAT/ L x TON). When the switch is opened, the magnetic field collapses, and the energy stored in the magnetic field is converted into a discharge current which flows through the inductor in the same direction as the charging current. Because there is no path for current to flow through the switch, the current must flow through the switch, the current must flow through the diode to supply the load and charge the output capacitor. Complete Step-Up Regulator A complete schematic of the minimum step-up application is shown in Figure 7. The ideal switch in the DC-to-DC Converter diagram is replaced by an open collector NPN transistor Q1. CF functions as the output filter capacitor, and D1 and LX replace D and L. When power is first applied, the current in R1 supplies bias current to pin 6 (IC). This current is stabilized by a unity gain current source amplifier and then used as bias current for the 1.31V bandgap reference. A very stable bias current generated by the bandgap is mirrored and used to bias the remainder of the chip. At the same time the RC4190 is starting up, current will flow through the inductor and the diode to charge the output capacitor to VBAT – VD. G E ILX LX R1 6 5 VBAT IC +VS Q1 B OSC GND 3 A CX 2 CX F ID (+) 4 LX D + – RL REF LBR 1 NC LBD 8 NC R3 D1 RC4190 VFB +1.31V 7 R2 + – CF C ILOAD VOUT = VREF ( R2 + 1) R3 (–) 65-2673A Figure 7. Complete Step-Up Regulator 6 RC4190 PRODUCT SPECIFICATION 1.4V A 0.5V (Internal) IL (Max) C 0mA 0.72V (Internal) 0V VBAT LX E VOUT – VBAT LX IMAX 0 mA IMAX F ID 0 mA VLX VOUT + VD VMAX 0.3V (Q1 SAT) 65-2674 CX OSC ILOAD B D VBEQ1 ILX G Figure 8. Step-Up Regulator Waveforms +VS R1 1M CT * 1 µF 7 VFB 6 IC 5 +VS R4 R5 Motorola MBR140P V OUT Q1 TIP73 R2 CF 4190 CX 2 LX Q2 2N3904 GND 3 R3 CX * May not be required R5 = 50 VS IMAX R4 = 10 R5 65-2675 Figure 9. High Power Step-Up Regulator (With the addition of a power transistor (TIP73) and a few components, the 4190 can accomodate load power up to 10W.) At this point, the feedback (pin 7) senses that the output voltage is too low, by comparing a division of the output voltage (set by the ratio of R2 to R3) to the +1.31V reference. If the output voltage is too low then the comparator output changes to a logical zero. The NOR gate then effectively ANDs the oscillator square wave with the comparator signal; if the comparator output is zero AND the oscillator output is low, then the NOR gate output is high and the switch transistor will be forced on. When the oscillator goes high again, the NOR gate output goes low and the switch transistor will turn off. This turning on and off of the switch transistor performs the same function that opening and closing the switch in the simple DC-to-DC Converter does; i.e., it stores energy in the inductor during the on time and releases it into the capacitor during the off time. The comparator will continue to allow the oscillator to turn the switch on and off until enough charge has been delivered to the capacitor to raise the feedback voltage above 1.31V. Thereafter, this feedback system will vary the duration of the on time in response to changes in load current or battery 7 PRODUCT SPECIFICATION RC4190 voltage (see Figure 8). If the load current increases (waveform C), then the transistor will remain on (waveform D) for a longer portion of the oscillator cycle (waveform B), thus allowing the inductor current (waveform E) to build up to a higher peak value. The duty cycle of the switch transistor varies in response to changes in load and time. The inductor value and oscillator frequency must be carefully tailored to the battery voltage, output current, and ripple requirements of the application (refer to the Design Equations Section). If the inductor value is too high or the oscillator frequency is too high, then the inductor current will never reach a value high enough to meet the load current drain and the output voltage will collapse. If the inductor value is too low or the oscillator frequency too low, then the inductor current will build up too high, causing excessive output voltage ripple, or over stressing of the switch transistor, or possibly saturating the inductor. voltage applied across the inductor will discharge into the load. As in the step-up case, the average inductor current equals the load current. The maximum inductor current IMAX will equal (VBAT – VOUT)/L times the maximum on time of the switch transistor (TON). Current flows to the load during both half cycles of the oscillator. Complete Step-Down Regulator Most step-down applications are better served by the RC4391 step-down and inverting switching regulator (refer to the RC4391 data sheet). However, there is a range of load power for which the RC4190 has an advantage over the RC4391 in step-down applications. From approximately 500 mW to 2W of load power, the RC4190 step-down circuit of Figure 6 offers a lower component count and