L7987L 61 V 2 A asynchronous step-down switching regulator with adjustable current limitation Datasheet - preliminary data Description HTSSOP16 (RTH = 40 °C/W) Features 2 A DC output current 4.5 V to 61 V operating input voltage RDS,ON = 300 m typ. Adjustable fSW (250 kHz - 1.5 MHz ) Low IQ-SHD (11 µA typ. from VIN) Low IQ (1 mA typ. - VIN 24 V - VOUT 3.3 V) Output voltage adjustable from 0.8 V to VIN Synchronization Adjustable soft-start time Adjustable current limitation Low dropout operation (12 µs max.) The L7987L device is a step-down monolithic switching regulator able to deliver up to 2 A DC. The output voltage adjustability ranges from 0.8 V to VIN. The wide input voltage range and the 100% duty cycle capability meet the fail safe specifications for industrial systems. The embedded switchover feature on the VBIAS pin maximizes the efficiency at light load. The adjustable current limitation, designed to select the inductor RMS current accordingly with the nominal output current, and the high switching frequency capability make the size of the application compact. Pulse-by-pulse current sensing with digital frequency foldback implements an effective constant current protection over the different application conditions. The peak current foldback decreases the stress of the power components in heavy short-circuit condition. The PGOOD open collector output can also implement output voltage sequencing during the power-up phase. Multiple devices can be synchronized sharing the SYNCH pin to prevent beating noise in low noise applications like sensors with A/D conversion. VBIAS improves efficiency at light load PGOOD open collector output Output voltage sequencing Digital frequency foldback in short-circuit Peak current foldback in short-circuit Auto-recovery thermal shutdown Applications Designed for 24 V bus Fail safe tolerant systems Programmable logic controllers (PLCs) June 2014 DocID026362 Rev 2 This is preliminary information on a new product now in development or undergoing evaluation. Details are subject to change without notice. 1/36 www.st.com Contents L7987L Contents 1 Application schematic and block diagram . . . . . . . . . . . . . . . . . . . . . . . 4 2 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.4 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.5 ESD protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4 Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5 4.1 Oscillator and synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 4.2 Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.3 Error amplifier and light-load management . . . . . . . . . . . . . . . . . . . . . . . 16 4.4 Low VIN operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.5 Overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.6 Overtemperature protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.1 Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.2 Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5.3 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 5.4 Compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.4.1 Type II compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.4.2 Type III compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 5.5 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5.6 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6 Demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 7 Package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2/36 DocID026362 Rev 2 L7987L Contents 8 Ordering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 DocID026362 Rev 2 3/36 36 Application schematic and block diagram 1 L7987L Application schematic and block diagram Figure 1. Application schematic Figure 2. Block diagram 9&& 9%,$6 9,1 %227 (1 (1 (1$%/( 95(* %227 &+$5*( 89/2 ,616 5() DQG %,$6 95$03 )% &855(17 6(16,1* ($ )% +6 '5,9(5 3:0 &203 (1 62)7 67$57 /2*,& DQG &21752/ 7(03 021,725 287 /6 '5,9(5 ,66 3* 66 &203 )% 3*22' 95$03 26&,//$725 $1'5$03 &203 )6: 0$67(5 6/$9( 6<1&+ ,/,0 )81&7,21 *1' ,/,0 $0 4/36 DocID026362 Rev 2 L7987L Pin settings 2 Pin settings 2.1 Pin connection Figure 3. Pin connection (top view) DocID026362 Rev 2 5/36 36 Pin settings 2.2 L7987L Pin description Table 1. Pin description Number Pin Description 1 VBIAS Auxiliary input that can be used to supply part of the analog circuitry to increase the efficiency at light load. Typically connected to the regulated output voltage or to an external voltage rail higher than 3 V. Connect to the signal GND if not used or bypass with a 1 F ceramic capacitor if supplied by the output voltage or by an auxiliary rail. 2 VIN DC input voltage 3 VIN DC input voltage 4 VCC Filtered DC input voltage to the internal circuitry. Bypass to the signal GND by a 1 F ceramic capacitor. 5 EN Active high enable pin. Connect to the VCC pin if not used. 6 SS An internal current generator (5 µA typ.) charges the external capacitor to implement the softstart. 7 SYNCH Master / slave synchronization 8 COMP 9 FB 10 FSW A pull-down resistor to GND selects the switching frequency. 11 ILIM A pull-down resistor to GND selects the peak current limitation. 