L7980

L7980 2 A step-down switching regulator Datasheet - production data  Industrial: – PLD, PLA, FPGA, chargers  Networking: XDSL, modems, DC-DC modules VFQFPN8 3 x 3 HSOP8 exposed pad  Computer: – Optical storage, hard disk drive, printers, audio/graphic cards  LED driving Features  2 A DC output current Description  4.5 V to 28 V input voltage The L7980 device is a step-down switching regulator with a 2.5 A (minimum) current limited embedded Power MOSFET, so it is able to deliver up to 2 A current to the load depending on the application conditions.  Output voltage adjustable from 0.6 V  250 kHz switching frequency, programmable up to 1 MHz  Internal soft-start and enable The input voltage can range from 4.5 V to 28 V, while the output voltage can be set starting from 0.6 V to VIN.  Low dropout operation: 100% duty cycle  Voltage feed-forward  Zero load current operation  Overcurrent and thermal protection  VFQFPN 3 x 3 - 8L and HSOP8 package Applications  Consumer: – STB, DVD, DVD recorder, car audio, LCD TV and monitors 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 QFN and the HSOP packages with exposed pad allow reducing the RthJA down to 60 °C/W and 40 °C/W respectively. Figure 1. Application circuit May 2014 This is information on a product in full production. DocID15181 Rev 6 1/42 www.st.com Contents L7980 Contents 1 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5 Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6 7 2/42 5.1 Oscillator and synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.2 Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 5.3 Error amplifier and compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.4 Overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.5 Enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 5.6 Hysteretic thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Application informations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.1 Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.2 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6.3 Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 6.4 Compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.4.1 Type III compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.4.2 Type II compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 6.5 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.6 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.7 Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Application ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 7.1 Positive buck-boost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 7.2 Inverting buck-boost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 DocID15181 Rev 6 L7980 Contents 8 Package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 9 Order codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 DocID15181 Rev 6 3/42 42 Pin settings L7980 1 Pin settings 1.1 Pin connection Figure 2. Pin connection (top view) OUT VCC SYNCH GND EN FSW COMP 1.2 FB Pin description Table 1. Pin description 4/42 No. 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 respect to the power turn on is present at the pin. When connected to an external signal at a frequency higher than the internal one, then the device is synchronized by the external signal, with zero phase shift. Connecting together the SYNCH pin of two devices, the one with higher frequency works as master and the other one as slave; so the two powers turn on have a phase shift of half a period. 3 EN A logical signal (active high) enable the device. With EN higher than 1.2 V the device is ON and with EN is lower than 0.3 V the device is OFF. 4 COMP 5 FB Feedback input. Connecting the output voltage directly to this pin the output voltage is regulated at 0.6 V. To have higher regulated voltages an external resistor divider is required from Vout to the FB pin. 6 FSW The switching frequency can be increased connecting an external resistor from the FSW pin and ground. If this pin is left floating the device works at its free-running frequency of 250 kHz. 7 GND Ground 8 VCC Unregulated DC input voltage Error amplifier output to be used for loop frequency compensation DocID15181 Rev 6 L7980 2 Maximum ratings Maximum ratings Table 2. Absolute maximum ratings Symbol 3 Parameter Vcc Input voltage OUT Output DC voltage Value Unit 30 -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 VFQFPN 1.5. HSOP 2 V W TJ Junction temperature range -40 to 150 °C Tstg Storage temperature range -55 to 150 °C Thermal data Table 3. Thermal data Symbol RthJA Parameter Maximum thermal resistance junction ambient(1) Value VFQFPN 60 HSOP 40 Unit °C/W 1. Package mounted on demonstration board. DocID15181 Rev 6 5/42 42 Electrical characteristics 4 L7980 Electrical characteristics TJ = 25 °C, VCC = 12 V, unless otherwise specified. Table 4. Electrical characteristics Values Symbol Parameter Test condition Unit Min. Operating input