simpler circuit than the comparable RC4391 circuit, particularly when stepping down a voltage greater than 30V. Since the switch transistor in the RC4190 is in parallel with the load, a method must be used to convert it to a series connection for step-down applications. The circuit of Figure 11 accomplishes this. The 2N2907 replaces S of Figure 10, and R6 and R7 are added to provide the base drive to the 2N2907 in the correct polarity to operate the circuit properly. Simple Step-Down Converter Figure 10 shows a step-down DC-to-DC Converter (VOUT ≤ VBAT) with no feedback control. S L (+) Greater Than 30V Step-Down Regulator VBAT D C RL VOUT (–) 65-1644 Figure 10. Simple Step-Down Converter Adding a zener diode in series with the base of the 2N2907 allows the battery voltage to increase by the value of the zener, with only a slight decrease in efficiency. As an example, if a 24V zener is used, the maximum battery voltage can go to 48V2 when using a RC4190. Refer to Figure 12. Notes: 1. The addition of the zener diode will not alter the maximum change of supply. With a 24V zener, the circuit will stop operating when the battery voltage drops below 24V + 2.2V = 26.2V. 2. Maximum battery voltage is 54V when using RM4190 (30V + 24V). When S is closed, the battery voltage minus the output voltage is applied across the inductor. All of the inductor current will flow into the load until the inductor current exceeds the load current. The excess current will then charge the capacitor and the output voltage will rise. When S is opened, the 2N2907 R6 Lx V OUT D1 1N914 R7 5 R1 V BAT R4 6 1 +VS IC LBR GND 3 CX 2 Cx R4 = VS - 1.31V 5 µA 260K ~ 50 R6 ~ IL 4190 4 LX VFB 7 R3 R2 CF R5 65-2676 R5 = 10 VS ~ R7 ~ IL Figure 11. Complete Step-Down Regulator 8 RC4190 PRODUCT SPECIFICATION 2N2907 R6 Lx V OUT D1 1N914 Z1 R2 R7 CF 5 R1 V BAT R4 6 1 IC +VS 4190 LBR GND 3 CX 2 4 LX VFB 7 R3 Cx R4 = VS - 1.31V 5 µA 260K 50 ~ R6 ~ IL 65-2677 R5 R5 = 10 VS R7 ~ ~ IL Figure 12. Step-Down Regulator Greater Than 30V Design Equations The inductor value and timing capacitor (CX) value must be carefully tailored to the input voltage, input voltage range, output voltage, and load current requirements of the application. The key to the problem is to select the correct inductor value for a given oscillator frequency, such that the inductor current rises to a high enough peak value (IMAX) to meet the average load current drain. The selection of this inductor value must take into account the variation of oscillator frequency from unit to unit and the drift of frequency over temperature. Use ±20% as a maximum change from the nominal oscillator frequency. The worst-case conditions for calculating ability to supply load current are found at the minimum supply voltage; use +VS (min) to calculate the inductor value. Worst-case conditions for ripple are at +VS (max). The value of the timing capacitor is set according to the following equation: 2.4 × 10 f O ( Hz ) = --------------------C X ( pF ) 6 Find a value for the start-up resistor R1: V S – 1.2V R1 = -----------------------5µA Find a value for the feedback resistors R2 and R3: V OUT – 1.31V R2 = ----------------------------------IA 1.31V R3 = -------------IA Where IA is the feedback divider current (recommended value is between 50 µA and 100 µA). Step-Up Design Procedure 1. 2. Select an operating frequency and timing capacitor as shown above (10 kHz to 40kHz is typical). Find the maximum on time (add 5 µS for the turn-off base recombination delay of Q1): 1 T ON = --------- + 5 µ s 2F O The squarewave output of the oscillator is internal and cannot be directly measured, but is equal in frequency to the triangle waveform measurable at pin 4. The switch transistor is normally on when the triangle waveform is ramping up and off when ramping down. Capacitor selection depends on the application; higher operating frequencies will reduce the output voltage ripple and will allow the use of an inductor with a physically smaller inductor core, but excessively high frequencies will reduce load driving capability and efficiency. 3. Calculate the peak inductor current IMAX (if this value is greater than 375 mA, then an external power transistor must be used in place of Q1): V OUT + V D – V S I MAX =  ---------------------------------------------------- 2I L  ( F O ) T ON [ V S – V SW ] where: VS = supply voltage VD = diode forward voltage IL = dc load current VSW = saturation voltage of Q1 (typ 0.5V) 9 PRODUCT SPECIFICATION RC4190 4. Find an inductance value for LX: 2. V S – V SW L X ( Henries ) =  ------------------------ T ON  I MAX  Build the circuit and apply the worst case conditions to it, i.e., the lowest battery voltage and the highest load current at the desired output voltage. Adjust the inductor value down until the desired output voltage is achieved, then go a little lower (approximately 20%) to cover manufacturing tolerances. Check the output voltage with an oscilloscope for ripply, at high supply voltages, at voltages as high as are expected. Also check for efficiency by monitoring supply and output voltages and currents [eff = (VOUT) (IOUT)/(+VS)(ISY) x 100%$]. If the efficiency is poor, go back to (1) and start over. If the ripple is excessive, then increase the output filter capacitor value or start over. 5. The inductor chosen must exhibit approximately this value at a current level equal to IMAX. Calculate a value for the output filter capacitor: 3. 