L7981 3 A step-down switching regulator Features ■ 3 A DC output current ■ 4.5 V to 28 V input voltage ■ Output voltage adjustable from 0.6 V ■ 250 kHz switching frequency, programmable up to 1 MHz ■ Internal soft-start and enable Description ■ Low dropout operation: 100% duty cycle ■ Voltage feed-forward ■ Zero load current operation ■ Overcurrent and thermal protection The L7981 is a step down switching regulator with 3.
Contents L7981 Contents 1 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 Thermal data . . . . . . . .
L7981 Contents 8 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 9 Order codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pin settings L7981 1 Pin settings 1.1 Pin connection Figure 2. Pin connection (top view) OUT SYNCH GND EN FSW COMP 1.2 FB Pin description Table 1. 4/44 VCC Pin description N. Type 1 OUT Description Regulator output 2 SYNCH Master/Slave synchronization. When it is left floating, a signal with a phase shift of half a period respect to the power turn on is present at the pin.
L7981 2 Maximum ratings Maximum ratings Table 2. Absolute maximum ratings Symbol 3 Parameter Vcc Input voltage OUT Output DC voltage Value Unit 30 -0.3 to VCC FSW, COMP, SYNCH Analog pin -0.3 to 4 EN Enable pin -0.3 to VCC FB Feedback voltage -0.3 to 1.5 PTOT Power dissipation at TA < 60°C VFQFPN V 1.5. W HSOP 2 TJ Junction temperature range -40 to 150 °C Tstg Storage temperature range -55 to 150 °C Thermal data Table 3.
Electrical characteristics 4 L7981 Electrical characteristics TJ=25 °C, VCC=12 V, unless otherwise specified. Table 4. Electrical characteristics Values Symbol Parameter Test condition Unit Min VCC Operating input voltage range (1) VCCON Turn on VCC threshold (1) VCCHYS VCC UVLO Hysteresis (1) RDSON Mosfet on resistance ILIM 4.5 Max 28 4.4 0.12 V 0.35 160 180 160 250 3.7 4.2 4.
L7981 Electrical characteristics Table 4. Electrical characteristics (continued) Values Symbol Parameter Test condition Unit Min VCH High level output voltage VFB<0.6V VCL Low level output voltage VFB>0.6V IO SOURCE Source COMP pin IO SINK GV Typ Max 3 V 0.1 VFB=0.5V, VCOMP=1V 17 mA Sink COMP pin VFB=0.7V, VCOMP=1V 25 mA Open loop voltage gain (2) 100 dB Synchronization function High input voltage 2 3.3 V Low input voltage 1 Slave sink current VSYNCH=2.
Functional description 5 L7981 Functional description The L7981 is based on a “voltage mode”, constant frequency control. The output voltage VOUT is sensed by the feedback pin (FB) compared to an internal reference (0.6 V) providing an error signal that, compared to a fixed frequency sawtooth, controls the on and off time of the power switch. The main internal blocks are shown in the block diagram in Figure 3.
L7981 5.1 Functional description Oscillator and synchronization Figure 4 shows the block diagram of the oscillator circuit. The internal oscillator provides a constant frequency clock. Its frequency depends on the resistor externally connect to FSW pin. In case the FSW pin is left floating the frequency is 250 kHz; it can be increased as shown in Figure 6 by external resistor connected to ground.
Functional description 10/44 L7981 Figure 5. Sawtooth: voltage and frequency feed forward; external synchronization Figure 6.
L7981 5.2 Functional description Soft-start The soft-start is essential to assure correct and safe start up of the step-down converter. It avoids inrush current surge and makes the output voltage increases monothonically. The soft -start is performed by a staircase ramp on the non-inverting input (VREF) of the error amplifier.
Functional description 5.3 L7981 Error amplifier and compensation The error amplifier (E/A) provides the error signal to be compared with the sawtooth to perform the pulse width modulation. Its non-inverting input is internally connected to a 0.6 V voltage reference, while its inverting input (FB) and output (COMP) are externally available for feedback and frequency compensation. In this device the error amplifier is a voltage mode operational amplifier so with high DC gain and low output impedance.