12 PGOOD 13 LX Switching node 14 LX Switching node 15 BOOT Connect an external capacitor (100 nF typ.) between BOOT and LX pins. The gate charge required to drive the internal n-DMOS is recovered by an internal regulator during the off-time 16 GND Signal GND - E.P. Exposed pad must be connected to signal GND. 6/36 Output of the error amplifier. The designed compensation network is connected at this pin. Inverting input of the error amplifier. The PGOOD open collector output is driven low when the output voltage, sensed on the FB pin, is out of regulation. DocID026362 Rev 2 L7987L 2.3 Pin settings Maximum ratings Table 2. Absolute maximum ratings Symbol Description Min. Max. Unit VIN -0.3 61 V VCC -0.3 61 V VBOOT - GND -0.3 65 V VBOOT - VLX -0.3 4 V VBIAS -0.3 VCC V EN -0.3 VCC V PGOOD -0.3 VCC V LX -0.3 VIN + 0.3 V SYNCH -0.3 5.5 V SS -0.3 3.6 V FSW -0.3 3.6 V COMP -0.3 3.6 V ILIM -0.3 3.6 V FB -0.3 3.6 V Operating temperature range -40 150 °C TSTG Storage temperature range -65 150 °C TLEAD Lead temperature (soldering 10 sec.) 260 °C 2 A BOOT TJ High-side RMS current IHS 2.4 Thermal data Table 3. Thermal data 2.5 Symbol Parameter Value Unit RthJA Thermal resistance junction-ambient (device soldered on the STMicroelectronics® demonstration board) 40 °C/W Value Unit HBM 2 KV CDM 500 V ESD protection Table 4. ESD protection Symbol ESD Test condition DocID026362 Rev 2 7/36 36 Electrical characteristics 3 L7987L Electrical characteristics All the population tested at TJ = 25 °C, VIN = VCC = 24 V and VEN = 3 V unless otherwise specified. The specification is guaranteed for TJ = -40 °C to +125 °C by characterization and statistical correlation. Table 5. Electrical characteristics Symbol Parameter VIN Operating input voltage range RDSON HS High-side RDSON fSW Switching frequency Selected switching frequency IPK ISKIP VFOLD Peak current limit Min. 61 V 0.48 ISW = 0.5 A 0.30 0.57 FSW floating; TJ = 25 °C 233 250 267 kHz FSW floating 225 250 275 kHz RFSW = 10 k 1350 1500 1650 kHz RILIM = 27 k; VFB = 0.6 V TBD 2.7 A 0.90 A RILIM = 27 k; VFB = GND Pulse skipping peak current 0.63 388 (1) Minimum off time 0.85 1.06 0.5 Feedback foldback level Minimum on time Unit 0.30 Static measure. Internal circuitry delay could impact the actual ISKIP value, as shown in Section 4: Functional description. TONMIN Max. ISW = 0.5 A; TJ = 25 °C RILIM = 100 k VFB = 0.6 V Maximum on time Typ. 4.5 Selected peak current limit TONMAX TOFFMIN Test condition 400 A A 412 mV 12 µs 120 ns 360 ns VCC / VBIAS VCCH VCCHYST VCC UVLO rising threshold 3.88 4.0 4.20 V VCC UVLO hysteresis 170 200 300 mV 2.85 2.9 2.95 V 50 80 110 mV Switch internal supply from VIN to VBIAS. VIN = VCC rising, VBIAS = 3.3 V 3.23 4.05 5.22 V Hysteresis 450 750 1050 mV VBIAS threshold Hysteresis SWO VCC - VBIAS threshold 8/36 Switch internal supply from VIN to VBIAS. VIN = 24 V, VBIAS ramping up DocID026362 Rev 2 L7987L Electrical characteristics Table 5. Electrical characteristics (continued) Symbol Parameter Test condition Min. Typ. Max. Unit Power consumption ISHTDWN Shutdown current from VIN VEN = GND 11 18 A IQUIESC Quiescent current from VIN and VCC LX floating, VFB = 1 V, VBIAS = GND, FSW floating 2.5 3.5 mA IQOPVIN Quiescent current from VIN and VCC LX floating, VFB = 1 V, VBIAS = 3.3 V, FSW floating 1.0 1.5 mA LX floating, VFB = 1 V, VBIAS = 3.3 V, FSW floating 1.6 2.5 mA IQOPVBIAS Quiescent current from VBIAS Enable VEN Device OFF level 0.10 0.32 V Device ON level 0.37 1.16 V Soft start TSSSETUP Soft start setup time ISS CH CSS charging current Delay from UVLO rising to switching activity 544 640 900 s From VSS = 0 to VREF 4.3 5.0 5.3 A Error amplifier VFB Voltage feedback TJ = 25 °C 0.792 0.800 0.808 V VFB Voltage feedback TJ = - 40 °C to +125 °C 0.788 0.800 0.812 V VCOMPH VFB = GND; SS floating VCOMPL VFB = 1 V; SS floating IFB FB biasing current AV0 2.0 Unity gain buffer configuration (FB connected to COMP).COMP voltage Output stage sinking capability variation due to IOSINK injection lower than ± 0.1 x VFB Error amplifier gain Unity gain buffer configuration (FB connected to COMP). No load on COMP pin. GBWP VSWT VFB = 800 mV VFB = GND; SS pin floating; VCOMP = 2 V IOSOURCE IOSINK 3.2 Sawtooth voltage FSW = 250 kHz 3.35 3.5 V 0.1 V 5 50 nA 3.1 5.0 mA 5 mA (1) 100 dB (1) 23 MHz Peak value VIN = 61 V 1.70 2.03 2.50 V Peak value VIN = 24 V 0.70 1.05 1.50 V Bottom value DocID026362 Rev 2 0.2 V 9/36 36 Electrical characteristics L7987L Table 5. Electrical characteristics (continued) Symbol Parameter Test condition Min. Typ. Max. Unit Synchronization (fan out: 5 slave devices max.) fSYN MIN Synchronization frequency VSYNOUT Master output amplitude VSYNOW Output pulse width VSYNIH SYNCH slave input threshold VSYNIL SYNCH slave input threshold ISYN VSYNIW Slave SYNCH pull-down current FSW floating 280 kHz ILOAD = 4 mA 2.6 V ILOAD = 0 A; pin SYNCH floating ILOAD = 0 A; pin SYNCH floating 180 225 3.45 V 270 ns 2.0 VSYNCH = 5 V 450 Input pulse width V 650 1.0 V 850 A 200 ns PGOOD VPGDTH VFB rising 0.675 0.696 0.717 V VPGDHYST PGOOD hysteresis VFB falling 30 mV VPGDLOW PGOOD low level IPGD = 1 mA, VFB = GND 30 mV IPGDLKG PGOOD rising threshold PGOOD leakage current VPGOOD = 61 V; VFB = 0.8 V 0.1 A Thermal shutdown TSHDWN THYS Thermal shutdown temperature (1) 170 °C Thermal shutdown hysteresis (1) 15 °C 1. Parameter guaranteed by design. Not production tested. 10/36 DocID026362 Rev 2 L7987L 4 Functional description Functional description The L7987L device is based on a voltage mode, constant frequency control loop. The output voltage VOUT, sensed by the feedback pin (FB), is compared to an internal reference (0.8 V) providing an error signal on the COMP pin. The COMP voltage level is then compared to a fixed frequency sawtooth ramp, which finally controls the on- and off-time of the power switch. The main internal blocks are shown in the block diagram in Figure 2 on page 4 and can be summarized as follow. 