L7981 STMicroelectronics, L7981 Datasheet

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... Application circuit July 2010 3 A step-down switching regulator VFQFPN8 3x3 Description The L7981 is a step down switching regulator with 3.7 A (minimum) current limited embedded power MOSFET able to deliver current to the load depending on the application conditions. The input voltage can range from 4 while the output voltage can be set starting from 0 ...

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... Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6.3 Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.4 Compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.4.1 6.4.2 6.5 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.6 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 6.7 Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 7 Application ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 7.1 Positive buck-boost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 7.2 Inverting buck-boost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2/44 Type III compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Type II compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Doc ID 15182 Rev 3 L7981 ...

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... L7981 8 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 9 Order codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Doc ID 15182 Rev 3 Contents 3/44 ...

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... The switching frequency can be increased connecting an external resistor from FSW pin and ground. If this pin is left floating the device works at its free-running frequency of 250 kHz. Ground Unregulated DC input voltage CC Doc ID 15182 Rev GND GND GND GND FSW FSW FSW FSW Description L7981 ...

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... L7981 2 Maximum ratings Table 2. Absolute maximum ratings Symbol Vcc OUT F , COMP, SYNCH TOT stg 3 Thermal data Table 3. Thermal data Symbol R thJA 1. Package mounted on demonstration board. Parameter Input voltage Output DC voltage Analog pin Enable pin Feedback voltage VFQFPN Power dissipation at T < 60°C ...

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... Values Min Typ Max 4.5 28 4.4 0.12 0.35 160 180 160 250 3.7 4.2 4.7 225 250 275 220 275 1.254 0 100 1000 (1) 0.593 0.6 0.607 2 0.3 1.2 7.5 10 7.4 8.2 9.1 2 L7981 Unit V mΩ A KHz V % KHz V mA μA V μA ms ...

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... L7981 Table 4. Electrical characteristics (continued) Symbol V High level output voltage CH V Low level output voltage CL I Source COMP pin O SOURCE I Sink COMP pin O SINK G Open loop voltage gain V Synchronization function High input voltage Low input voltage Slave sink current Master output amplitude ...

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... Functional description 5 Functional description The L7981 is based on a “voltage mode”, constant frequency control. The output voltage V is sensed by the feedback pin (FB) compared to an internal reference (0.6 V) providing OUT an error signal that, compared to a fixed frequency sawtooth, controls the on and off time of the power switch ...

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... L7981 5.1 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 ...

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... Functional description Figure 5. Sawtooth: voltage and frequency feed forward; external synchronization Figure 6. Oscillator frequency versus FSW pin resistor 10/44 Doc ID 15182 Rev 3 L7981 ...

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... L7981 5.2 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 (V error amplifier. So the output voltage slew rate is: ...

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... III compensation network has to be used (see compensation network selection). Anyway the methodology to compensate the loop is to introduce zeros to obtain a safe phase margin. 12/44 GBWP Slew rate Chapter 6.4 Doc ID 15182 Rev 3 L7981 100dB 4.5MHz 7V/μ 3.3V 17mA/25mA for details about the ...

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... L7981 5.4 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 “ ...

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... The thermal shutdown block generates a signal that turns off the power stage if the junction temperature goes above 150°C. Once the junction temperature goes back to about 130°C, the device restarts in normal operation. The sensing element is very close to the PDMOS area, so ensuring an accurate and fast temperature detection. 14/44 compatible. Doc ID 15182 Rev 3 L7981 ...

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... L7981 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 ...

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... ⋅ ⋅ -------------------------- - C = – --- - ⎝ ⎠ ⋅ η the minimum input capacitor (C PP_MAX ----------------------------------------------- - ⋅ IN_MIN 2 V PP_MAX Series Cap value (μF) UMK325BJ106MM-T GMK325BJ106MN-T GRM32ER71H475K Doc ID 15182 Rev 3 can be written as follows ⋅ --- - + 1 D – η ) value is: IN_MIN ⋅ Rated voltage ( 4.7 50 L7981 ...