voltage range (1) Turn on VCC threshold (1) VCCHYS VCC UVLO hysteresis (1) RDSON MOSFET on resistance VCC VCCON ILIM 4.5 Max. 28 4.4 0.12 V 0.35 160 180 160 250 2.5 3.0 3.5 225 250 275 (1) Maximum limiting current Typ. m A Oscillator FSW Switching frequency VFSW FSW pin voltage D FADJ (1) 275 1.254 Duty cycle Adjustable switching frequency 220 0 RFSW = 33 k KHz V 100 1000 % KHz Dynamic characteristics VFB Feedback voltage 4.5 V < VCC < 28 V(1) 0.593 0.6 0.607 V 2.4 mA 30 A DC characteristics IQ IQST-BY Quiescent current Duty cycle = 0, VFB = 0.8 V Total standby quiescent current 20 Enable EN threshold voltage EN current Device OFF level Device ON level 0.3 1.2 EN = VCC 7.5 10 8.2 9.1 V A Soft-start TSS Soft-start duration FSW pin floating 7.4 FSW = 1 MHz, RFSW = 33 k 2 ms Error amplifier VCH High level output voltage VFB < 0.6 V VCL Low level output voltage VFB > 0.6 V IO SOURCE Source COMP pin 6/42 VFB = 0.5 V, VCOMP = 1 V DocID15181 Rev 6 3 0.1 17 V mA L7980 Electrical characteristics Table 4. Electrical characteristics (continued) Values Symbol Parameter Test condition Unit Min. IO SINK GV Sink COMP pin VFB = 0.7 V, VCOMP = 1 V Open loop voltage gain (2) Typ. Max. 25 mA 100 dB Synchronization function High input voltage 2 3.3 Low input voltage 1 Slave sink current VSYNCH = 2.9 V Master output amplitude ISOURCE = 4.5 mA Output pulse width SYNCH floating Input pulse width 0.7 0.9 2.0 V mA V 110 70 ns Protection TSHDN Thermal shutdown 150 Hysteresis 30 °C 1. Specification referred to TJ from -40 to +125 °C. Specification in the -40 to +125 °C temperature range are assured by design, characterization and statistical correlation. 2. Guaranteed by design. DocID15181 Rev 6 7/42 42 Functional description 5 L7980 Functional description The L7980 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. They are:  A fully integrated oscillator that provides sawtooth to modulate the duty cycle and the synchronization signal. Its switching frequency can be adjusted by an external resistor. The voltage and frequency feed forward are implemented.  The soft-start circuitry to limit inrush current during the start-up phase.  The voltage mode error amplifier  The pulse width modulator and the relative logic circuitry necessary to drive the internal power switch.  The high-side driver for embedded P-channel Power MOSFET switch.  The peak current limit sensing block, to handle overload and short-circuit conditions.  A voltage regulator and internal reference. It supplies internal circuitry and provides a fixed internal reference.  A voltage monitor circuitry (UVLO) that checks the input and internal voltages.  A thermal shutdown block, to prevent thermal runaway. Figure 3. Block diagram VCC REGULATOR TRIMMING EN & BANDGAP EN 1.254V 3.3V 0.6V COMP UVLO PEAK CURRENT LIMIT THERMAL SOFTSTART SHUTDOWN E/A PWM DRIVER S Q R OUT OSCILLATOR FB 8/42 FSW GND DocID15181 Rev 6 SYNCH & PHASE SHIFT SYNCH L7980 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. In case the FSW pin is left floating the frequency is 250 kHz; it can be increased as shown in Figure 6 by external resistor connected to ground. To improve the line transient performance, keeping the PWM gain constant versus the input voltage, the voltage feed forward is implemented by changing the slope of the sawtooth according to the input voltage change (see Figure 5.a). The slope of the sawtooth also changes if the oscillator frequency is increased by the external resistor. In this way a frequency feed forward is implemented (Figure 5.b) in order to keep the PWM gain constant versus the switching frequency (see Section 6.4 on page 19 for PWM gain expression). On the SYNCH pin the synchronization signal is generated. This signal has a phase shift of 180° with respect to the clock. This delay is useful when two devices are synchronized connecting the SYNCH pin together. When SYNCH pins are connected, the device with higher oscillator frequency works as the master, so the slave device switches at the frequency of the master but with a delay of half a period. This minimizes the RMS current flowing through the input capacitor (see the L5988D datasheet). Figure 4. Oscillator circuit block diagram Clock FSW Clock Generator Synchronization SYNCH Ramp Generator Sawtooth The device can be synchronized to work at higher frequency feeding an external clock signal. The synchronization changes the sawtooth amplitude, changing the PWM gain (Figure 5.c). This changing has to be taken into account when the loop stability is studied. To minimize the change of the PWM gain, the free running frequency should be set (with a resistor on the FSW pin) only slightly lower than the external clock frequency. This preadjusting of the frequency will change the sawtooth slope in order to get negligible the truncation of sawtooth, due to the external synchronization. DocID15181 Rev 6 9/42 42 Functional description L7980 Figure 5. Sawtooth: voltage and frequency feed