L7986 3 A step-down switching regulator Datasheet - production data Applications • Consumer: STB, DVD, DVD recorder, car audio, LCD TV and monitors • Industrial: PLD, PLA, FPGA, chargers • Networking: XDSL, modems, DC-DC modules HSOP8 exp. pad VFDFPN10 3 x 3 mm • Computer: optical storage, hard disk drive, printers, audio/graphic cards Features • LED driving • 3 A DC output current Description • 4.5 V to 38 V input voltage The L7986/A is a step-down switching regulator with a 3.
Contents L7986 Contents 1 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 Thermal data . . . . . . . .
L7986 8 Contents Package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 8.1 VFDFPN10 (3x3x1.0 mm) package information . . . . . . . . . . . . . . . . . . . . 39 8.2 HSOP8 package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 9 Ordering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pin settings L7986 1 Pin settings 1.1 Pin connection Figure 2. Pin connection (top view) OUT VCC OUT OUT VCC SYNCH EN COMP GND FSW FB VCC SYNCH EN COMP GND FSW FB HSOP8 VFQFPN10 1.2 Pin description Table 1. Pin description No. No. (VFDFPN) (HSOP) 1-2 1 OUT 3 2 SYNCH 4 3 EN 5 4 COMP 6 4/45 Type 5 Description Regulator output Master/slave synchronization.
L7986 Maximum ratings 2 Maximum ratings Table 2. Absolute maximum ratings Symbol Parameter Vcc Input voltage OUT Output DC voltage Value 45 -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 3 Unit Power dissipation at TA < 60 °C VFDFPN 1.5 HSOP 2 V W TJ Junction temperature range -40 to 150 °C Tstg Storage temperature range -55 to 150 °C Value Unit Thermal data Table 3.
Electrical characteristics 4 L7986 Electrical characteristics TJ = 25 °C, VCC = 12 V, unless otherwise specified. Table 4. Electrical characteristics Values Symbol Parameter Test condition Unit Min. Operating input voltage range (1) Turn-on VCC threshold (1) VCCHYS VCC UVLO hysteseris (1) RDSON MOSFET on resistance VCC VCCON Typ. 4.5 Max. 38 4.5 0.1 V 0.4 200 ILIM mΩ (1) Maximum limiting current 400 3.7 4.2 4.
L7986 Electrical characteristics Table 4. Electrical characteristics (continued) Values Symbol Parameter Test condition Unit Min. IO SINK GV Sink COMP pin VFB = 0.7 V, VCOMP = 1 V Open-loop voltage gain (2) Typ. Max. 30 mA 100 dB Synchronization function VS_IN,HI High input voltage 2 VS_IN,LO Low input voltage tS_IN_PW Input pulse width ISYNCH,LO Slave sink current VSYNCH = 2.9 V VS_OUT,HI Master output amplitude ISOURCE = 4.
Functional description 5 L7986 Functional description The L7986 device 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.
L7986 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 connected to the FSW pin. If the FSW pin is left floating, the frequency is 250 kHz; it can be increased as shown in Figure 6 by an external resistor connected to ground.
Functional description L7986 Figure 5. Sawtooth: voltage and frequency feed-forward; external synchronization Figure 6. Oscillator frequency vs.
L7986 5.2 Functional description Soft-start The soft-start is essential to assure correct and safe startup of the step-down converter. It avoids inrush current surge and makes the output voltage increase monothonically. The soft-start is performed by a staircase ramp on the non inverting input (VREF) of the error amplifier.
Functional description 5.3 L7986 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.
L7986 Functional description This kind of overcurrent protection is effective if the output current is limited. To prevent the current from diverging, the current ripple in the inductor during the on-time must not be higher than the current ripple during the off-time.
Functional description L7986 Figure 8. Overcurrent protection 5.5 Enable function The enable feature allows to put the device into standby mode. With the EN pin lower than 0.3 V the device is disabled and the power consumption is reduced to less than 30 µA. With the 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.
L7986 Application information 6 Application information 6.1 Input capacitor selection The capacitor connected to the input must be capable of supporting 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 information L7986 In this case the equation of CIN as a function of the target VPP can be written as follows: Equation 10 IO D D C IN = --------------------------- ⋅ 1 – ---- ⋅ 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.
