LTC1775 High Power No RSENSE Current Mode Synchronous Step-Down Switching Regulator TM U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LTC®1775 is a synchronous step-down switching regulator controller that drives external N-channel power MOSFETs using few external components. Current mode control with MOSFET VDS sensing eliminates the need for a sense resistor and improves efficiency. Largely similar to the LTC1625, the LTC1775 has twice the maximum sense voltage for high current applications. The frequency of a nominal 150kHz internal oscillator can be synchronized to an external clock over a 1.5:1 frequency range. Highest Efficiency Current Mode Controller No Sense Resistor Required 300mV Maximum Current Sense Voltage Stable High Current Operation Dual N-Channel MOSFET Synchronous Drive Wide VIN Range: 4V to 36V Wide VOUT Range: 1.19V to VIN ±1% 1.19V Reference Programmable Fixed Frequency with Injection Lock Very Low Drop Out Operation: 99% Duty Cycle Forced Continuous Mode Control Pin Optional Programmable Soft Start Pin Selectable Output Voltage Foldback Current Limit Output Overvoltage Protection Logic Controlled Micropower Shutdown: IQ < 30µA Available in 16-Lead Narrow SSOP and SO Packages Burst ModeTM operation at low load currents reduces switching losses and low dropout operation extends operating time in battery-powered systems. A forced continuous mode control pin can assist secondary winding regulation by disabling Burst Mode operation when the main output is lightly loaded. Fault protection is provided by foldback current limiting and an output overvoltage comparator. An external capacitor attached to the RUN/SS pin provides soft start capability for supply sequencing. A wide supply range allows operation from 4V (4.3V for LTC1775I) to 36V at the input and 1.19V to VIN at the output. U APPLICATIO S ■ ■ ■ Notebook Computers Automotive Electronics Battery Chargers Distributed Power Systems , LTC and LT are registered trademarks of Linear Technology Corporation. No RSENSE and Burst Mode are trademarks of Linear Technology Corporation. U ■ TYPICAL APPLICATION Efficiency vs Load Current SYNC CSS 0.1µF + TK RUN/SS TG VPROG SW M1 SUD50N03-10 ITH SGND DB CMDSH-3 INTVCC BG VOSENSE PGND L1 6µH CB 0.33µF BOOST RC 10k CC 2.2nF VIN + CVCC 4.7µF D1 MBRS340 CIN 22µF 30V ×4 + M2 SUD50N03-10 VOUT 3.3V COUT 10A 680µF 6.3V 95 VIN = 10V f = 150kHz FCB = INTVCC 90 VOUT = 5V VOUT = 3.3V 85 80 75 1775 F01 Figure 1. High Efficiency Step-Down Converter 100 VIN 5V TO 28V EFFICIENCY (%) LTC1775 70 0.01 0.1 1 LOAD CURRENT (A) 10 1775 F01b 1 LTC1775 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Input Supply Voltage (VIN, TK) ................. 36V to – 0.3V Boosted Supply Voltage (BOOST) ............. 42V to – 0.3V Boosted Driver Voltage (BOOST – SW) ...... 7V to – 0.3V Switch Voltage (SW) ....................................36V to – 5V EXTVCC Voltage ...........................................7V to – 0.3V ITH Voltage ................................................2.7V to – 0.3V FCB, RUN/SS, SYNC Voltages .....................7V to – 0.3V VOSENSE, VPROG Voltages ........(INTVCC + 0.3V) to – 0.3V Peak Driver Output Current < 10µs (TG, BG) ............ 2A INTVCC Output Current ........................................ 50mA Operating Ambient Temperature Range LTC1775C .............................................. 0°C to 70°C LTC1775I (Note 5) .............................. – 40°C to 85°C Junction Temperature (Note 2) ............................. 125°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C ORDER PART NUMBER TOP VIEW EXTVCC 1 16 VIN SYNC 2 15 TK RUN/SS 3 14 SW FCB 4 13 TG ITH 5 12 BOOST SGND 6 11 INTVCC 10 BG VOSENSE 7 9 VPROG 8 GN PACKAGE 16-LEAD NARROW PLASTIC SSOP LTC1775CGN LTC1775CS LTC1775IGN LTC1775IS GN PART MARKING PGND 1775 1775I S PACKAGE 16-LEAD PLASTIC SO TJMAX = 125°C, θJA = 130°C/W (GN) TJMAX = 125°C, θJA = 110°C/W (S) Consult factory for Military grade parts. ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 15V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 10 50 nA 1.190 3.300 5.000 1.202 3.380 5.100 V V V 0.001 0.01 %/V – 0.020 0.035 – 0.2 0.2 % % 1.16 1.19 1.22 V –1 –2 µA 1.24 1.28 1.32 V Main Control Loop IINVOSENSE Feedback Current VPROG Pin Open, ITH = 1.19V (Note 3) VOUT Regulated Output Voltage 1.19V (Adjustable) Selected 3.3V Selected 5V Selected ITH = 1.19V (Note 3) VPROG Pin Open VPROG = 0V VPROG = INTVCC VLINEREG Reference Voltage Line Regulation VIN = 4V to 20V, ITH = 1.19V (Note 3), VPROG Pin Open VLOADREG Output Voltage Load Regulation ITH = 2V (Note 3) ITH = 0.5V (Note 3) ● ● VFCB Forced Continuous Threshold VFCB Ramping Negative ● IFCB Forced Continuous Bias Current VFCB = 1.19V VOVL Output Overvoltage Lockout VPROG Pin Open IPROG VPROG Input Current 3.3V VOUT 5V VOUT VPROG = 0V VPROG = 5V – 3.5 3.5 –7 7 µA µA Input DC Supply Current Normal Mode Shutdown EXTVCC = 5V (Note 4) VRUN/SS = 0V, 4V < VIN < 15V 500 15 30 µA µA 0.8 1.4 2 V VRUN/SS = 0V –1.2 –2.5 –4 µA VOSENSE = 1V, VPROG Pin Open 260 300 340 mV IQ VRUN/SS RUN/SS Pin Threshold IRUN/SS Soft Start Current Source ∆VSENSE(MAX) Maximum Current Sense Threshold 2 ● ● ● ● 1.178 3.220 4.900 LTC1775 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 15V unless otherwise noted. SYMBOL PARAMETER CONDITIONS TG tR TG tF TG Transition Time Rise Time Fall Time BG tR BG tF BG Transition Time Rise Time Fall Time MIN TYP MAX UNITS (Note 6) CLOAD = 3300pF CLOAD = 3300pF 50 50 150 150 ns ns (Note 6) CLOAD = 3300pF CLOAD = 3300pF 50 50 150 150 ns ns 5.2 5.4 V – 0.2 –1 % 180 300 mV Internal VCC Regulator VINTVCC Internal VCC Voltage 6V < VIN < 30V, VEXTVCC = 4V VLDOINT INTVCC Load Regulation ICC = 20mA, VEXTVCC = 4V VLDOEXT EXTVCC Voltage Drop ICC = 20mA, VEXTVCC = 5V VEXTVCC EXTVCC Switchover Voltage ICC = 20mA, VEXTVCC Ramping Positive fOSC Oscillator Freqency SYNC = 0V fH/fOSC Maximum Synchronized Frequency Ratio VSYNC SYNC Pin Threshold (Figure 4) RSYNC SYNC Pin Input Resistance ● ● 5.0 4.5 4.7 135 150 V Oscillator 165 kHz 1.2 V 1.5 Ramping Positive Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: LTC1775CGN/LTC1775IGN: TJ = TA + (PD • 130°C/W) LTC1775CS/LTC1775IS: TJ = TA + (PD • 110°C/W) Note 3: The LTC1775 is tested in a feedback loop that adjusts VOSENSE to achieve a specified error amplifier output voltage (ITH). 0.9 50 kΩ Note 4: Typical in application circuit with EXTVCC tied to VOUT = 5V, IOUT = 0A and FCB = INTVCC. Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. See Applications Information. Note 5: Minimum input supply voltage is 4.3V at – 40°C for industrial grade parts. Note 6: Rise and fall times are measured at 10% to 90% levels. 