Datasheet

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This is information on a product in full production.

October 2012 Doc ID 022098 Rev 3 1/43

L7986TA

3 A step-down switching regulator

Datasheet − production data

Features

■ 3 A DC output current

■ 4.5 V to 38 V input voltage

■ Output voltage adjustable from 0.6 V

■ 250 kHz switching frequency, programmable

up to 1 MHz

■ Internal soft-start and enable

■ Low dropout operation: 100% duty cycle

■ Voltage feed-forward

■ Zero load current operation

■ Overcurrent and thermal protection

■ HSOP8 package

■ Guarantee overtemperature range (-40 °C to

125 °C)

Applications

■ Automotive:

– Car audio, car infotainment

■ Industrial:

– PLD, PLA, FPGA, chargers

■ Networking: XDSL, modems, DC-DC modules

■ Computer:

– Optical storage, hard disk drive, printers

■ LED driving

Description

The L7986TA is a step-down switching regulator

with 3.7 A (min.) current limited embedded power

MOSFET, so it is able to deliver up to 3 A current

to the load depending on the application

conditions.

The input voltage can range from 4.5 V to 38 V,

while the output voltage can be set starting from

0.6 V to V

Requiring a minimum set of external components,

the device includes an internal 250 kHz switching

frequency oscillator that can be externally

adjusted up to 1 MHz.

The HSOP package with exposed pad allows the

reduction of R

thJA

down to 40 °C/W.

HSOP8 exposed pad

www.st.com

Summary of content (43 pages)