4.1 The fully integrated oscillator that provides the sawtooth ramp to modulate the duty cycle and the synchronization signal. Its switching frequency can be adjusted by an external resistor. The input voltage feed-forward is 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 N-channel Power MOSFET switch and bootstrap circuitry. A dedicated high resistance low-side MOSFET, for anti-boot discharge management purposes, is also present. The peak current limit sensing block, with programmable threshold, to handle overload and short-circuit conditions including current foldback and a thermal shutdown block, to prevent thermal runaway. The voltage regulator and internal reference, to supply the internal circuitry and provide a fixed internal reference. The switchover function from VCC to VBIAS can be implemented for higher efficiency. This block also implements a voltage monitor circuitry (UVLO) that checks the input and internal voltages. The output voltage monitor circuitry which releases the PGOOD signal if the sensed output voltage is above 87% of the target value. Oscillator and synchronization Figure 4 shows the block diagram of the oscillator circuit. The internal oscillator provides a constant frequency clock, whose frequency depends on the resistor externally connected between the FSW pin and ground. Figure 4. Oscillator and synchronization DocID026362 Rev 2 11/36 36 Functional description L7987L If the FSW pin is left floating, the programmed frequency is 250 kHz (typ.); if FSW pin is connected to an external resistor the programmed switching frequency can be increased up to 1.5 MHz, as shown in Figure 5. The required RFSW value (expressed in k) is estimated by Equation 1: Equation 1 12500 F SW = 250kHz + ---------------R FSW Figure 5. Switching frequency programmability To improve the line transient performance, keeping the PWM gain constant versus the input voltage, the input voltage feed-forward is implemented by changing the slope of the sawtooth ramp, according to the input voltage change (Figure 6 a). The slope of the sawtooth also changes if the oscillator frequency is programmed by the external resistor. In this way a frequency feed-forward is implemented (Figure 6 b) in order to keep the PWM modulator gain constant versus the switching frequency. 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 pins together. When SYNCH pins are connected, the device with a higher oscillator frequency works as master, so the slave device switches at the frequency of the master but with a delay of half a period. This helps reducing the RMS current flowing through the input capacitor. Up to five L7987Ls can be connected to the same SYNCH pin; however, the clock phase shift from master switching frequency to slaves input clock is 180°. The L7987L device can be synchronized to work at a higher frequency, in the range 250 kHz - 1500 kHz, providing an external clock signal on the SYNCH pin. The synchronization changes the sawtooth amplitude, also affecting the PWM gain (Figure 6 c). This change must be taken into account when the loop stability is studied. In order to minimize the change of PWM gain, the free running frequency should be set (with a resistor on the FSW pin) only slightly lower than the external clock frequency. 12/36 DocID026362 Rev 2 L7987L Functional description This pre-adjusting of the slave IC switching frequency keeps the truncation of the ramp sawtooth negligible. In case two or more (up to five) L7987L SYNCH pins are tied together, the L7987L IC with higher programmed switching frequency is typically the master device; however, the SYNCH circuit is also able to synchronize with a slightly lower external frequency, so the frequency pre-adjustment with the same resistor on the FSW pin, as suggested above, is required for a proper operation. DocID026362 Rev 2 13/36 36 Functional description L7987L Figure 6. Feed-forward 14/36 DocID026362 Rev 2 L7987L 4.2 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 increase monotonically. The soft-start is performed by charging an external capacitor, connected between the SS pin and ground, with a constant current (5 µA typ.). The SS voltage is used as reference of the switching regulator and the output voltage of the converter tracks the ramp of the SS voltage. When the SS pin voltage reaches 0.8 V level, the error amplifier switches to the internal 0.8 V ± 1% reference to regulate the output voltage. Figure 7. Soft-start During the soft-start period the current limit is set to the nominal value. The dVSS/dt slope is programmed in agreement with Equation 2: Equation 2 5A T SS I SS T SS C SS = ------------------------ = --------------------------V REF 0.8V Before starting the CSS capacitor charge, the soft-start circuitry turns-on the discharge switch shown in Figure 7 for TSSDISCH minimum time, in order to completely discharge the CSS capacitor. As a consequence, the maximum value for the soft-start capacitor, which assures an almost complete discharge in case of EN signal toggle, is provided by: Equation 3 T SSDISCH C SS –MAX ------------------------------------ 270nF 5 R SSDISCH given TSSDISCH = 530 s and RSSDISCH = 380 typical values. DocID026362 Rev 2 15/36 36 Functional description L7987L The enable feature allows to put the device into standby mode. With the EN pin lower than 0.32 V the device is disabled and the power consumption is reduced to less than 11 A (typ.). With the EN pin higher than 1.16 V, the device is enabled. If the EN pin is left floating, an internal pull-down current ensures that the voltage at the pin reaches the inhibit threshold and the device is disabled. The pin is also VCC compatible. 