forward; external synchronization Figure 6. Oscillator frequency versus FSW pin resistor 10/42 DocID15181 Rev 6 L7980 5.2 Functional description Soft-start The 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 increases monothonically. The soft-start is performed by a staircase ramp on the non inverting input (VREF) of the error amplifier. So the output voltage slew rate is: Equation 1 R1 SR OUT = SR VREF   1 + --------  R2 where SRVREF is the slew rate of the non inverting input, while R1and R2 is the resistor divider to regulate the output voltage (see Figure 7). The soft-start stair case consists of 64 steps of 9.5 mV each one, from 0 V to 0.6 V. The time base of one step is of 32 clock cycles. So the soft-start time and then the output voltage slew rate depend on the switching frequency. Figure 7. Soft-start scheme Soft-start time results: Equation 2 32  64 SS TIME = ----------------Fsw For example with a switching frequency of 250 kHz the SSTIME is 8 ms. DocID15181 Rev 6 11/42 42 Functional description 5.3 L7980 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. In this device the error amplifier is a voltage mode operational amplifier so with high DC gain and low output impedance. 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/s 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. Different kinds of compensation networks can be used depending on the ESR value of the output capacitor. In case the zero introduced by the output capacitor helps to compensate the double pole of the LC filter a type II compensation network can be used. Otherwise, a type III compensation network has to be used (see Section 6.4 on page 19 for details about the compensation network selection). Anyway the methodology to compensate the loop is to introduce zeros to obtain a safe phase margin. 5.4 Overcurrent protection The L7980 device implements the overcurrent 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” or “blanking time”. The masking time is about 200 ns. When the overcurrent is detected, two different behaviors are possible depending on the operating condition. 12/42 1. Output voltage in regulation. When the overcurrent is sensed, the Power MOSFET is switched off and the internal reference (VREF), that biases the non inverting input of the error amplifier, is set to zero and kept in this condition for a soft-start time (TSS, 2048 clock cycles). After this time, a new soft-start phase takes place and the internal reference begins ramping (see Figure 8.a). 2. Soft-start phase. If the overcurrent limit is reached the Power MOSFET is turned off implementing the pulse by pulse overcurrent protection. During the soft-start phase, under overcurrent condition, the device can skip pulses in order to keep the output current constant and equal to the current limit. If at the end of the “masking time” the current is higher than the overcurrent threshold, the Power MOSFET is turned off and it will skip one pulse. If, at the next switching on at the end of the “masking time” the DocID15181 Rev 6 L7980 Functional description current is still higher than the threshold, the device will skip two pulses. This mechanism is repeated and the device can skip up to seven pulses. While, if at the end of the “masking time” the current is lower than the overcurrent threshold, the number of skipped cycles is decreased of one unit. At the end of soft-start phase the output voltage is in regulation and if the overcurrent persists the behavior explained above takes place (see Figure 8.b). So the overcurrent protection can be summarized as an “hiccup” intervention when the output is in regulation and a constant current during the soft-start phase. If the output is shorted to ground when the output voltage is on regulation, the overcurrent is triggered and the device starts cycling with a period of 2048 clock cycles between “hiccup” (Power MOSFET off and no current to the load) and “constant current” with very short on-time and with reduced switching frequency (up to one eighth of normal switching frequency). See Figure 32 on page 33 for short-circuit behavior. Figure 8. Overcurrent protection strategy DocID15181 Rev 6 13/42 42 Functional description 5.5 L7980 Enable function The enable feature allows to put in standby mode the device. 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. 5.6 Hysteretic thermal shutdown 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. 