PAGE 1
L7986TA 3 A step-down switching regulator Datasheet − production data Features ■ 3 A DC output current ■ 4.5 V to 38 V input voltage ■ Output voltage adjustable from 0.
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Contents L7986TA Contents 1 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 Thermal data . . . . . . .
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L7986TA Contents 8 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 9 Ordering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Pin settings L7986TA 1 Pin settings 1.1 Pin connection Figure 1. 1.2 Pin description Table 1. 4/43 Pin connection (top view) 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, with respect to the power turn-on, is present at the pin.
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L7986TA 2 Maximum ratings Maximum ratings Table 2. Absolute maximum ratings Symbol 3 Parameter Vcc Input voltage OUT Output DC voltage Value Unit 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 Power dissipation at TA < 60 °C HSOP V 2 W TJ Junction temperature range -40 to 150 °C Tstg Storage temperature range -55 to 150 °C Thermal data Table 3.
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Electrical characteristics 4 L7986TA Electrical characteristics TJ=-40 °C to 125 °C, VCC=12 V, unless otherwise specified. Table 4. Electrical characteristics Values Symbol Parameter Test condition Unit Min. VCC Operating input voltage range VCCON Turn-on VCC threshold VCCHYS VCC UVLO hysteresis RDSON MOSFET on resistance ILIM Maximum limiting current Typ. 4.5 Max. 38 4.5 0.1 TJ=25 °C 3.7 V 0.4 200 400 4.2 4.7 mΩ A 3.5 4.
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L7986TA Electrical characteristics Table 4. Electrical characteristics (continued) Values Symbol Parameter Test condition Unit Min. Typ. Max. Error amplifier VCH High level output voltage VFB<0.6 V VCL Low level output voltage VFB>0.6 V Source COMP pin VFB=0.5 V, VCOMP=1 V 19 mA Sink COMP pin VFB=0.7 V, VCOMP=1 V 30 mA Open-loop voltage gain (1) 100 dB IO SOURCE IO SINK GV 3 V 0.1 Synchronization function VS_IN,HI High input voltage VS_IN,LO Low input voltage tS_IN_PW 2 3.
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Functional description 5 L7986TA Functional description The L7986TA 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 2.
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L7986TA 5.1 Functional description Oscillator and synchronization Figure 3 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 the FSW pin. If the FSW pin is left floating, the frequency is 250 kHz; it can be increased as shown in Figure 5 by an external resistor connected to ground.
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Functional description 10/43 L7986TA Figure 4. Sawtooth: voltage and frequency feed-forward; external synchronization Figure 5. Oscillator frequency vs.
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L7986TA 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.
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Functional description 5.3 L7986TA 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.
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L7986TA 5.4 Functional description Overcurrent protection The L7986TA implements overcurrent protection by 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.
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Functional description L7986TA If, with VIN=38 V, the switching frequency is set higher than 706 kHz, during short-circuit condition the system finds a different equilibrium with higher current. For example, with FSW=800 kHz and the output shorted to ground, the output current is limited around: Equation 5 V IN ⋅ F *SW – V F ⁄ T ON_MIN I OUT = -------------------------------------------------------------------------------------------------------------------------- = 4.
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L7986TA 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.
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Application information L7986TA Given the physical dimension, ceramic capacitors can well meet the requirements of the input filter sustaining a 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 10 IO C IN = ------------------------------- ⋅ V PP ⋅ F SW D ⎛1 – D ----⎞ ⋅ D + ---- ⋅ ( 1 – D ) ⎝ η η⎠ neglecting the small ESR of ceramic capacitors.
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L7986TA Application information 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 which is at minimum duty cycle (see Section 6.1 to calculate minimum duty).
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Application information L7986TA Equation 15 ΔI MAX ΔV OUT = ESR ⋅ ΔI MAX + -------------------------------------------8⋅ C ⋅ f OUT SW Usually the resistive component of the ripple is much higher than the capacitive one, if the output capacitor adopted is not a multi-layer ceramic capacitor (MLCC) with very low ESR value. 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.
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L7986TA Application information Equation 16 V IN G PW0 = --------Vs where VS is the sawtooth amplitude. As seen in Section 5.
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Application information L7986TA Equation 20 1 f LC = -----------------------------------------------------------------------------------, ESR 2π ⋅ L ⋅ C OUT ⋅ 1 + ---------------R OUT 1 f zESR = --------------------------------------------------2π ⋅ ESR ⋅ C OUT Equation 21 R OUT ⋅ L ⋅ C OUT ⋅ ( R OUT + ESR ) Q = ------------------------------------------------------------------------------------------------------- , L + C OUT ⋅ R OUT ⋅ E SR V OUT R OUT = --------------I OUT As seen in Section 5.
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L7986TA Application information Figure 9. Type III compensation network In Figure 10 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 10. 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.