4.3 Error amplifier and light-load management 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 0.8V voltage reference and 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, therefore, with high DC gain and low output impedance. The uncompensated error amplifier characteristics are summarized in Table 6. Table 6. Error amplifier characteristics Parameters Value Low frequency gain (A0) 100 dB GBWP 23 MHz Output voltage swing 0 to 3.5 V Source/sink current capability 2 mA / 5 mA In continuous conduction working 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. If 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 must be used (see Section 5.4 on page 23 for details on the compensation network design). In case of light load (i.e. if the output current is lower than the half of the inductor current ripple) the L7987L device enters pulse-skipping working mode. The HS MOS is kept off if the COMP level is below 200 mV (typ.); when this bottom level is reached the integrated switch is turned on until the inductor current reaches ISKIP value. So, in discontinuous conduction working mode (DCM), the HS MOS on-time is only related to the time necessary to charge the inductor up to ISKIP level. Due to current sensing comparator delay, the actual inductor charge current is slightly impacted by VIN and inductance level. In order to let the bootstrap capacitor recharge, in case of extremely light load the L7987L is able to pull-down the LX net through an integrated small LS MOS. In this way the bootstrap recharge current can flow from VIN through CBOOT, LX and the LS MOS. This mechanism is activated if the HS MOS has been kept turned-off for more than 3 ms (typ.). 16/36 DocID026362 Rev 2 L7987L 4.4 Functional description Low VIN operation In normal operation (i.e. VOUT programmed lower than input voltage) when the HS MOS is turned off, a minimum off time (TOFFMIN) interval is performed. In case the input voltage falls close or below the programmed output voltage (low dropout, LDO) the L7987L control loop is able to increase the duty cycle up to 100%. However, in order to keep the boot capacitor properly recharged, a maximum HS MOS on time is limited (TONMAX). When this limit is reached the HS MOS is turned-off and a pull-down resistor between LX and GND is turned on until one of the following conditions is met: A negative current limit (300 mA typ.) is reached A timeout (1 s typ.) is reached. So doing the L7987L device is able to work in low dropout operation, due to the advanced boot capacitor management, and the effective maximum duty cycle is about 12 µs / 13 µs = 92%. 4.5 Overcurrent protection The L7987L device implements an overcurrent protection by sensing the current flowing through the Power MOSFET. Due to the noise created by the switching activity of the Power MOSFET, the current sensing circuitry 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 120 ns. If the overcurrent limit is reached, the Power MOSFET is turned off implementing pulse-bypulse overcurrent protection. In the overcurrent condition, the device can skip turn-on pulses in order to keep the inductor current constant and equal to the current limit, assuming only a slight drift due to input and output voltage variation. If, at the end of the “masking time”, the current is higher than the overcurrent threshold, the Power MOSFET is turned off and one pulse is skipped. If, at the following switching on, when the “masking time” ends, the current is still higher than the overcurrent threshold, the device skips two pulses. This mechanism is repeated and the device can skip up to seven pulses (refer to Figure 8). If at the end of the “masking time” the current is lower than the overcurrent threshold, the number of skipped cycles is decreased by one unit. As a consequence, the overcurrent/short-circuit protection acts by switching off the Power MOSFET and reducing the switching frequency down to one eighth of the default switching frequency, in order to keep constant the output current close to the current limit. DocID026362 Rev 2 17/36 36 Functional description L7987L Figure 8. OCP and frequency scaling If the sensed output voltage, monitored through FB pin, falls below the VFOLD threshold (400 mV typ.) the peak current limit threshold is reduced to 1/3 of the nominal value. This additional feature helps to reduce the IC stress in case of output short-circuit. As soon as the FB pin increases above the VFOLD threshold, the full peak current limit threshold is restored. This fold back protection is disabled during the soft-start. This kind of overcurrent protection is effective if the inductor can be completely discharged during HS MOS turn-off time, in order to avoid the inductor current to run away. In case of output short-circuit the maximum switching frequency can be computed by Equation 4. Equation 4 8 V F + R DCR I LIM 1 F SW MAX ---------------------------------------------------------------------- ---------------------V IN – R ON + R DCR I LIM T ON MIN Assuming VF = 0.6 V the freewheeling diode direct voltage, RDCR = 70 m the inductor parasitic resistance, ILIM = IPK = 0.9 A the peak current limit during foldback protection, RON = 0.30 the HS MOS resistance and TON,MIN = 120 ns the minimum HS MOS on duration, the maximum FSW frequency which avoids the inductor current run away in case of output short-circuit and VIN = 61 V is 728 kHz. If the programmed switching frequency is higher than the above computed limit, an estimation of the inductor current in case of output short-circuit fault is provided by Equation 5: Equation 5 F SW T ON V IN – 8 V F I LIM = ----------------------------------------------------------------------------------------------------------------------8 R DCR + F SW T ON MIN R ON + R DCR 18/36 DocID026362 Rev 2 L7987L Functional description The peak current limit threshold (ILIM) can be programmed in the range 0.85 A - 3.6 A by selecting the proper RILIM resistor, as suggested in Equation 6: Equation 6 I PK R ILIM = 20k ---------I LIM IPK = 3.6 A (typ.) is the default L7987L current limit in case of RILIM not mounted. In any case, the maximum high-side MOS RMS current must not be allowed to exceed the value shown in Table 2 on page 7. Figure 9. Current limit and programming resistor The minimum programmed current limit can't be lower than ISKIP = 0.5 A (typical), also in case of foldback detection. 