6. 4. V S I MAX T ON  -------------------- + IL  V OUT  C F ( µ F ) = ------------------------------------------------VR where VR = ripple voltage (peak) 5. Step-Down Design Procedure 1. 2. 3. Select an operating frequency. Compensation Determine the maximum on time (TON) as in the stepup design procedure. Calculate IMAX: When large values (>50 kΩ) are used for the voltage setting resistors, R2 and R3 of Figure 7, stray capacitance at the VFB input can add a lag to the feedback response, destabilizing the regulator, increasing low frequency ripple, and lowering efficiency. This can often be avoided by minimizing the stray capacitance at the VFB node. It can also be remedied by adding a lead compensation capacitor of 100 pF to 10 nF in parallel with R2 in Figure 7. 2I L I MAX = ----------------------------------------------------------------------V S – V OUT  ( F O ) ( T ON )  ---------------------------- + 1  V OUT – V D 4. LX Calculate LX: V S – V OUT =  --------------------------- ( T ON )  I MAX  Inductors Efficiency and load regulation will improve if a quality high Q inductor is used. A ferrite pot core is recommended; the wind-yourself type with an air gap adjustable by washers or spacers is very useful for breadboarding prototypes. Care must be taken to choose a permeable enough core to handle the magnetic flux produced at IMAX; if the core saturates, then efficiency and output current capability are severely degraded and excessive current will flow though the switch transistor. A pot core inductor design section is provided later in this datasheet. An isolated AC current probe for an oscilloscope (example: Tektronix P6042) is an excellent tool for saturation problems; with it the inductor current can be monitored for nonlinearity at the peaks (a sign of saturation). 5. Calculate a value for the output filter capacitor: ( V S – V OUT ) I MAX T ON  ---------------------------------------------- + IL   V OUT C F ( µ F ) = --------------------------------------------------------------------------VR Alternate Design Procedure The design equations above will not work for the certain input/output voltage ratios, and for these circuits another method of defining component values must be used. If the slope of the current discharge waveform is much less than the slope of the current charging waveform, then the inductor current will become continuous (never discharging completely), and the equations will become extremely complex. So, if the voltage applied across the inductor during the charge time is greater than during the discharge time, used the design procedure below. For example, a step-down circuit with 20V input and 5V output will have approximately 15V across the inductor when charging, and approximately 5V when discharging. So in this example, the inductor current will be continuous and the alternate procedure will be necessary. 1. Select an operating frequency (a value between 10 kHz and 40 kHz is typical). Low Battery Detector An open collector signal transistor Q2 with comparator C2 provides the designer with a method of signaling a display or computer whenever the battery voltage falls below a programmed level (see Figure 8). This level is determined by the +1.3V reference level and by the selection of two external resistors according to the equation: R4 V TH = V REF  ------ + 1  R5  Where VTH = Threshold Voltage for Detection 10 RC4190 PRODUCT SPECIFICATION +Vs R4 1 LBR C2 R5 V REF 1.31V 65-1651 LBD 8 Q2 I LBD Another method of automatic shutdown without temperature limitations is the use of a zener diode in series with the IC pin and set resistor. When the battery voltage falls below VZ + 1.2V the circuit will start to shut down. With this connection and the low battery detector, the application can be designed to signal a display when the battery voltage has dropped to the first programmed level, then shut itself off as the battery reaches the zener threshold. The set current can also be turned off by forcing the IC pin to 0.2V or less using an external transistor or mechanical switch. An example of this is shown in Figure 15. In this circuit an external control voltage is used to determine the operating state of the RC4190. If the control voltage VC is a logic 1 at the input of the 4025 (CMOS Triple NOR Gate), the voltage at the IC pin will be less than 0.5V forcing the 4190 off (