3 LTC1775 U W TYPICAL PERFOR A CE CHARACTERISTICS Efficiency vs Load Current Efficiency vs Input Voltage 100 100 ILOAD = 5A 95 90 95 ILOAD = 5A 70 CONTINUOUS MODE 60 ILOAD = 500mA 85 80 VIN = 10V VOUT = 5V EXTVCC = VOUT 50 0.001 75 90 ILOAD = 500mA 85 80 75 VOUT = 5V FIGURE 1 CIRCUIT 70 0.1 0.01 1.0 LOAD CURRENT (A) EFFICIENCY (%) 80 BURST MODE 90 EFFICIENCY (%) EFFICIENCY (%) Efficiency vs Input Voltage 100 10 0 10 15 20 INPUT VOLTAGE (V) 5 25 VOUT = 3.3V FIGURE 1 CIRCUIT 70 30 0 10 15 20 INPUT VOLTAGE (V) 5 25 1775 • G02 1775 • G01 Load Regulation 1775 • G03 VIN – VOUT Dropout Voltage vs Load Current ITH Pin Voltage vs Load Current 3.0 0 400 FIGURE 1 CIRCUIT VIN = 20V VOUT = 5V FIGURE 1 CIRCUIT 2.5 – 0.1 30 FIGURE 1 CIRCUIT VOUT = 5V, 5% DROP 300 VITH (V) ∆VOUT (%) – 0.2 – 0.3 VIN – VOUT (mV) 2.0 1.5 CONTINUOUS MODE 1.0 100 – 0.4 BURST MODE 0.5 – 0.5 0 2 0 6 4 LOAD CURRENT (A) 8 10 0 8 6 10 4 LOAD CURRENT (A) 2 12 Input and Shutdown Current vs Input Voltage 400 20 EXTVCC = 5V 400 EXTVCC – INTVCC (mV) 30 0 – 0.5 5 20 15 25 10 INPUT VOLTAGE (V) 30 0 35 1775 • G07 300 200 100 10 0 10 VIN = 15V EXTVCC = 5V 0.5 ∆INTVCC (%) 600 SHUTDOWN CURRENT (µA) 40 SHUTDOWN 200 8 500 50 800 4 6 CURRENT LOAD (A) EXTVCC Switch Drop vs INTVCC Load Current VIN = 15V EXTVCC OPEN 2 1775 • G06 1.0 60 1000 INPUT CURRENT (µA) 0 INTVCC Load Regulation 1200 4 0 14 1775 • G05 1775 • G04 0 200 –1.0 0 20 10 30 40 INTVCC LOAD CURRENT (mA) 50 1775 • G08 0 0 10 30 40 20 INTVCC LOAD CURRENT (mA) 50 1775 • G09 LTC1775 U W TYPICAL PERFOR A CE CHARACTERISTICS Maximum Current Sense Voltage vs Temperature 300 250 200 150 100 50 0 0 0.2 0.4 0.6 DUTY CYCLE 0.8 320 300 250 300 200 SYNC = 0V 150 100 290 50 60 35 85 10 TEMPERATURE (°C) 110 135 0 –40 –15 60 35 85 10 TEMPERATURE (°C) 1775 • G11 1775 • G10 0 110 135 1775 • G12 RUN/SS Pin Current vs Temperature FCB Pin Current vs Temperature Soft Start 0 RUN/SS CURRENT (µA) – 0.5 FCB CURRENT (µA) SYNC = 1.5V 310 280 –40 –15 1.0 Oscillator Frequency vs Temperature FREQUENCY (kHz) 350 MAXIMUM CURRENT SENSE VOLTAGE (mV) MAXIMUM CURRENT SENSE VOLTAGE (mV) Maximum Current Sense Voltage vs Duty Cycle – 1.0 – 1.5 –1 RUN/SS 5V/DIV –2 VOUT 5V/DIV –3 IL 5A/DIV –4 – 2.0 –40 –15 60 35 85 10 TEMPERATURE (°C) 110 135 –5 –40 –15 60 35 85 10 TEMPERATURE (°C) 1775 • G13 110 VIN = 20V 20ms/DIV VOUT = 5V RLOAD = 0.5Ω FIGURE 1 CIRCUIT 135 1775 • G14 Transient Response (Burst Mode Operation) Transient Response VOUT 100mV /DIV 1530 G16 Burst Mode Operation VOUT 50mV /DIV VOUT 100mV /DIV ITH 100mV /DIV IL 5A/DIV IL 5A/DIV 100µs/DIV VIN = 20V VOUT = 5V ILOAD = 2A TO 8A FIGURE 1 CIRCUIT 1530 G18 IL 2A/DIV VIN = 20V 200µs/DIV VOUT = 5V ILOAD = 100mA TO 2A FIGURE 1 CIRCUIT 1530 G17 VIN = 20V 20µs/DIV VOUT = 5V ILOAD = 100mA FIGURE 1 CIRCUIT 1530 G15 5 LTC1775 U U U PI FU CTIO S EXTVCC (Pin 1): INTVCC Switch Input. When the EXTVCC voltage is above 4.7V, the switch closes and supplies INTVCC power from EXTVCC. Do not exceed 7V at this pin. Leaving VPROG open allows the output voltage to be set by an external resistive divider between the output and VOSENSE. SYNC (Pin 2): Synchronization Input for Internal Oscillator. The oscillator will nominally run at 150kHz when open, 225kHz when tied above 1.2V, and will lock over a 1.5:1 clock frequency range. PGND (Pin 9): Driver Power Ground. Connects to the source of the bottom N-channel MOSFET, the (–) terminal of CVCC and the (–) terminal of CIN. RUN/SS (Pin 3): Run Control and Soft Start Input. A capacitor to ground at this pin sets the ramp time to full current output (approximately 1s/µF). Forcing this pin below 1.4V shuts down the device. FCB (Pin 4): Forced Continuous Input. Tie this pin to ground to force synchronous operation at low load currents, to a resistive divider from the secondary output when using a secondary winding, or to INTVCC to enable Burst Mode operation at low load currents. ITH (Pin 5): Error Amplifier Compensation Point. The current comparator threshold increases with this control voltage, forcing inductor current to be roughly proportional to VITH. Nominal voltage range for this pin is 0V to 2.4V. SGND (Pin 6): Signal Ground. Connect to the (–) terminal of COUT. VOSENSE (Pin 7): Output Voltage Sense. Feedback input from the remotely sensed output voltage or from an external resistive divider across the output. VPROG (Pin 8): Output Voltage Programming. When VOSENSE is connected to the output, VPROG < 0.8V selects a 3.3V output and VPROG > 3.5V selects a 5V output. 6 BG (Pin 10): Bottom Gate Drive. Drives the gate of the bottom N-channel MOSFET between ground and INTVCC. INTVCC (Pin 11): Internal 5.2V Regulator Output. The driver and control circuits are powered from this voltage. Decouple this pin to power ground with a minimum of 4.7µF tantalum or other low ESR capacitor. BOOST (Pin 12): Topside Floating Driver Supply. The (+) terminal of the bootstrap capacitor connects here. This pin swings from a Schottky diode drop below INTVCC to VIN + INTVCC. TG (Pin 13): Top Gate Drive. Drives the top N-channel MOSFET with a voltage swing equal to INTVCC superimposed on the switch node voltage. SW (Pin 14): Switch Node. The (–) terminal of the bootstrap capacitor connects here. This pin swings from a diode drop below ground up to VIN. TK (Pin 15): Top MOSFET Kelvin Sense. MOSFET VDS sensing requires this pin to be routed to the drain of the top MOSFET separately from VIN. VIN (Pin 16): Main Supply Input. Decouple this pin to ground with an RC filter (1Ω, 0.1µF) for applications above 3A. LTC1775 W FU CTIO AL DIAGRA U U TK + 2 SYNC VIN 15 TA × 5.5 INTVCC + – INTVCC CIN – BA × 5.5 0.95V + ITH +– 5 OSC 0.7V RC – 1.19V – + gm = 1m BOOST + – CL – TOP R 0.5V VFB 12 B SLEEP CB TG SWITCH LOGIC/ DROPOUT COUNTER Ω EA 14 INTVCC 11 OVERVOLTAGE +– 0.6V – DB + CVCC BG FCNT 10 3µA M2 PGND + 3 M1 13 SW SHUTDOWN VIN CSS I1 – + RUN/SS S Q ITHB 0.6V I2 + CC1 + REV 9 6V 1.28V 1.19V REF – + VIN 16 OV 1.19V 5.2V LDO REG + SGND + F 6 – INTVCC 4.7V – 1µA L1 8 VPROG 7 VOSENSE 4 FCB 1 EXTVCC + VOUT COUT 1775 BD 7 LTC1775 U OPERATIO Main Control Loop The LTC1775 is a constant frequency, current mode controller for DC/DC step-down converters. In normal operation, the top MOSFET is turned on when the RS latch is set by the on-chip oscillator and is turned off when the current comparator I1 resets the latch. While the top MOSFET is turned off, the bottom MOSFET is turned on until either the inductor current reverses, as determined by