L7986 Application information Equation 13 V OUT + V F 1 – DMIN L MIN = ---------------------------- ⋅ ----------------------ΔI MAX F SW where FSW is the switching frequency, 1 / (TON + TOFF). For example, for VOUT = 5 V, VIN = 24 V, IO = 3 A and FSW = 250 kHz, the minimum inductance value to have ΔIL = 30% of IO is about 18 µH.
Application information L7986 The output capacitor is important also for loop stability: it fixes the double LC filter pole and the zero due to its ESR. Section 6.4 illustrates how to consider its effect in the system stability. For example, with VOUT = 5 V, VIN = 24 V, ΔIL = 0.9 A (resulting from the inductor value), in order to have a ΔVOUT = 0.01·VOUT, if the multi-layer ceramic capacitors are adopted, 10 µF are needed and the ESR effect on the output voltage ripple can be neglected.
L7986 Application information In this way the PWM modulator gain results constant and equal to: Equation 18 V IN 1 G PW0 = --------- = ---- = 18 Vs K The synchronization of the device with an external clock provided through the SYNCH pin can modify the PWM modulator gain (see Section 5.1 on page 9 to understand how this gain changes and how to keep it constant in spite of the external synchronization). Figure 9.
Application information L7986 As seen in Section 5.3 on page 12, two different kinds of network can compensate the loop. In the following two paragraphs the guidelines to select the type II and type III compensation network are illustrated. 6.4.
L7986 Application information 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. 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 follows: 1. Choose a value for R1, usually between 1 kΩ and 5 kΩ. 2.
Application information L7986 Equation 27 R1 R3 = ---------------------------, 4 ⋅ BW ----------------- – 1 f LC 1 C 3 = ----------------------------------------2π ⋅ R 3 ⋅ 4 ⋅ BW The suggested maximum system bandwidth is equal to the switching frequency divided by 3.5 (FSW / 3.5), anyway, lower than 100 kHz if the FSW is set higher than 500 kHz. For example, with VOUT = 5 V, VIN = 24 V, IO = 3 A, L = 18 µH, COUT = 22 µF, ESR < 1 mΩ, the type III compensation network is: R 1 = 4.
L7986 Application information Figure 12.
Application information 6.4.2 L7986 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 non-negligible ESR (> 30 mΩ), so with this kind of output capacitor the type II network combined with the zero of the ESR allows to stabilize the loop. In Figure 13 the type II network is shown. Figure 13.
L7986 Application information In Figure 14 the Bode diagram of the PWM and LC filter transfer function [GPW0 · GLC(f)] and the open-loop gain [GLOOP(f) = GPW0 · GLC(f) · GTYPEII(f)] are drawn. 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 follows: 1.
Application information L7986 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 the module and phase of the open-loop gain is shown. The bandwidth is about 21 kHz and the phase margin is 45°. Figure 15.
L7986 6.5 Application information 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 non-negligible RDS(on) of the power switch; these are equal to: Equation 33 2 P ON = R DSON ⋅ ( IOUT ) ⋅ D where D is the duty cycle of the application and the maximum RDSON overtemperature is 220 mΩ.
Application information L7986 Figure 16. Switching losses 6.6 Layout considerations The PC board layout of the switching the DC/DC regulator is very important to minimize the noise injected in high impedance nodes and interference generated by the high switching current loops. In a step-down converter the input loop (including the input capacitor, the power MOSFET and the freewheeling diode) is the most critical one. This is due to the fact that the high value pulsed currents are flowing through it.
L7986 Application information In Figure 17 a layout example is shown. Figure 17.
Application information 6.7 L7986 Application circuit In Figure 18 the demonstration board application circuit is shown. Figure 18. Demonstration board application circuit Table 9. Component list 30/45 Reference Part number Description Manufacturer C1 UMK325BJ106MM-T 10 μF, 50V Taiyo Yuden C2 GRM32ER61E226KE15 22 μF, 25V Murata C3 3.3 nF, 50V C4 33 nF, 50V C5 100 pF, 50V C6 470 nF, 50V R1 4.99 kΩ, 1%, 0.1 W 0603 R2 1.1 kΩ, 1%, 0.1 W 0603 R3 330 Ω, 1%, 0.1 W 0603 R4 1.
L7986 Application information Figure 19. PCB layout: L7986 and L7986A (component side) Figure 20. PCB layout: L7986 and L7986A (bottom side) Figure 21.
