L5987 STMicroelectronics, L5987 Datasheet
L5987
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L5987 Summary of contents
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... Figure 1. Application circuit June 2009 3 A step-down switching regulator VFQFPN8 3x3 mm Description The L5987 is a step-down switching regulator with 3.5 A (minimum) current limited embedded power MOSFET able to deliver current to the load depending on the application conditions (Section 5.7 The input voltage can range from 2 while the output voltage can be set starting from 0 ...
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... Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.3 Error amplifier and compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.4 Overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.5 Inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.6 Hysteretic thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.7 Maximum DC output current L5987A (HSOP8 5.8 Maximum DC output current L5987 (VFQFPN Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6.1 Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6.2 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 6.3 Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6 ...
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... L5987 8 Order codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Doc ID 14972 Rev 3 Contents 3/41 ...
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... Vout to FB pin. 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 14972 Rev GND GND FSW FSW FB FB Description L5987 ...
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... L5987 2 Maximum ratings Table 2. Absolute maximum ratings Symbol Vcc OUT F , COMP, SYNCH Analog pin SW INH FB P TOT stg 3 Thermal data Table 3. Thermal data Symbol R thJA 1. Package mounted on demonstration board. Parameter Input voltage Output DC voltage Inhibit pin Feedback voltage VFQFPN Power dissipation at T < ...
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... Values Min Typ Max 2.9 18 2.9 0.175 0.3 140 170 140 220 3.5 4.0 4.4 225 250 275 220 265 1.262 0 100 1000 (1) 0.593 0.6 0.607 2 0.6 1.9 7.5 10 7.4 8.2 9.1 2 L5987 Unit V mΩ A kHz V % kHz V mA μA V μA ms ...
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... L5987 Table 4. Electrical characteristics (continued) Symbol Error amplifier V High level output voltage CH V Low level output voltage CL I Bias source current FB 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 ...
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... Functional description 5 Functional description The L5987 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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... L5987 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 10/41 Doc ID 14972 Rev 3 L5987 ...
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... L5987 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 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. 12/41 Parameter GBWP Slew rate Chapter 6.4 Doc ID 14972 Rev 3 L5987 Value 100 dB 4.5 MHz 7 V/μ 3 mA/40 mA for details about the ...
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... L5987 5.4 Overcurrent protection The L5987 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” ...
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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. 14/41 Doc ID 14972 Rev 3 L5987 ...
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... A. So the L5987A can deliver 3 A with 100% of duty cycle. 5.8 Maximum DC output current L5987 (VFQFPN) The L5987 can manage DC output currents However the rated RMS current of its internal power switch is 2.5 A. Since the current flows through the integrated power element only during the on time, the ...
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... For duty cycles lower than 69%, the RMS current does not limit the maximum DC output current For duty cycles higher than the 69% the maximum DC output current is limited by the RMS current (see 16/ OUT O DS(on) Equation 5) Doc ID 14972 Rev 3 = 220 mΩ 0.35 V and DCR = 30 mΩ, F L5987 ...
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... L5987 6 Application information 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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... OFF L is the conduction OFF + T )). The maximum current ripple, at OFF 1 D – F MIN ⋅ ---------------------- - OFF = 250 kHz the minimum SW Δ ------- - 2 Inductor value (μH) Saturation current (A) 3 3 3.84 to 5.34 8 3.75 to 6.25 2 1.5 to 3.3 3.6 to 5.2 6 4.1 to 5.7 L5987 ...
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... L5987 6.3 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 ...
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... Compensation network The compensation network has to assure stability and good dynamic performance. The loop of the L5987 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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... L5987 Equation 17 where: Equation 18 Equation 19 As seen in Chapter 5.3 following paragraph the guidelines to select the Type II and Type III compensation network are illustrated. 6.4.1 Type III compensation network The methodology to stabilize the loop consists of placing two zeros to compensate the effect of the LC double pole, so increasing phase margin; then to place one pole in the origin to minimize the dc error on regulated output voltage ...
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... Choose a value for R 2. Choose a gain (R 22/ ----------------------------- - ⋅ ⋅ 2π ( · G (f) · G LOOP PW0 LC , usually between 1 kΩ and 5 kΩ order to have the required bandwidth (BW), that means Doc ID 14972 Rev ------------------------------------------- - ⋅ ⋅ ⋅ ------------------- - 2π · G PW0 (f)) are drawn. TYPEIII L5987 (f)) LC ...
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... L5987 Equation 22 where K is the feed forward constant and 1/K is equals Calculate C 4 Equation 23 4. Calculate C 5 Equation 24 5. Set also the first pole at four times the system bandwidth and also the second zero at the output filter double pole: Equation 25 The suggested maximum system bandwidth is equals to the switching frequency divided by 3 ...