the current reversal comparator I2, or the next cycle begins. Inductor current is measured by sensing the VDS potential across the conducting MOSFET. The output of the appropriate sense amplifier (TA or BA) is selected by the switch logic and applied to the current comparator. The voltage on the ITH pin sets the comparator threshold corresponding to peak inductor current. The error amplifier EA adjusts this voltage by comparing the feedback signal VFB from the output voltage with the internal 1.19V reference. The VPROG pin selects whether the feedback voltage is taken directly from the VOSENSE pin or is derived from an on-chip resistive divider. When the load current increases, it causes a drop in the feedback voltage relative to the reference. The ITH voltage then rises until the average inductor current again matches the load current. The internal oscillator can be synchronized to an external clock applied to the SYNC pin and can lock to a frequency between 100% and 150% of its nominal 150kHz rate. When the SYNC pin is left open, it is pulled low internally and the oscillator runs at its normal rate. If this pin is taken above 1.2V, the oscillator will run at its maximum 225kHz rate. Pulling the RUN/SS pin low forces the controller into its shutdown state and turns off both MOSFETs. Releasing the RUN/SS pin allows an internal 3µA current source to charge up an external soft start capacitor CSS. When this voltage reaches 1.4V, the controller begins switching, but with the ITH voltage clamped at approximately 0.8V. As CSS continues to charge, the clamp is raised until full range operation is restored. The top MOSFET driver is powered from a floating bootstrap capacitor CB. This capacitor is normally recharged from INTVCC through a diode DB when the top MOSFET is turned off. As VIN decreases towards VOUT, the converter 8 will attempt to turn on the top MOSFET continuously (‘’dropout’’). A dropout counter detects this condition and forces the top MOSFET to turn off for about 500ns every tenth cycle to recharge the bootstrap capacitor. An overvoltage comparator OV guards against transient overshoots and other conditions that may overvoltage the output. In this case, the top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared. Foldback current limiting for an output shorted to ground is provided by a transconductance amplifer CL. As VFB drops below 0.6V, the buffered ITH input to the current comparator is gradually pulled down to a 0.95V clamp. This reduces peak inductor current to about one fifth of its maximum value. Low Current Operation The LTC1775 is capable of Burst Mode operation at low load currents. If the error amplifier drives the ITH voltage below 0.95V, the buffered ITH input to the current comparator will remain clamped at 0.95V. The inductor current peak is then held at approximately 60mV/RDS(ON)(TOP). If ITH then drops below 0.5V, the Burst Mode comparator B will turn off both MOSFETs. The load current will be supplied solely by the output capacitor until ITH rises above the 50mV hysteresis of the comparator and switching is resumed. Burst Mode operation is disabled by comparator F when the FCB pin is brought below 1.19V. This forces continuous operation and can assist secondary winding regulation. INTVCC/EXTVCC Power Power for the top and bottom MOSFET drivers and most of the internal circuitry of the LTC1775 is derived from the INTVCC pin. When the EXTVCC pin is left open, an internal 5.2V low dropout regulator supplies the INTVCC power from VIN. If EXTVCC is raised above 4.7V, the internal regulator is turned off and an internal switch connects EXTVCC to INTVCC. This allows a high efficiency source, such as the primary or a secondary output of the converter itself, to provide the INTVCC power. LTC1775 U W U U APPLICATIO S I FOR ATIO The LTC1775 requires two external N-channel power MOSFETs, one for the top (main) switch and one for the bottom (synchronous) switch. Important parameters for the power MOSFETs are the breakdown voltage V(BR)DSS, threshold voltage VGS(TH), on-resistance RDS(ON), reverse transfer capacitance CRSS and maximum current ID(MAX). The gate drive voltage is set by the 5.2V INTVCC supply. Consequently, logic level threshold MOSFETs must be used in LTC1775 applications. If low input voltage operation is expected (VIN < 5V), then sub-logic level threshold MOSFETs should be used. Pay close attention to the V(BR)DSS specification for the MOSFETs as well; many of the logic level MOSFETs are limited to 30V or less. The MOSFET on-resistance is chosen based on the required load current. The maximum average output current IO(MAX) is equal to the peak inductor current less half the peak-to-peak ripple current ∆IL. The peak inductor current is inherently limited in a current mode controller by the current threshold ITH range. The corresponding maximum VDS sense voltage is about 300mV under normal conditions. The LTC1775 will not allow peak inductor current to exceed 300mV/RDS(ON)(TOP). The following equation is a good guide for determining the required RDS(ON)(MAX) at 25°C (manufacturer’s specification), allowing some margin for ripple current, current limit and variations in the LTC1775 and external component values: RDS(ON)(MAX) ≅ 240mV IO(MAX) (ρT ) ( ) 2.0 ρT NORMALIZED ON RESISTANCE Power MOSFET Selection The ρT is a normalized term accounting for the significant variation in RDS(ON) with temperature, typically about 0.4%/°C as shown in Figure 2. Junction to ambient temperature TJA is around 20°C in most applications. For a maximum ambient temperature of 70°C, using ρ90°C ≅ 1.3 in the above equation is a reasonable choice. This equation is plotted in Figure 3 to illustrate the dependence of maximum output current on RDS(ON). Some popular MOSFETs are shown as data points. 