L7981 5.4 Functional description Overcurrent protection The L7981 implements the over current protection sensing current flowing through the power MOSFET. Due to the noise created by the switching activity of the power MOSFET, the current sensing is disabled during the initial phase of the conduction time. This avoids an erroneous detection of a fault condition. This interval is generally known as “masking time” or “blanking time”. The masking time is about 200 ns.
Functional description Figure 8. 5.5 L7981 Over current protection strategy Enable function The enable feature allows to put in stand-by mode the device.With EN pin lower than 0.3 V the device is disabled and the power consumption is reduced to less than 30 µA. With EN pin lower than 1.2 V, the device is enabled. If the EN pin is left floating, an internal pull down ensures that the voltage at the pin reaches the inhibit threshold and the device is disabled. The pin is also VCC compatible. 5.
L7981 Application informations 6 Application informations 6.1 Input capacitor selection The capacitor connected to the input has to be capable to support the maximum input operating voltage and the maximum RMS input current required by the device. The input capacitor is subject to a pulsed current, the RMS value of which is dissipated over its ESR, affecting the overall system efficiency.
Application informations L7981 Given the physical dimension, ceramic capacitors can meet well the requirements of the input filter sustaining an higher input RMS current than electrolytic / tantalum types. In this case the equation of CIN as a function of the target VPP can be written as follows: Equation 7 IO D C IN = --------------------------- ⋅ ⎛ 1 – D ----⎞ ⋅ D + ---- ⋅ ( 1 – D ) V PP ⋅ F SW ⎝ η η⎠ neglecting the small ESR of ceramic capacitors. Considering η=1, this function has its maximum in D=0.
L7981 Application informations Equation 9 V IN – V OUT V OUT + V F ΔI L = ------------------------------ ⋅ T ON = ---------------------------- ⋅ T OFF L L Where TON is the conduction time of the internal high side switch and TOFF is the conduction time of the external diode (in CCM, FSW=1/(TON + TOFF)). The maximum current ripple, at fixed Vout, is obtained at maximum TOFF that is at minimum duty cycle (see previous section to calculate minimum duty).
Application informations 6.3 L7981 Output capacitor selection The current in the capacitor has a triangular waveform which generates a voltage ripple across it. This ripple is due to the capacitive component (charge or discharge of the output capacitor) and the resistive component (due to the voltage drop across its ESR). So the output capacitor has to be selected in order to have a voltage ripple compliant with the application requirements.
L7981 6.4 Application informations Compensation network The compensation network has to assure stability and good dynamic performance. The loop of the L7981 is based on the voltage mode control. The error amplifier is a voltage operational amplifier with high bandwidth. So selecting the compensation network the E/A will be considered as ideal, that is, its bandwidth is much larger than the system one. The transfer functions of PWM modulator and the output LC filter are studied (see Figure 10.).
Application informations L7981 Equation 16 s 1 + ------------------------2π ⋅ f zESR G LC ( s ) = ------------------------------------------------------------------------2 s s 1 + ---------------------------+ ⎛⎝ -------------------⎞⎠ 2π ⋅ f LC 2π ⋅ Q ⋅ f LC where: Equation 17 1 f LC = -----------------------------------------------------------------------, ESR 2π ⋅ L ⋅ C OUT ⋅ 1 + --------------R OUT 1 f zESR = ------------------------------------------2π ⋅ ESR ⋅ C OUT Equation 18 R OUT ⋅ L ⋅ C OUT ⋅ (
L7981 Application informations Equation 20 f P0 = 0, 1 -, f P1 = ----------------------------2π ⋅ R 3 ⋅ C 3 1 f P2 = ------------------------------------------C4 ⋅ C5 ------------------2π ⋅ R 4 ⋅ C4 + C5 Figure 10. Type III compensation network In Figure 11 the Bode diagram of the PWM and LC filter transfer function (GPW0 · GLC(f)) and the open loop gain (GLOOP(f) = GPW0 · GLC(f) · GTYPEIII(f)) are drawn. Figure 11.