Application information L7986 Figure 22. Junction temperature vs. output current VIN = 24 V VQFN Figure 23. Junction temperature vs. output current VIN = 12 V HSOP VQFN VOUT=5V VOUT=5V VOUT=3.3V VOUT=3.3V VOUT=1.8V VOUT=1.8V HSOP VIN=12V FSW=250KHz TAMB=25 C VIN=24V FSW=250KHz TAMB=25 C Figure 24. Junction temperature vs. output current VIN = 5 V Figure 25. Efficiency vs. output current VO = 1.8 V 85 VQFN HSOP Vo=1.8V FSW=250kHz 80 75 VOUT=1.8V 70 VOUT=1.2V 65 E ff [%] VOUT=3.
L7986 Application information Figure 28. Load regulation Figure 29. Line regulation 3.350 3.3600 Vin=5V 3.345 Io=1A 3.3550 Vin=12V Io=2A Vin=24V Io=3A 3.340 3.3500 3.3450 VOUT [V] VOUT [V] 3.335 3.330 3.325 3.3400 3.3350 3.320 3.3300 3.315 3.3250 3.310 3.3200 3.305 0.00 0.50 1.00 1.50 2.00 2.50 3.00 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 VIN [V] Io [A] Figure 30. Load transient: from 0.4 A to 2 A Figure 31. Soft-start IL 0.
Application ideas L7986 7 Application ideas 7.1 Positive buck-boost The L7986 device can implement the step-up/down converter with a positive output voltage. Figure 34 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 38 V. Figure 34.
L7986 Application ideas Equation 39 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 a short time can exceed the maximum output current calculated by equation 39, also the peak current in the power MOSFET must be calculated. The peak current, shown in equation 40, must be lower than the minimum current limit (3.7 A).
Application ideas L7986 Equation 41 V OUT + 2 ⋅ V D D = -------------------------------------------------------------------------------------------V IN – VSW – VSWE + V OUT + 2 ⋅ V D where VD is the voltage drop across the diodes, VSW and VSWE across the internal and external power MOSFET. 7.2 Inverting buck-boost The L7986 can implement the step-up/down converter with a negative output voltage. Figure 34.
L7986 Application ideas As in the positive buck-boost, the maximum output current according to application conditions is shown in Figure 37. The dashed line considers a more accurate estimation of the duty cycles given by the equation 44, where power losses across diodes and the internal power MOSFET are taken into account. Equation 44 V OUT – V D D = ----------------------------------------------------------------– V IN – V SW + V OUT – V D Figure 37. Maximum output current according to switch max.
Package information 8 L7986 Package information 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.
L7986 8.1 Package information VFDFPN10 (3x3x1.0 mm) package information Figure 38. VFDFPN10 (3x3x1.
Package information L7986 Table 10. VFDFPN10 (3x3x1.0 mm) package mechanical data mm Symbol A Min. Typ. Max. 0.80 0.90 1.00 0.02 0.05 0.65 0.80 A1 A2 0.55 A3 0.20 b 0.18 0.25 0.30 D 2.85 3.00 3.15 D2 2.20 E 2.85 E2 1.40 e L 3.00 3.15 1.75 0.50 0.30 ddd 40/45 2.70 0.40 0.50 0.
L7986 8.2 Package information HSOP8 package information Figure 39. HSOP8 package outline ' PP 7\S ( PP 7\S $0 Y DocID022450 Rev 7 41/45 45
Package information L7986 Table 11. HSOP8 package mechanical data mm Symbol Min. Typ. A 1.70 A1 0.00 A2 1.25 b 0.31 0.51 c 0.17 0.25 D 4.80 4.90 5.00 E 5.80 6.00 6.20 E1 3.80 3.90 4.00 e 0.150 1.27 h 0.25 0.50 L 0.40 1.27 k 0.00 8.00 ccc 42/45 Max. 0.
L7986 9 Ordering information Ordering information Table 12.
Revision history 10 L7986 Revision history Table 13. Document revision history 44/45 Date Revision Changes 07-Nov-2011 1 Initial release. 01-Mar-2012 2 Section 8: Package information has been updated. 15-Oct-2012 3 In Section 5.6 changed temperature value from 130 to 120 °C. 19-Mar-2014 4 Updated text below Equation 4 on page 13 and in Equation 5 on page 13 (replaced “DRC” by “DCR”). Updated Figure 34: Positive buck-boost regulator on page 34 (replaced by new figure).
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