1.5 1.0 0.5 0 – 50 50 100 0 JUNCTION TEMPERATURE (°C) 150 1775 F02 Figure 2. RDS(ON) vs Temperature 25 MAXIMUM OUTPUT CURRENT (A) The basic LTC1775 application circuit is shown in Figure 1. External component selection is primarily determined by the maximum load current and begins with the selection of the sense resistance for the desired current level. Since the LTC1775 senses current using the on-resistance of the power MOSFET, the maximum application current primarily determines the choice of MOSFET. The operating frequency and the inductor are chosen based largely on the desired amount of ripple current. Finally, CIN is selected for its ability to handle the RMS current into the converter and COUT is chosen with low enough ESR to meet the output voltage ripple specification. IRL3803 20 15 SUD50N03-10 10 FDS8936A 5 Si9936 0 0 0.02 0.06 0.04 RDS(ON) (Ω) 0.08 0.10 1775 F03 Figure 3. Maximum Output Current vs RDS(ON) at VGS = 4.5V The 300mV maximum sense voltage of the LTC1775 allows a large current to be obtained from power MOSFET switches. It also causes a significant amount of power dissipation in those switches and careful attention must be 9 LTC1775 U W U U APPLICATIO S I FOR ATIO paid to the resulting thermal issues. Under DC conditions, the maximum power that can be dissipated by a MOSFET switch limits the current through it: IDS(MAX)= P RDS(ON) = TJ(MAX) – TA θ JA RDS(ON) ρTJ(MAX) For example, the SUD50N03-10 with TJ(MAX) = 175°C, TA =70°, θJA = 30° C/W, RDS(ON) = 0.019Ω, ρTJ(MAX) = 1.8 can operate with a maximum DC current of 10A. In a switching application, the actual power dissipation is increased by the transition losses and is reduced by the switch duty cycle. When the LTC1775 is operating in continuous mode, the duty cycles for the MOSFETs are: VOUT VIN V –V Bottom Duty Cycle = IN OUT VIN Top Duty Cycle = Operating Frequency and Synchronization The choice of operating frequency and inductor value is a trade-off between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET switching losses, both gate charge loss and transition loss. However, lower frequency operation requires more inductance for a given amount of ripple current. The internal oscillator runs at a nominal 150kHz frequency when the SYNC pin is left open or connected to ground. Pulling the SYNC pin above 1.2V will increase the frequency by 50%. The oscillator will injection lock to a clock signal applied to the SYNC pin with a frequency between 165kHz and 200kHz. The clock high level must exceed 1.2V for at least 1µs and no longer than 4µs as shown in Figure 4. The top MOSFET turn-on will synchronize with the rising edge of the clock. 1µs < tON < 4µs 7V The MOSFET power dissipations at maximum output current are: 1.2V PTOP V 2 = OUT (IO(MAX) )(ρT (TOP) )(RDS(ON) ) VIN 2 + (k)(VIN )(IO(MAX) )(C RSS )(f) 1775 F04 0 5µs < t < 6µs V –V 2 PBOT = IN OUT (IO(MAX) )(ρT (BOT ) )(RDS(ON) ) VIN Both MOSFETs have I2R losses and the PTOP equation includes an additional term for transition losses, which are largest at high input voltages. The constant k = 1.7 can be used to estimate the amount of transition loss. The bottom MOSFET losses are greatest at high input voltage or during a short circuit when the duty cycle is nearly 100%. The temperature rise of the MOSFETs depends on the effective thermal resistance θJA of the heat sink used in the application. Check the temperature of the MOSFET when testing applications and use appropriate heat sinking such as board power planes to spread the heat. 10 Figure 4. SYNC Clock Waveform Inductor Value Selection Given the desired input and output voltages, the inductor value and operating frequency directly determine the ripple current: V V ∆IL = OUT 1 – OUT VIN ( f)(L) Lower ripple current reduces losses in the inductor, ESR losses in the output capacitors and output voltage ripple. Thus, highest efficiency operation is obtained at low frequency with small ripple current. To achieve this, however, requires a large inductor. LTC1775 U W U U APPLICATIO S I FOR ATIO A reasonable starting point is to choose a ripple current that is about 40% of IO(MAX). Note that the largest ripple current occurs at the highest VIN. To guarantee that ripple current does not exceed a specified maximum, the inductor should be chosen according to: VOUT V L≥ 1 – OUT VIN(MAX) ( f)(∆IL(MAX) ) Burst Mode Operation Considerations The choice of RDS(ON) and inductor value also determines the load current at which the LTC1775 enters Burst Mode operation. When bursting, the controller clamps the peak inductor current to approximately: IBURST(PEAK) 60mV = RDS(ON) The corresponding average current depends on the amount of ripple current. Lower inductor values (higher ∆IL) will reduce the load current at which Burst Mode operation begins. The output voltage ripple can increase during Burst Mode operation if ∆IL is substantially less than IBURST. This will primarily occur when the duty cycle is very close to unity (VIN is close to VOUT) or if very large value inductors are chosen. This is generally only a concern in applications with VOUT ≥ 5V. At high duty cycles, a skipped cycle causes the inductor current to quickly descend to zero. However, it takes multiple cycles to ramp the current back up to IBURST(PEAK). During this interval, the output capacitor must supply the load current and enough charge may be lost to cause significant droop in the output voltage. It is a good idea to keep ∆IL comparable to IBURST(PEAK). Otherwise, one might need to increase the output capacitance in order to reduce the voltage ripple or else disable Burst Mode operation by forcing continuous operation with the FCB pin. Fault Conditions: Current Limit and Output Shorts The LTC1775 current comparator can accommodate a maximum sense voltage of 300mV. This voltage and the sense resistance determine the maximum allowed peak inductor current. The corresponding output current limit is: ILIMIT = ( 300mV 1 – ∆IL RDS(ON) ( ρT) 2 ) The current limit value should be checked to ensure that ILIMIT(MIN) > IO(MAX). The minimum value of current limit generally occurs with the largest VIN at the highest ambient temperature, conditions which cause the highest power dissipation in the top MOSFET. Note that it is important to check for self-consistency between the assumed junction temperature of the top MOSFET and the resulting value of ILIMIT which heats the junction. Caution should be used when setting the current limit based upon RDS(ON) of the MOSFETs. The maximum current limit is determined by the minimum MOSFET onresistance. Data sheets typically specify nominal and maximum values for RDS(ON), but not a minimum. A reasonable, but perhaps overly conservative, assumption is that the minimum RDS(ON) lies the same amount below the typical value as the maximum RDS(ON) lies above it. Consult the MOSFET manufacturer for further guidelines. The LTC1775 includes current foldback to help further limit load current when the output is shorted to ground. If the output falls by more than half, then the maximum sense voltage is progressively lowered from 300mV to about 80mV. Under short-circuit conditions with very low duty cycle, the LTC1775 will begin skipping cycles in order to limit the short-circuit current. In this situation the bottom MOSFET RDS(ON) will control the inductor current valley rather than the top MOSFET controlling the inductor current peak. The short-circuit ripple current is determined by the minimum on-time tON(MIN) of the LTC1775 (approximately 0.5µs), the input voltage, and inductor value: ∆IL(SC) = tON(MIN) VIN /L. The resulting short-circuit current is: ISC = 80mV 1 + ∆ IL(SC) ( ρT ) 2 (RDS(ON)(BOT) ) 11 LTC1775 U W U U APPLICATIO S I FOR ATIO Normally, the top and bottom MOSFETs will be of the same type. A bottom MOSFET with lower RDS(ON) than the top may be chosen if the resulting increase in short-circuit current is tolerable. However, the bottom MOSFET should never be chosen to have a higher nominal RDS(ON) than the top MOSFET. Inductor Core Selection Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite, molypermalloy or Kool Mµ® cores. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on the inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core loss and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means that inductance collapses rapidly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Molypermalloy (from Magnetics, Inc.) is a very good, low loss core material for toroids, but it is more expensive than ferrite. A reasonable compromise from the same manufacturer is Kool Mµ. Toroids are very space efficient, especially when you can use several layers of wire. Because they generally lack a bobbin, mounting is more difficult. However, designs for surface mount are available which do not increase the height significantly. Schottky Diode Selection The Schottky diode D1 shown in Figure 1 conducts during the dead time between the conduction of the power MOSFETs. This prevents the body diode of the bottom MOSFET from turning on and storing charge during the dead time, which could cost as much as 1% in efficiency. A 1A Schottky diode is generally a good size for 3A to 5A Kool Mµ is a registered trademark of Magnetics, Inc. 12 regulators. The diode may be omitted if the efficiency loss can be tolerated. Parasitic Lead Inductance Effects Because the LTC1775 is designed to operate with relatively large currents through single (or multiple) MOSFET switches, the lead inductance of these power switches can become a significant concern. The table below shows typical values of lead inductance for some common packages: MOSFET Package Lead Inductance TO-220 4nH to 12nH DDPAK 4nH DPAK 1.5nH SO-8 1nH Of particular concern are switches in TO-220 packages which can have a series inductance of between 4nH and 12nH depending upon the depth of insertion into the circuit board. When the main (top) switch is turned on, the lead inductance LP forms a voltage divider with the power inductor L1. The voltage VLP across this parasitic adds to the voltage from the switch on-resistance and increases the current sense voltage. VLP = (VIN – VOUT)LP/L1 The result is lower value of current limit than would have been expected otherwise. For example, a 10nH lead inductance with a 5µH power inductor has 50mV across it when VIN = 30V and VOUT = 5V. Thus, the 300mV current limit will be reached when the switch voltage due to onresistance is only 250mV, a 17% reduction. This effect is most noticeable at higher input voltages. Lead inductance also reduces the benefit of the Schottky diode D1 by delaying commutation of the inductor current from the diode over to the synchronous (bottom) switch. With the diode forward biased when the synchronous switch turns on, there is only about 500mV applied across the lead and trace inductance between the switch and the diode. It takes about 400ns to commutate a 20A current in this case. This delay reduces efficiency and can also increase the foldback current limit of the LTC1775. The LTC1775 U W U U APPLICATIO S I FOR ATIO Schottky diode must be placed next to the synchronous switch to minimize this effect. One also might consider using a power switch with an integrated Schottky diode, or omitting the diode altogether in high current applications. CIN and COUT Selection In continuous mode, the drain current of the top MOSFET is approximately a square wave of duty cycle VOUT / VIN. To prevent large input voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS current is given by: V V IRMS ≅ IO(MAX) OUT IN − 1 VIN VOUT 1/ 2 This formula has a maximum at VIN = 2VOUT, where IRMS = IO(MAX)/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor or to choose a capacitor rated at a higher temperature than required. Several capacitors may also be placed in parallel to meet size or height requirements in the design. The selection of COUT is primarily determined by the ESR required to minimize voltage ripple. The output ripple ∆VOUT is approximately bounded by: 1 ∆VOUT ≤ ∆IL ESR + (8 )(f )(COUT ) Since ∆IL increases with input voltage, the output ripple is highest at maximum input voltage. Typically, once the ESR requirement is satisfied the capacitance is adequate for filtering and has the required RMS current rating. Manufacturers such as Nichicon, United Chemicon and Sanyo should be considered for high performance throughhole capacitors. The OS-CON (organic semiconductor dielectric) capacitor available from Sanyo has the lowest product of ESR and size of any aluminum electrolytic at a somewhat higher price. An additional ceramic capacitor in parallel with OS-CON capacitors is recommended to reduce the effect of their lead inductance. In surface mount applications, multiple capacitors placed in parallel may be required to meet the ESR, RMS current handling and load step requirements. Dry tantalum, special polymer and aluminum electrolytic capacitors are available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important to only use types that have been surge tested for use in switching power supplies. Several excellent surge-tested choices are the AVX TPS and TPSV or the KEMET T510 series. Aluminum electrolytic capacitors have significantly higher ESR, but can be used in cost-driven applications providing that consideration is given to ripple current ratings and long term reliability. Other capacitor types include Nichicon PL, NEC Neocap, Panasonic SP and Sprague 595D series. INTVCC Regulator An internal P-channel low dropout regulator produces the 5.2V supply which powers the drivers and internal circuitry within the LTC1775. The INTVCC pin can supply a maximum RMS current of 50mA and must be bypassed to ground with a minimum of 4.7µF tantalum or low ESR electrolytic capacitance. Good bypassing is necessary to supply the high transient currents required by the MOSFET gate drivers. High input voltage applications in which large MOSFETs are being driven at high frequencies may cause the LTC1775 to exceed its maximum junction temperature rating. Most of the supply current drives the MOSFET gates unless an external EXTVCC source is used. The junction temperature can be estimated from the equations given in Note 2 of the Electrical Characteristics. For example, the LTC1775CGN is limited to less than 14mA from a 30V supply: TJ = 70°C + (14mA)(30V)(130°C/W) = 125°C To prevent the maximum junction temperature from being exceeded, the input supply current must be checked when operating in continuous mode at high VIN. Relief can be provided by using the EXTVCC pin to provide the gate drive current. 