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Application information L7986TA Equation 25 1 C 4 = ---------------------------------π ⋅ R 4 ⋅ f LC 4. Calculate C5 by placing the second pole at four times the system bandwidth (BW): Equation 26 C4 C 5 = ------------------------------------------------------------------------2π ⋅ R 4 ⋅ C 4 ⋅ 4 ⋅ BW – 1 5.
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L7986TA Application information Figure 11.
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Application information 6.4.2 L7986TA 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 12 the type II network is shown. Figure 12.
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L7986TA Application information Figure 13. 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, in order to have values of C4 and C5 not comparable with parasitic capacitance of the board. 2.
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Application information 4. L7986TA Then calculate C3 in order to place the second pole at four times the system bandwidth (BW): Equation 33 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, and ESR=35 mΩ, the type II compensation network is: Equation 34 R 1 = 1.1kΩ, R 2 = 150Ω, R 4 = 4.
PAGE 27
L7986TA Application information Figure 14. Open-loop gain Bode diagram with electrolytic/tantalum output capacitor 6.5 Thermal considerations The thermal design is important to prevent the thermal shutdown of the device if junction temperature goes above 150 °C.
PAGE 28
Application information L7986TA where D is the duty cycle of the application and the maximum RDSon overtemperature is 220 mΩ. Note that the duty cycle is theoretically given by the ratio between VOUT and VIN, but actually it is quite higher in order to compensate the losses of the regulator. So the conduction losses increase compared with the ideal case.
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L7986TA Application information Figure 15. Switching losses 6.6 Layout considerations The PC board layout of the switching 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.
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Application information L7986TA Figure 16.
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L7986TA 6.7 Application information Application circuit In Figure 17 the demonstration board application circuit is shown. Figure 17. Demonstration board application circuit Table 9. Component list Reference Part number Description Manufacturer C1 UMK325BJ106MM-T 10 μF, 50 V Taiyo Yuden C2 GRM32ER61E226KE15 22 μF, 25 V Murata C3 3.3 nF, 50 V C4 33 nF, 50 V C5 100 pF, 50 V C6 470 nF, 50 V 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.
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Application information L7986TA Figure 18. PCB layout: L7986TA (component side) Figure 19. PCB layout: L7986TA (bottom side) Figure 20.
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L7986TA Application information Figure 21. Junction temperature vs. output current VQFN Figure 22. Junction temperature vs. output current HSOP VQFN VOUT=5V HSOP VOUT=5V VOUT=3.3V VOUT=3.3V VOUT=1.8V VOUT=1.8V VIN=24V FSW=250KHz TAMB=25 C Figure 23. Junction temperature vs. output current VIN=12V FSW=250KHz TAMB=25 C Figure 24. Efficiency vs. output current 95 VQFN Vo=5.0V FSW=250kHz HSOP 90 VOUT=3.3V 85 VOUT=1.8V Eff [%] VOUT=1.
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Application information L7986TA Figure 27. Load regulation Figure 28. Line regulation 3.3600 3.350 Io=1A Vin=5V 3.345 3.3550 Vin=12V Io=2A Io=3A 3.340 3.3500 3.335 3.3450 VOUT [V] VOUT [V] Vin=24V 3.330 3.3400 3.325 3.3350 3.320 3.3300 3.315 3.3250 3.310 3.3200 5.0 3.305 0.00 0.50 1.00 1.50 2.00 2.50 10.0 15.0 20.0 25.0 30.0 35.0 40.0 VIN [V] 3.00 Io [A] Figure 29. Load transient: from 0.4 A to 3 A Figure 30. Soft-start IL 0.
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L7986TA Application ideas 7 Application ideas 7.1 Positive buck-boost The L7986TA 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 38 V. Figure 33.
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Application ideas L7986TA Equation 41 I OUT I SW = ------------- < 3 A 1–D where ISW is the average current in the embedded power MOSFET in the on-time. To choose the right value of the inductor and to manage transient output current, which for a short time can exceed the maximum output current calculated by Equation 41, also the peak current in the power MOSFET must be calculated. The peak current, shown in Equation 42, must be lower than the minimum current limit (3.7 A).
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L7986TA Application ideas Equation 43 V OUT + 2 ⋅ V D D = -------------------------------------------------------------------------------------------------V IN – V SW – V SWE + V OUT + 2 ⋅ VD 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 L7986TA can implement the step-up/down converter with a negative output voltage. Figure 33.
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Application ideas L7986TA The GND pin of the device is connected to the output voltage so, given the output voltage, input voltage range is limited by the maximum voltage the device can withstand across VCC and GND (38 V). Therefore, if the output is -5 V the input voltage can range from 4.5 V to 33 V. As in the positive buck-boost, the maximum output current according to application conditions is shown in Figure 36.
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L7986TA 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. Table 10. HSOP8 mechanical data mm Dim Min. Typ. A Max. 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.
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Package mechanical data L7986TA Figure 37.
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L7986TA 9 Ordering information Ordering information Table 11.
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Revision history 10 L7986TA Revision history Table 12. 42/43 Document revision history Date Revision Changes 25-Oct-2011 1 Initial release. 01-Mar-2012 2 Section 8: Package mechanical data has been updated. 16-Oct-2012 3 In Section 5.
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L7986TA Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale.