A7986A 3 A step-down switching regulator for automotive applications Datasheet - production data Applications • Dedicated to automotive applications • Automotive LED driving Description HSOP8 exposed pad Features • Qualified following AEC-Q100 requirements • 3 A DC output current • 4.5 V to 38 V input voltage • Output voltage adjustable from 0.
Contents A7986A Contents 1 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 Thermal data . . . . . . .
A7986A Contents 8 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 9 Ordering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pin settings A7986A 1 Pin settings 1.1 Pin connection Figure 2. Pin connection (top view) 1.2 Pin description Table 1. Pin description 4/42 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 in respect to the power turn-on is present at the pin.
A7986A 2 Maximum ratings Maximum ratings Table 2. Absolute maximum ratings Symbol 3 Parameter Value VCC Input voltage 45 OUT Output DC voltage -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 HSOP8 Unit V 2 W TJ Junction temperature range -40 to 150 °C Tstg Storage temperature range -55 to 150 °C Thermal data Table 3.
Electrical characteristics 4 A7986A Electrical characteristics TJ = - 40 °C to 125 °C, VCC = 12 V, unless otherwise specified. Table 4. Electrical characteristics Values Symbol Parameter Test conditions Unit Min. VCC Operating input voltage range VCCON Turn-on VCC threshold VCCHYS VCC UVLO hysteresis RDS(on) MOSFET on-resistance ILIM Maximum limiting current Typ. 4.5 Max. 38 4.5 0.1 0.4 200 TJ = 25 °C V 400 3.7 5.2 3.5 5.
A7986A Electrical characteristics Table 4. Electrical characteristics (continued) Values Symbol Parameter Test conditions Unit Min. Typ. Max. 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 = 0.75 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.
Functional description 5 A7986A Functional description The A7986A 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.
A7986A 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 A7986A Figure 5. Sawtooth: voltage and frequency feed-forward; external synchronization Figure 6. Oscillator frequency vs.
A7986A 5.2 Functional description Soft-start Soft-start is essential to assure a correct and safe startup of the step-down converter. It avoids inrush current surge and makes the output voltage increase monothonically. Soft-start is performed by a staircase ramp on the non-inverting input (VREF) of the error amplifier.
Functional description 5.3 A7986A Error amplifier and compensation The error amplifier (EA) 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.
A7986A 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 A7986A Figure 8. Overcurrent protection 5.5 Enable function The enable feature allows the device to be put into standby mode. With the EN pin is lower than 0.3 V the device is disabled and the power consumption is reduced to less than 30 μA. With the EN pin is 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.
A7986A 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 A7986A Equation 10 IO D D C IN = --------------------------- ⋅ 1 – ---- ⋅ D + ---- ⋅ ( 1 – D ) VPP ⋅ FSW η η neglecting the small ESR of ceramic capacitors. Considering η = 1, this function has its maximum in D = 0.
A7986A Application information Equation 13 VOUT + V F 1 – D MIN L MIN = ---------------------------- ⋅ ----------------------ΔIMAX FSW 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 A7986A 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 6.4 illustrates how to consider its effect in the system stability. For example, with VOUT = 5 V, VIN = 24 V, ΔIL = 0.
A7986A Application information Equation 17 VS = K ⋅ V IN 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 trough the SYNCH pin can modify the PWM modulator gain (see Section 5.1 to understand how this gain changes and how to keep it constant in spite of the external synchronization). Figure 9.
Application information A7986A Equation 21 R OUT ⋅ L ⋅ C OUT ⋅ ( R OUT + ESR ) Q = ------------------------------------------------------------------------------------------ , L + COUT ⋅ R OUT ⋅ E SR V OUT R OUT = -------------IOUT As seen in Section 5.3, two different kinds of network can compensate the loop. In the two following paragraphs, the guidelines to select the Type II and Type III compensation network are illustrated. 6.4.
A7986A Application information 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. 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 A7986A 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 C5 = -------------------------------------------------------------2π ⋅ R 4 ⋅ C 4 ⋅ 4 ⋅ BW – 1 5.
A7986A Application information Figure 12. Open loop gain Bode diagram with ceramic output capacitor 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.
Application information A7986A Figure 13. Type II compensation network The singularities of the network are: Equation 29 1 f Z1 = ------------------------------, 2π ⋅ R 4 ⋅ C 4 f P0 = 0, 1 fP1 = -------------------------------------------C4 ⋅ C5 2π ⋅ R 4 ⋅ -------------------C4 + C5 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.
A7986A Application information 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.
Application information A7986A Figure 15.
A7986A 6.5 Application information Thermal considerations The thermal design is important to prevent the thermal shutdown of the 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 RDS(on) of the power switch; these are equal to: Equation 34 2 P ON = R DS ( on ) ⋅ ( IOUT ) ⋅ D where D is the duty cycle of the application and the maximum RDS(on) over temperature is 220 mΩ.
Application information A7986A 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 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 current is flowing through it.
A7986A Application information Figure 17.
Application information 6.7 A7986A Application circuit In Figure 18 the demonstration board application circuit is shown. Figure 18. Demonstration board application circuit Table 9. Component list Reference 30/42 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.
A7986A Application information Figure 19. PCB layout: A7986A (component side) Figure 20. PCB layout: A7986A (bottom side) Figure 21.
Application information A7986A 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 HSOP VOUT=5V VOUT=3.3V VOUT=3.3V VOUT=1.8V VOUT=1.8V VIN=24V FSW=250KHz TAMB=25 C Figure 24. Junction temperature vs. output current @ Vin = 5 V VIN=12V FSW=250KHz TAMB=25 C Figure 25. Efficiency vs. output current @ Vin = 12 V 95 VQFN Vo=5.0V FSW=250kHz HSOP 90 VOUT=3.3V 85 VOUT=1.8V Eff [%] VOUT=1.
A7986A Application information Figure 28. Load regulation Figure 29. 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 30. Load transient: from 0.4 A to 2 A Figure 31. Soft-start IL 0.
Application ideas A7986A 7 Application ideas 7.1 Positive buck-boost The A7986A 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.
A7986A Application ideas Equation 40 I OUT I SW = ------------- < 2 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, which for a short time can exceed the maximum output current calculated by Equation 40, also the peak current in the power MOSFET must be calculated. The peak current, shown in Equation 41, must be lower than the minimum current limit (3.7 A).
Application ideas A7986A Equation 42 V OUT + 2 ⋅ VD 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 A7986A can implement the step-up/down converter with a negative output voltage.
A7986A 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 Equation 45, where power losses across diodes and the internal power MOSFET are taken into account. Equation 45 V OUT – V D D = ----------------------------------------------------------------– V IN – V SW + V OUT – V D Figure 37. Maximum output current according to switch max.
Package mechanical data 8 A7986A 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 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.
A7986A Package mechanical data Figure 38.
Ordering information 9 A7986A Ordering information Table 11.
A7986A 10 Revision history Revision history Table 12. Document revision history Date Revision Changes 13-Feb-2012 1 Initial release. 20-Mar-2012 2 Section 8: Package mechanical data has been updated. 16-Oct-2012 3 In Section 5.6 changed temperature value from 130 to 120 °C. 04-Jul-2013 4 Updated values for VFB parameter in Table 4: Electrical characteristics. 12-Aug-2013 5 Changed VFB parameter in Table 4 from 0.594 to 0.588.
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