13 LTC1775 U W U U APPLICATIO S I FOR ATIO VPUMP ≈ 2(VOUT – VD) < 7V EXTVCC Connection + The LTC1775 contains an internal P-channel MOSFET switch connected between the EXTVCC and INTVCC pins. Whenever the EXTVCC pin is above 4.7V the internal 5.2V regulator shuts off, the switch closes and INTVCC power is supplied via EXTVCC until EXTVCC drops below 4.5V. This allows the MOSFET gate drive and control power to be derived from the output or other external source during normal operation. When the output is out of regulation (start-up, short circuit) power is supplied from the internal regulator. Do not apply greater than 7V to the EXTVCC pin and ensure that EXTVCC ≤ VIN. Significant efficiency gains can be realized by powering INTVCC from the output, since the VIN current supplying the driver and control currents will be scaled by a factor of Duty Cycle/Efficiency. For 5V regulators this simply means connecting the EXTVCC pin directly to VOUT. However, for 3.3V and other lower voltage regulators, additional circuitry is required to derive INTVCC power from the output. The following list summarizes the four possible connections for EXTVCC: 1. EXTVCC left open (or grounded). This will cause INTVCC to be powered from the internal 5.2V regulator resulting in a low current efficiency penalty of up to 10% at high input voltages. 2. EXTVCC connected directly to VOUT. This is the normal connection for a 5V regulator and provides the highest efficiency. 1µF VIN + CIN VIN BAT85 BAT85 0.22µF TK TG LTC1775 BAT85 VN2222LL SW L1 EXTVCC VOUT + COUT BG PGND 1775 F05b Figure 5b: Capacitive Charge Pump for EXTVCC 3. EXTVCC connected to an output-derived boost network. For 3.3V and other low voltage regulators, efficiency gains can still be realized by connecting EXTVCC to an output-derived voltage which has been boosted to greater than 4.7V. This can be done with either an inductive boost winding as shown in Figure 5a or a capacitive charge pump as shown in Figure 5b. 4. EXTVCC connected to an external supply. If an external supply is available in the 5V to 7V range (EXTVCC < VIN), it may be used to power EXTVCC. Figure 6 shows how one can easily generate a suitable EXTVCC voltage from VIN. This circuit still derives the gate drive current from VIN, but it removes the power dissipation from the LTC1775 internal regulator and increases the gate drive voltage. VIN VIN + CIN VIN TK OPTIONAL EXTVCC CONNECTION 5V < VSEC < 7V TG LTC1775 SW EXTVCC R4 1N4148 • + T1 1:N FCB R3 VSEC • + CSEC 1µF VOUT EXTVCC 1775 F06 Figure 6. EXTVCC Power Supplied from VIN COUT PGND 1775 F05a Figure 5a: Secondary Output Loop and EXTVCC Connection 14 Q1 D1 6.8V BG SGND R1 Note that RDS(ON) also varies with the gate drive level. If gate drives other than the 5.2V INTVCC are used, this must be accounted for when selecting the MOSFET RDS(ON). Particular care should be taken with applications where EXTVCC is connected to the output. When the output LTC1775 U W U U APPLICATIO S I FOR ATIO voltage is between 4.7V and 5.2V, INTVCC will be connected to the output and the gate drive is reduced. The resulting increase in RDS(ON) will also lower the current limit. Even applications with VOUT > 5.2V will traverse this region during start-up and must take into account the reduced current limit. An external bootstrap capacitor (CB in the functional diagram) connected to the BOOST pin supplies the gate drive voltage for the topside MOSFET. This capacitor is charged through diode DB from INTVCC when the SW node is low. Note that the voltage across CB is about a diode drop below INTVCC. When the top MOSFET turns on, the switch node voltage rises to VIN and the BOOST pin rises to approximately VIN + INTVCC. During dropout operation, CB supplies the top driver for as long as ten cycles between refreshes. Thus, the boost capacitance needs to store about 100 times the gate charge required by the top MOSFET. In many applications 0.1µF to 0.47µF is adequate. When adjusting the gate drive level , the final arbiter is the total input current for the regulator. If you make a change and the input current decreases, then you improved the efficiency. If there is no change in input current, then there is no change in efficiency. The LTC1775 drivers are adequate for driving up to about 30nC into MOSFET switches. When using large single, or multiple, MOSFET switches, external buffers may be required to provide additional gate drive capability. Special purpose gate driver circuits such as the LTC1693 are ideal in such cases. Alternately, the external buffer circuit shown in Figure 7 can be used. Note that the bipolar devices INTVCC BOOST Q1 FMMT619 VOUT 0V 3.3V INTVCC 5V Open Adjustable Remote sensing of the output voltage is provided by the VOSENSE pin. For fixed 3.3V and 5V output applications an internal resistive divider is used and the VOSENSE pin is connected directly to the output voltage as shown in Figure 8a. When using an external resistive divider, the VPROG pin is left open and the VOSENSE pin is connected to feedback resistors as shown in Figure 8b. The output voltage is set by the divider as: R2 VOUT = 1.19 V 1 + R1 VOUT COUT SGND 1775 F08a Figure 8a. Fixed 3.3V or 5V VOUT LTC1775 OPEN Q4 FMMT720 PGND VOSENSE + VOUT GATE OF M2 BG LTC1775 VPROG INTVCC CONNECT FOR VOUT = 3.3V Q3 FMMT619 GATE OF M1 SW VPROG CONNECT FOR VOUT = 5V External Gate Drive Buffer Q2 FMMT720 Output Voltage Programming The LTC1775 has a pin selectable output voltage determined by the VPROG pin as follows: Topside MOSFET Driver Supply (CB, DB) TG reduce the signal swing at the gate by a diode drop. Thus, the LTC1775 requires an increased EXTVCC voltage of about 6V (such as provided by the Figure 6 circuit) when using this driver. VPROG R2 + COUT VOSENSE R1 SGND 1775 F07 1775 F08b Figure 7. Optional External Gate Driver Figure 8b. Adjustable VOUT 15 LTC1775 U W U U APPLICATIO S I FOR ATIO Run/Soft Start Function The RUN/SS pin is a dual purpose pin that provides a soft start function and a means to shut down the LTC1775. Soft start reduces surge currents from VIN by gradually increasing the controller’s current limit ITH(MAX). This pin can also be used for power supply sequencing. Pulling the RUN/SS pin below 1.4V puts the LTC1775 into a low quiescent current shutdown (IQ < 30µA). This pin can be driven directly from logic as shown in Figure 9. Releasing the RUN/SS pin allows an internal 3µA current source to charge up the soft-start capacitor CSS. If RUN/SS has been pulled all the way to ground there is a delay before starting of approximately: When the voltage on RUN/SS reaches 1.4V the LTC1775 begins operating with a clamp on ITH at 0.8V. As the voltage on RUN/SS increases to approximately 3.1V, the clamp on ITH is raised until its full 2.4V range is restored. This takes an additional 0.5s/µF. During this time the load current will be folded back to approximately 80mV/RDS(ON) until the output reaches half of its final value. Diode D1 in Figure 9 reduces the start delay while allowing CSS to charge up slowly for the soft start function. This diode and CSS can be deleted if soft start is not needed. The RUN/SS pin has an internal 6V zener clamp (See Functional Diagram). RUN/SS RUN/SS D1 CSS CSS 1775 F09 Figure 9. RUN/SS Pin Interfacing FCB Pin Operation When the FCB pin drops below its 1.19V threshold, continuous synchronous operation is forced. In this case, the top and bottom MOSFETs continue to be driven regardless of the load on the main output. Burst Mode operation is disabled and current reversal under light loads is allowed in the inductor. 16 The secondary output voltage VSEC is normally set as shown in Figure 5a by the turns ratio N of the transformer: VSEC ≅ (N + 1)VOUT However, if the controller goes into Burst Mode operation and halts switching due to a light main load current, then VSEC will droop. An external resistor divider from VSEC to the FCB pin sets a minimum voltage VSEC(MIN): R4 VSEC(MIN) ≅ 1.19 V 1 + R3 1.4V tDELAY = CSS = (0.5s / µF ) CSS 3µA 3.3V OR 5V In addition to providing a logic input to force continuous operation, the FCB pin provides a means to regulate a flyback winding output. It can force continuous synchronous operation when needed by the flyback winding, regardless of the main output load. If VSEC drops below this level, the FCB voltage forces continuous operation until VSEC is again above its minimum. Minimum On-Time Considerations Minimum on-time tON(MIN) is the smallest amount of time that the LTC1775 is capable of turning the top MOSFET on and off again. It is determined by internal timing delays and the amount of gate charge required to turn on the top MOSFET. Low duty cycle applications may approach this minimum on-time limit and care should be taken to ensure that: tON(MIN) < VOUT ( VIN)( f) If the duty cycle falls below what can be accommodated by the minimum on-time, the LTC1775 will begin to skip cycles. The output voltage will continue to be regulated, but the ripple current and ripple voltage will increase. The minimum on-time for the LTC1775 is generally about 0.5µs. However, as the peak sense voltage (IL(PEAK) • RDS(ON)) decreases, the minimum on-time gradually increases up to about 0.7µs. This is of particular concern in forced continuous applications with low ripple current at light loads. If the duty cycle drops below the minimum on-time limit in this situation, a significant amount of LTC1775 U W U U APPLICATIO S I FOR ATIO cycle skipping can occur with correspondingly larger current and voltage ripple. Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power (×100%). Percent efficiency can be expressed as: %Efficiency = 100% – (L1 + L2 + L3 + ...) losses ranging from 2% to 8% as the output current increases from 0.5A to 2A for a 5V output. I2R losses cause the efficiency to drop at high output currents. 3. Transition losses apply only to the topside MOSFET, and only when operating at high input voltages (typically 20V or greater). Transition losses can be estimated from: Transition Loss = (1.7)(VIN2)(IO(MAX))(CRSS)(f) where L1, L2, etc. are the individual losses as a percentage of input power. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Although all dissipative elements in the circuit produce losses, four main sources usually account for most of the losses in LTC1775 circuits: 4. LTC1775 VIN supply current. The VIN current is the DC supply current to the controller excluding MOSFET gate drive current. Total supply current is typically about 850µA. If EXTVCC is connected to 5V, the LTC1775 will draw only 330µA from VIN and the remaining 520µA will come from EXTVCC. VIN current results in a small (< 1%) loss which increases with VIN. 1. INTVCC current. This is the sum of the MOSFET driver and control currents. The driver current results from switching the gate capacitance of the power MOSFETs. Each time a MOSFET gate is switched on and then off, a packet of gate charge Qg moves from INTVCC to ground. The resulting current out of INTVCC is typically much larger than the control circuit current. In continuous mode, IGATECHG = f(Qg(TOP) + Qg(BOT)). Other losses including CIN and COUT ESR dissipative losses, Schottky conduction losses during dead time and inductor core losses, generally account for less than 2% total additional loss. By powering EXTVCC from an output-derived source, the additional VIN current resulting from the driver and control currents will be scaled by a factor of Duty Cycle/ Efficiency. For example, in a 20V to 5V application at 400mA load, 10mA of INTVCC current results in approximately 3mA of VIN current. This reduces the loss from 10% (if the driver was powered directly from VIN) to about 3%. 