Application informations L7981 Equation 21 BW R 4 = ---------- ⋅ K ⋅ R 1 f LC where K is the feed forward constant and 1/K is equals to 9. 3. Calculate C4 by placing the zero at 50% of the output filter double pole frequency (fLC): Equation 22 1 C 4 = --------------------------π ⋅ R 4 ⋅ f LC 4. Calculate C5 by placing the second pole at four times the system bandwidth (BW): Equation 23 C4 C 5 = ------------------------------------------------------------2π ⋅ R 4 ⋅ C 4 ⋅ 4 ⋅ BW – 1 5.
L7981 Application informations Figure 12.
Application informations 6.4.2 L7981 Type II compensation network If the equivalent series resistance (ESR) of the output capacitor introduces a zero with a frequency lower than the desired bandwidth (that is: 2π∗ESR∗COUT>1/BW), this zero helps stabilize the loop. Electrolytic capacitors show not negligible ESR (>30 mΩ), so with this kind of output capacitor the type II network combined with the zero of the ESR allows stabilizing the loop. In Figure 13 the type II network is shown. Figure 13.
L7981 Application informations Figure 14. Open loop gain: module bode diagram The guidelines for positioning the poles and the zeroes and for calculating the component values can be summarized as follow: 1. Choose a value for R1, usually between 1kΩ and 5kΩ, in order to have values of C4 and C5 not comparable with parasitic capacitance of the board. 2.
Application informations L7981 Equation 28 C4 C 5 = ------------------------------------------------------------2π ⋅ R 4 ⋅ C 4 ⋅ 4 ⋅ BW – 1 For example with VOUT=5 V, VIN=24 V, IO=3 A, L=18 μH, COUT=330 μF, ESR=35 mΩ, the type II compensation network is: R 1 = 1.1kΩ, R 2 = 150Ω, R 4 = 4.99kΩ, C 4 = 82nF, C 5 = 68pF In Figure 15 is shown the module and phase of the open loop gain. The bandwidth is about 21 kHz and the phase margin is 45°.
L7981 Application informations Figure 15.
Application informations 6.5 L7981 Thermal considerations The thermal design is important to prevent the thermal shutdown of device if junction temperature goes above 150°C. The three different sources of losses within the device are: a) conduction losses due to the not negligible RDSon of the power switch; these are equal to: Equation 29 2 P ON = R DSON ⋅ ( I OUT ) ⋅ D Where D is the duty cycle of the application and the maximum RDSon over temperature is 220 mΩ.
L7981 Application informations of heat. The RthJA measured on the demonstration board described in the following paragraph is about 60°C/W for the VFQFPN package and about 40°C/W for the HSOP package. Figure 16. Switching losses 6.6 Layout considerations The PC board layout of switching DC/DC regulator is very important to minimize the noise injected in high impedance nodes and interferences generated by the high switching current loops.
Application informations L7981 Figure 17.
L7981 6.7 Application informations Application circuit In Figure 18 the demonstration board application circuit is shown. Figure 18. Demonstration board application circuit (rev 1.0) Table 9. Component list (rev 1.0) Reference Part number Description Manufacturer C1 UMK325BJ106MM-T 10 μF, 50V Taiyo Yuden C2 GRM32ER61E226KE15 22 μF, 25V Murata C3 2.2 nF, 50V C4 22 nF, 50V C5 470 pF, 50V C6 470 nF, 50V R1 4.99 kΩ, 1%, 0.1W 0603 R2 1.1 kΩ, 1%, 0.1W 0603 R3 249 Ω, 1%, 0.
Application informations L7981 Figure 19. PCB layout: L7981 and L7981A (component side) Figure 20. PCB layout: L7981 and L7981A (bottom side) Figure 21.
L7981 Application informations Figure 22. Junction temperature vs. output current Figure 23. Junction temperature vs. output current Figure 24. Junction temperature vs. output current Figure 25. Efficiency vs. output current 92 V IN =12V VIN =18V 87 Eff [%] VIN =24V 82 77 VOUT=5.0 V fsw=250 kHz 72 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Io [A] Figure 26. Efficiency vs.output current Figure 27. Efficiency vs.