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... Application information Figure 13. Open loop gain Bode diagram with ceramic output capacitor 24/41 Doc ID 14972 Rev 3 L5987 ...
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... L5987 6.4.2 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 ...
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... R = ⎝ ⎠ ESR f = ------------------------------------------- - ⋅ ESR 2π ESR C by placing the zero one decade below the output filter double pole ------------------------------ - 4 ⋅ 2π order to place the second pole at four times the system bandwidth 3 Doc ID 14972 Rev ⋅ ⋅ -------- - ⋅ OUT ⋅ L5987 ...
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... L5987 Equation 29 For example with V ESR = 35 mΩ, the type II compensation network is: In Figure 16 is shown the module and phase of the open loop gain. The bandwidth is about 32 kHz and the phase margin is 45° ------------------------------------------------------------- - ⋅ ⋅ 5 2π μ 3 OUT 1.5kΩ ...
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... Application information Figure 16. Open loop gain Bode diagram with electrolytic/tantalum output capacitor 28/41 Doc ID 14972 Rev 3 L5987 ...
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... L5987 6.5 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 equal to: Equation 30 Where D is the duty cycle of the application and the maximum 220 mΩ ...
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... Thanks to the exposed pad of the device, the ground plane helps to reduce the thermal resistance junction to ambient large ground plane enhances the thermal performance of the converter allowing high power conversion. In Figure 18 a layout example is shown. 30/41 measured on the demonstration board described in the following Doc ID 14972 Rev 3 L5987 ...
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... L5987 Figure 18. Layout example Doc ID 14972 Rev 3 Application information 31/41 ...
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... W 0603 330 Ω, 1%, 0.1 W 0603 2 kΩ, 1%, 0.1 W 0603 100k. STPS2L25V 5.2 μH, 30%, 5.28 A, MSS1038-522NL DCR MAX Doc ID 14972 Rev 3 L1 5.2uH MSS1038 Vout=1.8V C2 22u D1 25V STPS2L25 R1 4.99K 2.2nF 330 2.49k Description Manufacturer MURATA MURATA STMicroelectronics Coilcraft = 22 mΩ L5987 ...
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... L5987 Figure 20. PCB layout: L5987 and L5987A (component side) Figure 21. PCB layout: L5987 and L5987A (bottom side) Figure 22. PCB layout: L5987 and L5987A (front side) Doc ID 14972 Rev 3 Application information 33/41 ...
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... Io [A] Io [A] Doc ID 14972 Rev 3 output current =12V =12V 0.3 0.3 0.8 0.8 1.3 1.3 1.8 1.8 2.3 2.3 Io [ =2. =1. =1. =3. 0.3 0.8 1.3 1.8 2.3 Io [A] L5987 = =250kHz =250kHz SW SW =5V = =3.3V =3. =2.5V =2. 2.8 2.8 2.8 ...
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... L5987 Figure 29. Load regulation 1.2 1 0.8 0.6 0.4 0 0.5 Figure 31. Load transient: from 0 OUT OUT 100mV/div 100mV/div AC coupled AC coupled I I 1A/div 1A/div L L Figure 33. Short-circuit behavior OUT 10V/div OUT 10V/div OUT 10V/div OUT 10V/div 1V/div 0.5V/div 1V/div 0.5V/div OUT OUT OUT ...
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... Package mechanical data 7 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 trademark. 36/41 Doc ID 14972 Rev 3 L5987 www.st.com. ECOPACK ...
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... L5987 Table 10. VFQFPN8 ( 1.08 mm) mechanical data Dim ddd Figure 34. Package dimensions mm Min Typ Max 0.80 0.90 1.00 0.02 0.05 0.70 0.20 0.18 0.23 0.30 2.95 3.00 3.05 2.23 2.38 2.48 2.95 3.00 3.05 1.65 1.70 1.75 0.50 0.35 0.40 0.45 0.08 ...
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... Doc ID 14972 Rev 3 inch Min Typ Max 0.0669 0.00 0.0059 0.0492 0.0122 0.0201 0.0067 0.0098 0.1890 0.1929 0.1969 0.2283 0.2441 0.1496 0.1575 0.0098 0.0197 0.0157 0.0500 0.3150 0.0039 L5987 ...
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... L5987 8 Order codes Table 12. Order codes Order codes L5987 L5987A L5987TR L5987ATR Package VFQFPN8 HSOP8 VFQFPN8 HSOP8 Doc ID 14972 Rev 3 Order codes Packaging Tube Tape and reel 39/41 ...
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... Revision history 9 Revision history Table 13. Document revision history Date 29-Aug-2008 29-Jan-2009 19-Jun-2009 40/41 Revision 1 Initial release 2 Updated: Equation 22 3 Updated Table 4 on page 5 Doc ID 14972 Rev 3 Changes and Figure 6 on page 10 L5987 ...
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... L5987 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. ...