4.6 Overtemperature protection It is recommended that the device never exceeds the maximum allowable junction temperature. This temperature increase is mainly caused by the total power dissipated from the integrated Power MOSFET. To avoid any damage to the device when reaching high temperature, the L7987L device implements a thermal shutdown feature: when the junction temperature reaches 170 °C (typ.) the device turns off the Power MOSFET and shuts down. When the junction temperature drops to 155 °C (typ.), the device restarts with a new softstart sequence. DocID026362 Rev 2 19/36 36 Application information L7987L 5 Application information 5.1 Input capacitor selection The input capacitor must be rated for the maximum input operating voltage and the maximum RMS input current. Since the step-down converters input current is a sequence of pulses from 0 A to IOUT, the input capacitor must absorb the equivalent RMS current which can be up to the load current divided by two (worst case, with duty cycle of 50%). For this reason, the quality of these capacitors must be very high to minimize the power dissipation generated by the internal ESR, thereby improving system reliability and efficiency. The RMS input current (flowing through the input capacitor) is roughly estimated by: Equation 7 I CIN RMS I OUT D 1 – D Actual DC/DC conversion duty cycle, D = VOUT/VIN, is influenced by a few parameters: Equation 8 V OUT + V F D MAX = ---------------------------------------------------V IN MIN – V SW MAX V OUT + V F D MIN = ---------------------------------------------------V IN MAX – V SW MIN where VF is the freewheeling diode forward voltage and VSW the voltage drop across the internal high-side MOSFET. Considering the range DMIN to DMAX it is possible to determine the maximum ICIN,RMS flowing through the input capacitor. The input capacitor value must be dimensioned to safely handle the input RMS current and to limit the VIN and VCC ramp-up slew-rate to 0.5 V/s maximum, in order to avoid the device active ESD protections turn-on. Different capacitors can be considered: 20/36 Electrolytic capacitors These are the most commonly used due to their low cost and wide range of operative voltage. The only drawback is that, considering ripple current rating requirements, they are physically larger than other capacitors. Ceramic capacitors If available for the required value and voltage rating, these capacitors usually have a higher RMS current rating for a given physical dimension (due to the very low ESR). The drawback is their high cost. Tantalum capacitors Small, good quality tantalum capacitors with very low ESR are becoming more available. However, they can occasionally burn if subjected to very high current, for example when they are connected to the power supply. DocID026362 Rev 2 L7987L Application information The amount of the input voltage ripple can be roughly overestimated by Equation 9. Equation 9 D 1 – D I OUT V IN PP = ---------------------------------------------- + R ES IN I OUT C IN F SW In case of MLCC ceramic input capacitors, the equivalent series resistance (RES,IN) is negligible. In addition to the above considerations, a ceramic capacitor with an appropriate voltage rating and with a value 1 F or higher should always be placed across VIN and power ground and across VCC and the IC GND pins, as close as possible to the L7987L device. This solution is necessary for spike filtering purposes. 5.2 Output capacitor selection The output capacitor is very important in order to satisfy the output voltage ripple requirement. Using a small inductor value is useful to reduce the size of the choke but increases the current ripple. So, to reduce the output voltage ripple, a low ESR capacitor is required. Nevertheless, the ESR of the output capacitor introduces a zero in the open loop gain, which helps to increase the phase margin of the system. If the zero goes to very high frequency, typical drawback in case of ceramic output capacitor application, a type III compensation network must be designed. The current in the output capacitor has a triangular waveform which generates a voltage ripple across it. This ripple is due to the capacitive component (charge and discharge of the output capacitor) and the resistive component (due to the voltage drop across its ESR). So the output capacitor must be selected in order to have a voltage ripple compliant with the application requirements. The amount of the voltage ripple can be estimated starting from the current ripple obtained by the inductor selection. Assuming IL the inductor current ripple, the output voltage ripple is roughly overestimated by Equation 10. Equation 10 I L V OUT PP I L R ES OUT + ----------------------------------------8 F SW C OUT 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. The output capacitor is also the key component that provides the current to the load during a load transient which exceeds the system bandwidth. So, if the high slew rate load transient is required by the application, the output capacitor must be designed in order to sustain the load transient or absorbs the energy stored in the inductor until the converter reacts. In fact, even if the controller detects immediately the load variation and sets the duty cycle at 100% or 0%, the output current slope is limited by the inductor value, the input and output voltage. DocID026362 Rev 2 21/36 36 Application information L7987L The output voltage has a drop or overshoot that depends on the ESR and capacitive charge/discharge, as roughly estimated in Equation 11: Equation 11 L I OUT V OUT –LT I OUT R ES OUT + I OUT ---------------------------------------2 C OUT V L where VL is the voltage applied to the inductor during the load appliance or load release. Equation 12 D MAX V IN – V OUT V L = V OUT MLCC capacitors have typically low ESR to minimize the ripple but also have low capacitance that does not minimize the voltage deviation during dynamic load variations. Electrolytic capacitors, on the other hand, have a large capacitance which minimizes voltage deviation during load transients whereas they do not show the same ESR values as the MLCCs, resulting then in higher ripple voltages. A mix between an electrolytic and MLCC capacitor can be used to minimize ripple as well as reducing voltage deviation in dynamic mode. The high bandwidth error amplifier of the L7987L and external compensation feature let design a wide range of output filter configurations (including all MLCC solutions) and perform fast transient response. 