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... L7981 Equation 9 Where T is the conduction time of the internal high side switch and T ON time of the external diode (in CCM, F fixed Vout, is obtained at maximum T to calculate minimum duty). So fixing ΔI minimum inductance value can be calculated: Equation 10 where F is the switching frequency, 1/(T SW For example for V value to have Δ ...

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... OUT Series Cap value (μF) GRM32 22 to 100 GRM31 ECJ EEFCD TPA/B/C 100 to 470 C3225 22 to 100 Doc ID 15182 Rev 3 ΔI MAX + ------------------------------------ - ⋅ ⋅ OUT SW Rated voltage (V) ESR (mΩ 6.3 6 6.3 L7981 < 5 < 5 < 5 < 5 ...

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... 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. ...

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... In the two , They expression are ----------------------------------------------- - ⋅ ⋅ 2π Doc ID 15182 Rev 3 s ------------------------- - ⋅ 2π f zESR ⎛ ⎞ ------------------ - + ⎝ ⎠ ⋅ 2π ------------------------------------------- - ⋅ ⋅ zESR 2π ESR C OUT ) ESR V OUT , R = -------------- OUT I OUT ----------------------------- - ⋅ ⋅ Z2 2π L7981 ...

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... L7981 Equation 20 Figure 10. Type III compensation network In Figure 11 the Bode diagram of the PWM and LC filter transfer function (G and the open loop gain (G 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 follow: 1 ...

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... A, L=18 μ = OUT 680Ω R 200Ω R 3.3kΩ Doc ID 15182 Rev 3 ⋅ ⋅ ⋅ ⋅ – ---------------------------------------- - ⋅ ⋅ ⋅ 3 2π set higher than 500 kHz. SW =22 μF, ESR<1 mΩ, the type OUT , , , C 3.3nF C 22nF = = 3 4 L7981 ): LC C 220pF = 5 ...

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... L7981 Figure 12. Open loop gain bode diagram with ceramic output capacitor Doc ID 15182 Rev 3 Application informations 23/44 ...

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... The singularities of the network are: In Figure 14 the Bode diagram of the PWM and LC filter transfer function (G and the open loop gain (G 24/ ----------------------------- - = ⋅ ⋅ 2π ( · G (f) · G LOOP PW0 LC Doc ID 15182 Rev ------------------------------------------- - ⋅ ⋅ ⋅ ------------------- - 2π · G PW0 (f)) are drawn. TYPEII L7981 (f)) LC ...

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... L7981 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 R and C5 not comparable with parasitic capacitance of the board. 2. Choose a gain (R Equation 25 Where f ...

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... The bandwidth is about 21 kHz and the phase margin is 45°. 26/ ------------------------------------------------------------- - ⋅ ⋅ 5 2π L=18 μ = OUT 1.1kΩ 150Ω Doc ID 15182 Rev 3 4 ⋅ ⋅ – 1 =330 μF, ESR=35 mΩ, the OUT , , 4.99kΩ 82nF C = 68pF 4 5 L7981 ...

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... L7981 Figure 15. Open loop gain bode diagram with electrolytic/tantalum output capacitor Doc ID 15182 Rev 3 Application informations 27/44 ...

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... FALL ⋅ =2.4 mA). Q can be calculated as Rth the sum of the power losses just seen. TOT Doc ID 15182 Rev 3 of the power switch; these are DSon 2 ) ⋅ OUT over temperature is DSon OUT ⋅ ⋅ ⋅ OUT ⋅ P TOT L7981 and Figure 16. ...

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... L7981 of heat. The Rth JA 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 ...

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... Application informations Figure 17. Layout example 30/44 Doc ID 15182 Rev 3 L7981 ...

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... L7981 6.7 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 10 μF, 50V UMK325BJ106MM-T 22 μF, 25V GRM32ER61E226KE15 2.2 nF, 50V 22 nF, 50V 470 pF, 50V 470 nF, 50V 4.99 kΩ, 1%, 0.1W 0603 1.1 kΩ ...

