L7985 2 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 VFDFPN10 3 x 3 mm Computer: optical storage, hard disk drive, printers, audio/graphic cards HSOP8 exp. pad LED driving Features Description 2 A DC output current The L7985/A is a step-down switching regulator with a 2.
Contents L7985 Contents 1 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 Thermal data . . . . . . . .
L7985 8 Contents Package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 8.1 VFDFPN10 (3 x 3 x 1.0 mm) package information . . . . . . . . . . . . . . . . . . 38 8.2 HSOP8 package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 9 Ordering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pin settings L7985 1 Pin settings 1.1 Pin connection Figure 2. Pin connection (top view) 287 9&& 287 9&& 6<1&+ (1 &203 *1' )6: )% 287 9&& 6<1&+ (1 &203 *1' )6: )% +623 9)4)31 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 4/44 Type 5 Description Regulator output Master/slave synchronization.
L7985 2 Maximum ratings 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 3 -0.3 to 4 EN Enable pin -0.3 to VCC FB Feedback voltage -0.3 to 1.5 PTOT 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 L7985 Electrical characteristics TJ = 25 °C, VCC = 12 V, unless otherwise specified. Table 4. Electrical characteristics Values Symbol Parameter Test conditions Unit Min. Operating input voltage range (1) Turn on VCC threshold (1) VCCHYS VCC UVLO hysteseris (1) RDSON MOSFET on-resistance VCC VCCON ILIM Typ. 4.5 Max. 38 4.5 0.1 0.4 200 (1) Maximum limiting current V 400 mΩ 2.5 3.0 3.
L7985 Electrical characteristics Table 4. Electrical characteristics (continued) Values Symbol Parameter Test conditions 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 L7985 Functional description The L7985 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.
L7985 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 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 6 by an external resistor connected to ground.
Functional description L7985 Figure 5. Sawtooth: voltage and frequency feed-forward; external synchronization Figure 6. Oscillator frequency vs.
L7985 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 L7985 The uncompensated error amplifier characteristics are the following: Table 5. Uncompensated error amplifier characteristics Parameter Value Low frequency gain 100 dB GBWP 4.5 MHz Slew rate 7 V/µs Output voltage swing 0 to 3.3 V Maximum source/sink current 17 mA/25 mA In continuous conduction mode (CCM), the transfer function of the power section has two poles due to the LC filter and one zero due to the ESR of the output capacitor.
L7985 Functional description If the output voltage is shorted, VOUT 0, IOUT = ILIM, D/FSW = TON_MIN, (1 - D)/FSW 1/FSW. So, from Equation 3, the maximum switching frequency that guarantees to limit the current results: Equation 4 V F + DCR I LIM 1 F *SW = ------------------------------------------------------------------------------- --------------------- V IN – R DSON + DCR I LIM T ON_MIN With RDSON = 300 mΩ, DCR = 0.08 Ω, the worst condition is with VIN = 38 V, ILIM = 2.
Functional description 5.5 L7985 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.
L7985 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 L7985 Equation 10 IO D D C IN = --------------------------- 1 – ---- D + ---- 1 – D V PP F SW neglecting the small ESR of the ceramic capacitors. Considering = 1, this function has its maximum in D = 0.
L7985 Application information Equation 13 V OUT + V F 1 – D MIN L MIN = ---------------------------- ----------------------I MAX F SW where FSW is the switching frequency, 1 / (TON + TOFF). For example, for VOUT = 5 V, VIN = 24 V, IO = 2 A and FSW = 250 kHz, the minimum inductance value to have IL = 30% of IO is about 28 µH.
Application information L7985 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, IL = 0.6 A (resulting by the inductor value), in order to have a VOUT = 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.
L7985 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 L7985 As seen in Section 5.3 on page 11, 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.
L7985 Application information 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. Choose a gain (R4/R1) in order to have the required bandwidth (BW), that means: Equation 24 BW R 4 = ---------- K R 1 f LC where K is the feed-forward constant and 1/K is equal to 18. 3.
Application information L7985 For example, with VOUT = 5 V, VIN = 24 V, IO = 2 A, L = 22 µH, COUT = 22 µF, and ESR < 1 mΩ, the type III compensation network is: R 1 = 4.99k R 2 = 680 R 3 = 270 R 4 = 1.1k C 3 = 4.7nF C 4 = 47nF C 5 = 1nF In Figure 12 the module and phase of the open-loop gain is shown. The bandwidth is about 32 kHz and the phase margin is 51 °. Figure 12.
L7985 6.4.2 Application information 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: 2ESR 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 the stabilization of the loop. In Figure 13 the type II network is shown.
Application information L7985 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 shown. 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.
L7985 Application information Equation 32 C4 C 5 = -------------------------------------------------------------2 R 4 C 4 4 BW – 1 For example with VOUT = 5 V, VIN = 24 V, IO = 2 A, L = 22 µH, COUT = 330 F, ESR = 70 mΩ the type II compensation network is: R 1 = 1.1k R 2 = 150 R 4 = 4.99k C 4 = 180nF C 5 = 180pF In Figure 15 the module and phase of the open-loop gain is shown. The bandwidth is about 36 kHz and the phase margin is 53 °. Figure 15.
Application information 6.5 L7985 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 non-negligible RDSON of the power switch; these are equal to: Equation 33 2 P ON = R DSON I OUT D where D is the duty cycle of the application and the maximum RDSON overtemperature is 220 m.
L7985 Application information Figure 16. 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.
Application information L7985 In Figure 17 a layout example is shown. Figure 17.
L7985 6.7 Application information Application circuit In Figure 18 the demonstration board application circuit is shown. Figure 18. Demonstration board application circuit Table 9. Component list Reference Part number Description Manufacturer C1 UMK325BJ106MM-T 10 F, 50 V Taiyo Yuden C2 GRM32ER61E226KE15 22 F, 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.
Application information L7985 Figure 19. PCB layout: L7985 and L7985A (component side) Figure 20. PCB layout: L7985 and L7985A (bottom side) Figure 21.
L7985 Application information Figure 22. Junction temperature vs. output current VIN = 24 V VQFN Figure 23. Junction temperature vs. output current VIN = 12 V VQFN HSOP VOUT=5V VOUT=5V VOUT=3.3V VOUT=3.3V VOUT=1.8V VOUT=1.8V HSOP VIN=24V 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 Vo=1.8V FSW=250kHz 80 HSOP 75 VOUT=1.8V 70 VOUT=1.2V 65 Eff [%] VOUT=3.
Application information L7985 Figure 28. Load regulation Figure 29. Line regulation 3.345 3.3500 Vin=5V 3.340 Io=1A Vin=12V 3.3450 Vin=24V Io=2A 3.3400 3.330 VOUT [V] VOUT [V] 3.335 3.325 3.3350 3.3300 3.320 3.3250 3.315 3.310 0.00 3.3200 0.50 1.00 1.50 5.0 2.00 10.0 15.0 20.0 25.0 Figure 30. Load transient: from 0.4 A to 2 A 35.0 40.0 Figure 31. Soft-start VOUT 100mV/div AC coupled VOUT 500mV/div IL 500mA/div VIN=24V VOUT=3.
L7985 Application ideas 7 Application ideas 7.1 Positive buck-boost The L7985 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 a 12 V output voltage with input voltage from 4.5 V to 38 V. Figure 34.
Application ideas L7985 Equation 39 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 can exceed the maximum output current calculated by equation 39 for a short time, 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 (2.5 A).
L7985 Application ideas Equation 41 V OUT + 2 V D D = -------------------------------------------------------------------------------------------V IN – V SW – V SWE + 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 L7985 device can implement the step-up/down converter with a negative output voltage.
Application ideas L7985 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 37. The dashed line considers a more accurate estimation of the duty cycles given by equation 44, where power losses across diodes and the internal power MOSFET are taken into account.
L7985 8 Package information 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.
Package information 8.1 L7985 VFDFPN10 (3 x 3 x 1.0 mm) package information Figure 38. VFDFPN10 (3 x 3 x 1.
L7985 Package information Table 10. VFDFPN10 (3 x 3 x 1.0 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 2.70 3.00 3.15 1.75 0.50 0.3 ddd 0.40 0.5 0.
Package information 8.2 L7985 HSOP8 package information Figure 39. HSOP8 package outline ' PP 7\S ( PP 7\S 40/44 DocID022446 Rev 8
L7985 Package information Table 11. HSOP8 package mechanical data mm Symbol 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 3.80 3.90 4.00 e 0.15 1.27 h 0.25 0.50 L 0.40 1.27 k 0.00 8.00 ccc 0.
Ordering information 9 L7985 Ordering information Table 12.
L7985 10 Revision history Revision history Table 13. Document revision history Date Revision Changes 07-Nov-2011 1 Initial release. 01-Mar-2012 2 Section 8: Package information has been updated. 16-Oct-2012 3 In Section 5.6 changed temperature value from 130 to 120 °C. 18-Mar-2014 4 Updated text below Equation 4 on page 13 (replaced “DRC” by “DCR”). Numbered on page 22, Equation 28 on page 23, and Equation 32 on page 25. Updated Section 6.4.
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