5.3 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, must 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 conduction mode (CCM), the required inductance value can be calculated by Equation 13: Equation 13 V OUT V OUT 1 – -------------- V IN L = -------------------------------------------------L L F SW In order to guarantee a maximum current ripple in every condition, Equation 13 must be evaluated in case of maximum input voltage, assuming VOUT fixed. Increasing the value of the inductance help to reduce the current ripple but, at the same time, strongly impacts the converter response time to a dynamic load change. The response time is the time required by the inductor to change its current from the initial to the final value. Until the inductor has finished its charging (or discharging) time, the output current is supplied (or recovered) by the output capacitors. Further, if the compensation network is properly designed, during a load variation the device is able to properly change the duty cycle so improving the control loop transient response. When this condition is reached the response time is only limited by the time required to change the inductor current, basically by VIN, VOUT and L. Minimizing the response time, at the end, can help to decrease the output filter total cost and to reduce the application area. 22/36 DocID026362 Rev 2 L7987L 5.4 Application information Compensation network The compensation network must assure stability and good dynamic performance. The loop of the L7987L device is based on the voltage mode control. The error amplifier is an operational amplifier with high bandwidth. So, by selecting the compensation network the E/A is considered as ideal, that is, its bandwidth is much larger than the system one. Figure 10. Switching regulator control loop simplified model / 9,1 ,QWHUQDO VZLWFK 5 '& 3RZHU VHFWLRQ 5 (6 52 &2 3:0 =6 96 58 95() ($ &203 =) 5' &RPSHQVDWLRQ QHWZRUN $0 The transfer function of the PWM modulator, from the error amplifier output (COMP pin) to the LX pin results in an almost constant gain, due to the voltage feed-forward which generates a sawtooth with amplitude VS directly proportional to the input voltage: Equation 14 V IN 1 G PWO = --------- = --------- = 30 VS k FF The synchronization of the device with an external clock provided through the SYNCH pin can modify the PWM modulator gain (see Section 4.1 on page 11 to understand how this gain changes and how to keep it constant in spite of the external synchronization). DocID026362 Rev 2 23/36 36 Application information L7987L The transfer function of the power section (i.e. the L-CO filters and the output load) is the ratio of the parallel of CO and RO and the sum of L and the parallel of CO and RO, including L and CO parasitics: Equation 15 S 1 CO s RE RO RO C RD S RO RE CO 1 C 1 C D RD L R s S SRE RE CO CO s CO s C S RD RE RO RO O C RO L s S RE 2 RO CO L s s C GL ︵ ︶ given L, RDC, CO, RES and RO the parameters shown in Figure 10. The power section transfer function can be rewritten as follows: Equation 16 s 1 + --------------------------2 f zESR G LC s = G LCO ----------------------------------------------------------------------------2- ; s s 1 + ------------------------------- + -------------------- 2 Q f LC 2 f LC RO G LCO = -------------------------- 1 R O + R DC Equation 17 1 f zESR = -------------------------------- ; 2 C O R ES 1 1 f LC = ------------------------------------------------------ ----------------------------------------------------R O + R ES R O + R ES 2 LC O -------------------------- 2 LC O -----------------------R O + R DC RO Equation 18 LC O R O R O + R ES LC O R O + R DC R O + R ES Q = ----------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------L + C O R O R DC + R O R ES + R ES R DC L + C O R O R ES with the assumption that the inductor parasitic resistance, RDC, is negligible compared to RO. The closed loop gain is then given by: Equation 19 GLOOP(s) = GLC(s) • GPWO(s) • GCOMP(s) As noted in Section 5.2 on page 21, two different kinds of network can compensate the loop, depending on the value of fzESR, lower or higher than the regulator required bandwidth. In Section 5.4.1 and Section 5.4.2 the guidelines to select the type II and type III compensation network are illustrated. 