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... Application informations Figure 19. PCB layout: L7981 and L7981A (component side) Figure 20. PCB layout: L7981 and L7981A (bottom side) Figure 21. PCB layout: L7981 and L7981A (front side) 32/44 Doc ID 15182 Rev 3 L7981 ...

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... L7981 Figure 22. Junction temperature vs. output current Figure 24. Junction temperature vs. output current Figure 26. Efficiency vs.output current = =12V IN V =24V 0.0 0.5 1.0 1.5 Io [A] Figure 23. Junction temperature vs. output Figure 25. Efficiency vs. output current 0.0 Figure 27. Efficiency vs. output current =3.3 V OUT 55 fsw=250 kHz 50 0.0 2 ...

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... Figure 31. Soft-start V =24V IN V =3.3V OUT C =47uF OUT L=10uH F =520k SW Time base 5ms/div Time base 5ms/div Doc ID 15182 Rev 3 Io=1A Io=2A Io= [ 1A/div 1A/div OUT OUT Time base 1ms/div Time base 1ms/div L7981 1V/div 0.5V/div ...

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... 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 ...

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... The dashed line considers a more accurate estimation of the duty cycles given by 37, where power losses across diodes, external power MOSFET, internal power MOSFET are taken into account. 36/44 I OUT < ------------ - – Equation I r OUT ⋅ < ------------ - 3.7A SW, – OUT ⋅ ----------------------------------- - – ⋅ ⋅ OUT SW Doc ID 15182 Rev 3 L7981 35, also the peak Equation Equation ...

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... V D power MOSFET. 7.2 Inverting buck-boost The L7981 can implement the step up/down converter with a negative output voltage. Figure 33. shows the schematic to regulate - further external components are added to the standard buck topology. The relationship between input and output voltage is: ...

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... Figure 36. Maximum output current according to switch max peak current (3.0 A 38/44 Equation 39. Figure 36. The dashed line considers a more accurate estimation of Equation 40, where power losses across diodes and internal power V – V OUT ---------------------------------------------------------------- - – V – OUT Doc ID 15182 Rev 3 Equation – L7981 ...

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... L7981 8 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: ECOPACK trademark. Doc ID 15182 Rev 3 Package mechanical data ® www.st.com. 39/44 ...

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... Doc ID 15182 Rev 3 L7981 inch Min Typ Max 0.0315 0.0354 0.0394 0.0008 0.0020 0.0276 0.0079 0.0071 0.0091 0.0118 0.1161 0.1181 0.1200 0.0878 0.0937 ...

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... L7981 Table 11. HSOP8 mechanical data Dim ccc Figure 38. Package dimensions mm Min Typ Max 1.70 0.00 0.15 1.25 0.31 0.51 0.17 0.25 4.80 4.90 5.00 5.80 6.00 6.20 3.80 3.90 4.00 1.27 0.25 0.50 0.40 1. 0.10 Doc ID 15182 Rev 3 Package mechanical data ...

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... Order codes 9 Order codes Table 12. Order codes Order codes L7981 L7981A L7981TR L7981ATR 42/44 Package VFQFPN8 HSOP8 VFQFPN8) HSOP8 Doc ID 15182 Rev 3 L7981 Packaging Tube Tube Tape and reel Tape and reel ...

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... L7981 10 Revision history Table 13. Document revision history Date 19-Nov-2008 17-Mar-2009 01-Jul-2010 Revision 1 Initial release. 2 Content reworked to improve readability, no technical changes 3 Added application information Doc ID 15182 Rev 3 Revision history Changes 43/44 ...

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... Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan - Malaysia - Malta - Morocco - Philippines - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America 44/44 Please Read Carefully: © 2010 STMicroelectronics - All rights reserved STMicroelectronics group of companies www.st.com Doc ID 15182 Rev 3 L7981 ...