14/42 DocID15181 Rev 6 L7980 Application informations 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. So the input capacitor must have an RMS current rating higher than the maximum RMS input current and an ESR value compliant with the expected efficiency. The maximum RMS input current flowing through the capacitor can be calculated as: Equation 3 2 2 2D D I RMS = I O  D – --------------- + ------2  Where IO is the maximum DC output current, D is the duty cycle, is the efficiency. Considering , this function has a maximum at D = 0.5 and it is equal to IO/2. In a specific application the range of possible duty cycles has to be considered in order to find out the maximum RMS input current. The maximum and minimum duty cycles can be calculated as: Equation 4 V OUT + V F D MAX = ------------------------------------V INMIN – V SW and Equation 5 V OUT + V F D MIN = -------------------------------------V INMAX – V SW Where VF is the forward voltage on the freewheeling diode and VSW is voltage drop across the internal PDMOS. The peak to peak voltage across the input capacitor can be calculated as: Equation 6 IO D D V PP = -------------------------   1 – ----  D + ----   1 – D  + ESR  I O C IN  F SW    where ESR is the equivalent series resistance of the capacitor. DocID15181 Rev 6 15/42 42 Application informations L7980 Given the physical dimension, ceramic capacitors can meet well 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 7 IO D D C IN = ---------------------------   1 – ----  D + ----   1 – D  V PP  F SW    neglecting the small ESR of ceramic capacitors. Considering = 1, this function has its maximum in D = 0.5, thus, given the maximum peak to peak input voltage (VPP_MAX), the minimum input capacitor (CIN_MIN) value is: Equation 8 IO C IN_MIN = -----------------------------------------------2  V PP_MAX  F SW Typically CIN is dimensioned to keep the maximum peak-peak voltage in the order of 1% of VINMAX In Table 6 some multi layer ceramic capacitors suitable for this device are reported: Table 6. Input MLCC capacitors Manufacture Taiyo Yuden Murata Series Cap value (F) Rated voltage (V) UMK325BJ106MM-T 10 50 GMK325BJ106MN-T 10 35 GRM32ER71H475K 4.7 50 A ceramic bypass capacitor, as close to the VCC and GND pins as possible, so that additional parasitic ESR and ESL are minimized, is suggested in order to prevent instability on the output voltage due to noise. The value of the bypass capacitor can go from 100 nF to 1 µF. 6.2 Inductor selection The inductance value fixes the current ripple flowing through the output capacitor. So the minimum inductance value in order to have the expected current ripple has to be selected. The rule to fix the current ripple value is to have a ripple at 20% - 40% of the output current. In the continuous current mode (CCM), the inductance value can be calculated by Equation 9: Equation 9 V IN – V OUT V OUT + V F I L = ------------------------------  T ON = ----------------------------  T OFF L L 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 that is at minimum duty cycle (see Section 6.1 to calculate minimum duty). 16/42 DocID15181 Rev 6 L7980 Application informations So fixing IL = 20% to 30% of the maximum output current, the minimum inductance value can be calculated: Equation 10 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 H. The peak current through the inductor is given by: Equation 11 I L I L PK = I O + -------2 So if the inductor value decreases, the peak current (that has to be lower than the current limit of the device) increases. The higher is the inductor value, the higher is the average output current that can be delivered, without reaching the current limit. In Table 7 some inductor part numbers are listed. Table 7. Inductors Manufacturer Coilcraft Wurth SUMIDA 6.3 Series Inductor value (H) Saturation current (A) MSS1038 3.8 to 10 3.9 to 6.5 MSS1048 12 to 22 3.84 to 5.34 MSS1060 22 to 47 5 to 6.8 PD Type L 8.2 to 15 3.75 to 6.25 PD Type M 2.2 to 4.7 4 to 6 PD4 Type X 22 to 47 2.6 to 3.5 CDRH6D226/HP 1.5 to 3.3 3.6 to 5.2 CDR10D48MN 6.6 to 12 4.1 to 5.7 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. The amount of the voltage ripple can be calculated starting from the current ripple obtained by the inductor selection. DocID15181 Rev 6 17/42 42 Application informations L7980 Equation 12 I MAX V OUT = ESR  I MAX + ------------------------------------8  C OUT  f 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. In Section 6.4, it will be illustrated 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 capacitor are adopted, 10 µF are needed and the ESR effect on the output voltage ripple can be neglected. In case of not negligible ESR (electrolytic or tantalum capacitors), the capacitor is chosen taking into account its ESR value. So in case of 220 with ESR = 50 m, the resistive component of the drop dominates and the voltage ripple is 33 mV The output capacitor is also important to sustain the output voltage when a load transient with high slew rate is required by the load. When the load transient slew rate exceeds the system bandwidth the output capacitor provides the current to the load. So if the high slew rate load transient is required by the application the output capacitor and system bandwidth have to be chosen in order to sustain the load transient. In Table 8 some capacitor series are listed. Table 8. Output capacitors Manufacturer Series Cap value (F) Rated voltage (V) ESR (m) GRM32 22 to 100 6.3 to 25