24/36 DocID026362 Rev 2 L7987L 5.4.1 Application information Type II compensation network If the equivalent series resistance (RES) of the output capacitor introduces a zero with a frequency lower than the desired bandwidth (that is: 2• RES • CO > 1 / BW), this zero helps stabilize the loop. Electrolytic capacitors show non-negligible ESR (> 30 m typically), so with this kind of output capacitor the type II network combined with the zero of the ESR allows to stabilize the loop. Figure 11. Type II compensation network 9 287 95() 58 ($ &203 )% 5) 5' &) &3 =) $0 The type II compensation network transfer function, from VOUT to COMP, is computed in Equation 20. Equation 20 Z ( s) 1 1 sC F RF GCOMPII ( s) F RU RU s (C F C P ) 1 sC F C P RF s 2 f Z 1 s 1 2 f P1 1 s 2 f P 0 Equation 21 f Z1 1 ; 2 C F RF f P0 1 ; 2 C F C P RU f P1 1 2 C F C P RF The following suggestions can be followed for a quite common compensation strategy, assuming that CP << CF. Starting from Equation 19, in case of type II compensation network and electrolytic output capacitors the control loop gain module at s = 2 • FBW allows to fix the RF/RU ratio: Equation 22 1 f LC RF 1 GLOOP , II ( s 2 f BW ) 1 k FF f zESR RU f BW 2 DocID026362 Rev 2 25/36 36 Application information L7987L After choosing the regulator bandwidth (typically FBW < 0.2 • FSW ) and a value for RU, usually between 1 k and 50 k, in order to achieve CF and CP not comparable with parasitic capacitance of the board, the RF required value is computed by Equation 22. Select CF in order to place FZ1 below FLC (typically 0.1 • FLC) Select CP in order to place FP1 at 0.5 • FSW Equation 23 1 1 C F = ------------------------------------------------ ;C P = ------------------------------------------------2 R F 0.1 f LC 2 R F 0.5 f SW The resultant control loop and other transfer functions gain are shown in Figure 12. Figure 12. Type II compensation - bode plot ($JDLQ */223,, *&203,, */&,, I= I/& I](65 I%: I3 $0 26/36 DocID026362 Rev 2 L7987L 5.4.2 Application information Type III compensation network If FzESR is higher than the target loop bandwidth, as usually happens if the output filter is based on MLCC ceramic capacitors, a type III compensation network must be designed. Figure 13. Type III compensation network =6 &6 95() 9 287 58 56 ($ &203 )% 5) 5' &) &3 =) $0 The type III compensation network transfer function, from VOUT to COMP, is computed in Equation 24. Equation 24 s s 1 1 f f 2 2 Z F ( s) Z1 Z2 1 GCOMPIII ( s) RU // Z S ( s ) s s s 1 1 2 f P 0 2 f P1 2 f P 2 In addition to what shown in Equation 21, two more singularities are proper of this compensation network: Equation 25 1 f Z2 = ------------------------------------------------------- ; 2 C s R U + R S 1 f P2 = ---------------------------2 C S R S DocID026362 Rev 2 27/36 36 Application information L7987L The following suggestions can be followed for a quite common compensation strategy, assuming that CP << CF and RS << RU. Starting from Equation 19 on page 24, in case of type III compensation network and MLCC ceramic output capacitors the control loop gain module at s = 2 • FBW allows to fix the RF/RU ratio: Equation 26 GLOOP, III ( s 2 f BW ) 1 f LC RF 1 k FF f BW RU After choosing the regulator bandwidth (typically FBW < 0.2 • FSW ) and a value for RU, usually between 1 k and 50 k, in order to achieve CF and CP not comparable with parasitic capacitance of the board, the RF required value is computed by Equation 26. Select CF in order to place FZ1 below FLC (typically 0.1 • FLC) Select CP in order to place FP1 at 0.5 • FSW Equation 27 1 C F = ------------------------------------------------ ; 2 R F 0.1 f LC 1 C P = ------------------------------------------------2 R F 0.5 f SW Select CS in order to place FZ2 at FLC Select RS in order to place FP2 at 0.5 • FSW Equation 28 1 C S = ---------------------------------- ; 2 R U f LC 1 R S = -------------------------------------------------2 C S 0.5 f SW 28/36 DocID026362 Rev 2 L7987L Application information The resultant control loop and other transfer functions gain are shown in Figure 14. Figure 14. Type III compensation - bode plot ($JDLQ */223,,, *&203,,, */&,,, I= I/& I= I%: I3 I3 I](65 $0 5.5 Thermal considerations The thermal design is important to prevent the thermal shutdown of the device if junction temperature goes above 170 °C (typ.). The three different sources of losses within the device are: Conduction losses due to the non-negligible RDSON of the power switch; these are equal to: Equation 29 PHS,ON = RHS,ON • D • (IOUT)2 where D is the duty cycle of the application and the maximum RDSON in the full temperature range is 570 m. Note that the duty cycle is theoretically given by the ratio between VOUT and VIN, but actually it is quite higher in order to compensate the losses of the regulator. So the conduction losses increase compared with the ideal case; Switching losses due to Power MOSFET turn ON and OFF; these can be calculated as: Equation 30 T RISE + T FALL P HS SW = V IN I OUT ------------------------------------------- f SW V IN I OUT T TR f SW 2 where TRISE and TFALL are the overlap times of the voltage across the power switch (VDS) and the current flowing into it during turn ON and turn OFF phases. DocID026362 Rev 2 29/36 36 Application information L7987L TTR is the equivalent switching time. For this device the typical value for the equivalent switching time is 20 ns. Quiescent current losses, calculated as Equation 31 PQ = VIN • IQOPVIN + VBIAS • IQOPVBIAS where IQOPVIN and IQOPVBIAS are the L7987L quiescent current in case of separate bias supply. If the switchover feature is not used, the IC quiescent current is the only one from VIN, IQUIESC, as summarized in Table 5 on page 8. The junction temperature TJ can be calculated as: Equation 32 TJ = TA + Rth,JA • PTOT where TA is the ambient temperature and PTOT is the sum of the power losses just seen. RthJA is the equivalent thermal resistance junction to ambient of the device; it can be calculated as the parallel of many paths of heat conduction from the junction to the ambient. For this device the path through the exposed pad is the one conducting the largest amount of heat. The RthJA, measured on the demonstration board described in Section 5.6, is about 40 °C/W for the HTSSOP16 package. 5.6 Layout considerations The PCB layout of the switching DC/DC regulators is very important to minimize the noise injected in high impedance nodes and interference generated by the high switching current loops. Two separated ground areas must be considered: the signal ground and the power ground. 