Application informations L7981 Figure 28. Load regulation Figure 29. Line regulation 1.6 0.1 Vcc=5V 1.4 0.0 Vcc=12V 1.2 Vcc=24V Δ V FB /V FB [%] Δ VFB /VF B [%] -0.1 1 0.8 0.6 -0.2 -0.3 -0.4 0.4 Io=1A Io=2A -0.5 0.2 Io=3A -0.6 0 5 0 0.5 1 1.5 2 2.5 10 3 15 20 25 VCC [V] Io [A] Figure 30. Load transient: from 0.4 A to 3 A Figure 31. Soft-start IL 1A/div VOUT 200mV/div AC coupled VOUT 0.5V/div 1V/div VIN=24V VOUT=3.
L7981 Application ideas 7 Application ideas 7.1 Positive buck-boost The L7981 can implement the step up/down converter with a positive output voltage. Figure 33. shows the schematic: one power MOSFET and one Schottky diode are added to the standard buck topology to provide 12 V output voltage with input voltage from 4.5 V to 28 V. Figure 33.
Application ideas L7981 Equation 35 I OUT I SW = ------------- < 3 A 1–D where ISW is the average current in the embedded power MOSFET in the on time. To chose the right value of the inductor and to manage transient output current, that for short time can exceed the maximum output current calculated by Equation 35, also the peak current in the power MOSFET has to be calculated. The peak current, showed in Equation 36, must be lower than the minimum current limit (3.
L7981 Application ideas Figure 34. Maximum output current according to max DC switch current (3.0 A): VO=12 V Equation 37 V OUT + 2 ⋅ V D D = ------------------------------------------------------------------------------------------V IN – V SW – V SWE + V OUT + 2 ⋅ V D where VD is the voltage drop across diodes, VSW and VSWE across the internal and external power MOSFET. 7.2 Inverting buck-boost The L7981 can implement the step up/down converter with a negative output voltage. Figure 33.
Application ideas L7981 As in the positive one, in the inverting buck-boost the current flowing through the power MOSFET is transferred to the load only during the OFF time. So according to the maximum DC switch current (3.0 A), the maximum output current can be calculated from the Equation 35, where the duty cycle is given by Equation 39. Figure 35.
L7981 8 Package mechanical data Package mechanical data In order to meet environmental requirements, ST offers these devices in different grades of ECOPACK® packages, depending on their level of environmental compliance. ECOPACK® specifications, grade definitions and product status are available at: www.st.com. ECOPACK is an ST trademark.
Package mechanical data Table 10. L7981 VFQFPN8 (3x3x1.08mm) mechanical data mm inch Dim. Min Typ Max Min Typ Max 0.80 0.90 1.00 0.0315 0.0354 0.0394 A1 0.02 0.05 0.0008 0.0020 A2 0.70 0.0276 A3 0.20 0.0079 A b 0.18 0.23 0.30 0.0071 0.0091 0.0118 D 2.95 3.00 3.05 0.1161 0.1181 0.1200 D2 2.23 2.38 2.48 0.0878 0.0937 0.0976 E 2.95 3.00 3.05 0.1161 0.1181 0.1200 E2 1.65 1.70 1.75 0.0649 0.0669 0.0689 e L 0.50 0.35 0.40 ddd 0.0197 0.45 0.
L7981 Package mechanical data Table 11. HSOP8 mechanical data mm inch Dim Min Typ A Max Min Typ 1.70 Max 0.0669 A1 0.00 A2 1.25 b 0.31 0.51 0.0122 0.0201 c 0.17 0.25 0.0067 0.0098 D 4.80 4.90 5.00 0.1890 E 5.80 6.00 6.20 0.2283 0.2441 E1 3.80 3.90 4.00 0.1496 0.1575 e 0.15 0.00 0.0059 0.0492 0.1929 0.1969 1.27 h 0.25 0.50 0.0098 0.0197 L 0.40 1.27 0.0157 0.0500 k 0 8 0.3150 0.10 0.0039 ccc Figure 38.
Order codes 9 L7981 Order codes Table 12.
L7981 10 Revision history Revision history Table 13. Document revision history Date Revision Changes 19-Nov-2008 1 Initial release.
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