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 high value pulsed currents are flowing through it. In order to minimize the EMI, this loop must be as short as possible. The input loop, including also the output capacitor, must be referred to the power ground. All the other components are referred to the signal ground. The feedback pin (FB) connection to the external resistor divider is a high impedance node, so the interference can be minimized by placing the routing of the feedback node as far as possible from the high current paths. To reduce the pick-up noise, the resistor divider must be placed very close to the device. To filter the high frequency noise, a small bypass capacitor (1 F or higher) must be added as close as possible to the input voltage pin of the device for both VIN and VCC pins. Thanks to the exposed pad of the device, the ground plane helps to reduce the junction to ambient thermal resistance; so a wide ground plane enhances the thermal performance of the converter, allowing high power conversion. The exposed pad must be connected to the signal GND pin. The connection to the ground plane must be achieved by taking care of the above mentioned input loop, in order to avoid high current flowing through the signal GND. Refer to Section 6 for the L7987L layout example. 30/36 DocID026362 Rev 2 L7987L 6 Demonstration board Demonstration board In this section the L7987L demonstration board is described. The default settings are: Programmed VOUT = 5 V Max. IOUT = 2 A FSW = 500 kHz VBIAS = VOUT Soft-start 5.3 ms Figure 15. L7987L demonstration board schematic Table 7. L7987L demonstration board component list Reference Part Package Note Manufacturer P/N C1, C2 4.7 F 1210 X7S/100 V/10% TDK C3225X7S2A475K C3, C4 1 F 0805 X7S/100 V/10% TDK C2012X7S2A105K C5 47 F 1210 X5R/16 V/20% TDK C3225X7S0J476M C6, C12 N. M. C7 100 nF 0603 X7R/16 V C8 680 pF 0603 C0G/50 V C9 6.8 nF 0603 X7R/50 V C10 33 nF 0603 X7R/16 V C11 68 pF 0603 C0G/50 V R1, R10 N. M. R2 100 k 0603 R4 0 0603 R5 910 0603 1% tolerance R6 10 k 0603 1% tolerance R7 68 k 0603 1% tolerance DocID026362 Rev 2 31/36 36 Demonstration board L7987L Table 7. L7987L demonstration board component list (continued) Reference Part Package Note Manufacturer P/N R8 47 k 0603 1% tolerance R9 27 k 0603 1% tolerance R11 13 k 0603 1% tolerance L1 15 H 5x5 3.7 A sat./ 70 m Coilcraft XAL5050-153 D1 STPS2L60 SMB flat 60 V - 2 A Schottky rectifier STMicroelectronics STPS2L60UF U1 L7987L HTSSOP16 STMicroelectronics L7987L Figure 16. L7987L demonstration board layout (top and bottom) 32/36 DocID026362 Rev 2 L7987L 7 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. DocID026362 Rev 2 33/36 36 Package information L7987L Figure 17. HTSSOP16 package outline Table 8. HTSSOP16 package mechanical data Dimensions (mm) Symbol Min. Max. A 1.20 A1 0.15 A2 0.80 b 0.19 0.30 c 0.09 0.20 D 4.90 5.00 5.10 D1 2.80 3.00 3.20 E 6.20 6.40 6.60 E1 4.30 4.40 4.50 E2 2.80 3.00 3.20 e L k 1.00 1.05 0.65 0.45 L1 0.60 0.75 1.00 0.00 aaa 34/36 Typ. 8.00 0.10 DocID026362 Rev 2 L7987L 8 Ordering information Ordering information Table 9. Order codes 9 Order code Package Packaging L7987L HTSSOP16 Tube L7987LTR HTSSOP16 Tape and reel Revision history Table 10. Document revision history Date Revision Changes 15-May-2014 1 Initial release. 03-Jun-2014 2 Updated main title: 61 V 2 A asynchronous step-down switching regulator with adjustable current limitation on page 1 (replaced “3 A” by “2 A”). DocID026362 Rev 2 35/36 36 L7987L Please Read Carefully: 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. Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no liability whatsoever relating to the choice, selection or use of the ST products and services described herein. No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. If any part of this document refers to any third party products or services it shall not be deemed a license grant by ST for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoever of such third party products or services or any intellectual property contained therein. UNLESS OTHERWISE SET FORTH IN ST’S TERMS AND CONDITIONS OF SALE ST DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY WITH RESPECT TO THE USE AND/OR SALE OF ST PRODUCTS INCLUDING WITHOUT LIMITATION IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION), OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT. ST PRODUCTS ARE NOT DESIGNED OR AUTHORIZED FOR USE IN: (A) SAFETY CRITICAL APPLICATIONS SUCH AS LIFE SUPPORTING, ACTIVE IMPLANTED DEVICES OR SYSTEMS WITH PRODUCT FUNCTIONAL SAFETY REQUIREMENTS; (B) AERONAUTIC APPLICATIONS; (C) AUTOMOTIVE APPLICATIONS OR ENVIRONMENTS, AND/OR (D) AEROSPACE APPLICATIONS OR ENVIRONMENTS. WHERE ST PRODUCTS ARE NOT DESIGNED FOR SUCH USE, THE PURCHASER SHALL USE PRODUCTS AT PURCHASER’S SOLE RISK, EVEN IF ST HAS BEEN INFORMED IN WRITING OF SUCH USAGE, UNLESS A PRODUCT IS EXPRESSLY DESIGNATED BY ST AS BEING INTENDED FOR “AUTOMOTIVE, AUTOMOTIVE SAFETY OR MEDICAL” INDUSTRY DOMAINS ACCORDING TO ST PRODUCT DESIGN SPECIFICATIONS. PRODUCTS FORMALLY ESCC, QML OR JAN QUALIFIED ARE DEEMED SUITABLE FOR USE IN AEROSPACE BY THE CORRESPONDING GOVERNMENTAL AGENCY. Resale of ST products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by ST for the ST product or service described herein and shall not create or extend in any manner whatsoever, any liability of ST. ST and the ST logo are trademarks or registered trademarks of ST in various countries. Information in this document supersedes and replaces all information previously supplied. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners. © 2014 STMicroelectronics - All rights reserved STMicroelectronics group of companies 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 www.st.com 36/36 DocID026362 Rev 2