2. DC I2R Losses. Since there is no separate sense resistor, DC I2R losses arise only from the resistances of the MOSFETs and inductor. In continuous mode the average output current flows through L, but is “chopped” between the top MOSFET and the bottom MOSFET. If the two MOSFETs have approximately the same RDS(ON), then the resistance of one MOSFET can simply be summed with the resistance of L to obtain the DC I2R loss. For example, if each RDS(ON) = 0.05Ω and RL = 0.15Ω, then the total resistance is 0.2Ω. This results in Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to (∆ILOAD)(ESR), where ESR is the effective series resistance of COUT, and COUT begins to charge or discharge. The regulator loop acts on the resulting feedback error signal to return VOUT to its steady-state value. During this recovery time VOUT can be monitored for overshoot or ringing which would indicate a stability problem. The ITH pin external components shown in Figure 1 will provide adequate compensation for most applications. A second, more severe transient is caused by connecting loads with large (> 1µF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive in order to limit the inrush current to the load. 17 LTC1775 U W U U APPLICATIO S I FOR ATIO Automotive Considerations: Plugging into the Cigarette Lighter As battery-powered devices go mobile, there is a natural interest in plugging into the cigarette lighter in order to conserve or even recharge battery packs during operation. But before you connect, be advised: you are plugging into the supply from hell. The main power line in an automobile is the source of a number of nasty potential transients, including load dump, reverse and double battery. Load dump is the result of a loose battery cable. When the cable breaks connection, the field collapse in the alternator can cause a positive spike as high as 60V which takes several hundred milliseconds to decay. Reverse battery is just what it says, while double battery is a consequence of tow truck operators finding that a 24V jump start cranks cold engines faster than 12V. The network shown in Figure 10 is the most straightforward approach to protect a DC/DC converter from the ravages of an automotive power line. The series diode prevents current from flowing during reverse battery, while the transient suppressor clamps the input voltage during load dump. Note that the transient suppressor should not conduct during double-battery operation, but must still clamp the input voltage below breakdown of the converter. Although the LTC1775 has a maximum input voltage of 36V, most applications will be limited to 30V by the MOSFET V(BR)DSS. Design Example As a design example, take a supply with the following specifications: VIN = 6V to 22V (15V nominal), VOUT = 5V, IO(MAX) = 10A. The required RDS(ON) can immediately be estimated: RDS(ON) = 240mV = 0.018Ω (10 A)(1.3) A 0.019Ω Siliconix SUD50N03-10 MOSFET (θJA = 30°C/W) is close to this value. For 40% ripple current at maximum VIN the inductor should be: L≥ 5V 5V 1– = 6.4µH (150kHz)(0.4)(10 A) 22V Choosing a Magnetics 55380-A2 core with 8 turns of 15 gauge wire yields a 6µH inductor. The resulting maximum ripple current will be: ∆IL(MAX) = 5V 5V 1– = 4.3A (150kHz)(6µH) 22V Next, check that the minimum value of the current limit is acceptable. Assume a junction temperature about 20°C above the 70°C ambient with ρ90°C = 1.3. ILIMIT ≥ 300mV 1 – 4. 3A = 10 A (0.019 Ω)(1.3) 2 Now double-check the assumed TJ: 50A IPK RATING 12V PTOP = VIN LTC1775 TRANSIENT VOLTAGE SUPPRESSOR GENERAL INSTRUMENT 1.5KA24A ( ) (1.3)(0.019Ω) + 2 (1.7)(22V) (10A)(170pF)(150kHz) 5V 10 A 22V 2 = 0.56W + 0.21W = 0.77mW PGND 1775 F10 TJ = 70°C + (0.77W)(30°C/W) = 93°C Figure 10. Automotive Application Protection 18 Since ρ(93°C) ≅ ρ(90°C), the solution is self-consistent. LTC1775 U W U U APPLICATIO S I FOR ATIO A short circuit to ground will result in a folded back current of: ISC = CIN is chosen for an RMS current rating of at least 5A at temperature. COUT is chosen with an ESR of 0.013Ω for low output ripple. The output ripple in continuous mode will be highest at the maximum input voltage and is approximately: 80mV 1 (15V)(0.5µs) + = 6.2A (0.013Ω)(1.1) 2 6µH ∆VO = (∆IL(MAX))(ESR) = (4.3A)(0.013Ω) = 56mV with a typical value of RDS(ON) and ρ(50°C) = 1.1. The resulting power dissipated in the bottom MOSFET is: PBOT = The complete circuit is shown in Figure 11. 15V – 5 V (6 .2A)2 (1.1)(0.013Ω) = 0. 37W 15V which is less than under full load conditions. RF 1Ω VOUT 1 2 3 LTC1775 EXTVCC VIN SYNC TK RUN/SS 4 CC1 CSS FCB 2.2nF 0.1µF RC INTVCC 10k 5 ITH CC2 220pF 6 SGND 7 8 VOSENSE VPROG SW TG BOOST INTVCC BG PGND 16 + VIN 6V TO 22V + CF 0.1µF 15 M1 SUD50N03-10 L1 6µH 14 13 CIN 22µF 30V ×4 VOUT 5V 10A 12 DB CMDSH-3 11 10 9 + CVCC 4.7µF CB 0.33µF D1 MBRS340 + M2 SUD50N03-10 COUT 680µF 6.3V 1775 F11 CIN: SANYO 30SC22M COUT: SANYO 6SP680M L1: MAGENTICS 55380-A2, 8 TURNS, 15 GAUGE Figure 11. 5V/10A Fixed Output from Design Example 19 LTC1775 U W U U APPLICATIO S I FOR ATIO PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1775. These items are also illustrated graphically in the layout diagram of Figure 12. Check the following in your layout: 1) Connect the TK lead directly to the drain of the topside MOSFET. Then connect the drain to the (+) plate of CIN. This capacitor provides the AC current to the top MOSFET. 2) The power ground pin connects directly to the source of the bottom N-channel MOSFET. Then connect the source to the anode of the Schottky diode (if used) and (–) plate of CIN, which should have as short lead lengths as possible. 3) The LTC1775 signal ground pin should connect to the (–) plate of COUT. Connect the (–) plate of COUT to power ground at the source of the bottom MOSFET. All smallsignal components (R2, CSS, CC, etc.) should return directly to SGND. 4) Keep the switch node SW away from sensitive smallsignal nodes. Ideally the switch node should be placed on the opposite side of the power MOSFETs from the LTC1775. 5) Connect the INTVCC decoupling capacitor CVCC closely to the INTVCC pin and the power ground pin. This capacitor carries the MOSFET gate drive current. 6) Does the VOSENSE pin connect as close as possible to the load? In adjustable applications, the resistive divider (R1, R2) must be connected between the load and signal ground. Place the divider near the LTC1775 in order to keep the high impedance VOSENSE node short. 7) For applications with multiple switching power converters connected to the same VIN, ensure that the input filter capacitance for the LTC1775 is not shared with the other converters. AC input current from another converter will cause substantial input voltage ripple that may interfere with proper operation of the LTC1775. A few inches of PC trace or wire (LTRACE ≈ 100nH) between CIN and the VIN supply is sufficient to prevent sharing. LTRACE OPTIONAL 5V EXTVCC CONNECTION CSS CC1 EXT CLK INTVCC RC 1 2 3 4 5 LTC1775 EXTVCC VIN SYNC TK RUN/SS SW FCB TG ITH BOOST SGND INTVCC 15 M1 14 L1 13 CB R2 R1 7 OPEN 8 VOSENSE VPROG BG PGND VIN 12 DB 6 + 16 CVCC 11 + + 10 M2 9 D1 CIN – – OUTPUT DIVIDER REQUIRED WITH VPROG OPEN + BOLD LINES INDICATE HIGH CURRENT PATHS Figure 12. LTC1775 Layout Diagram 20 VOUT COUT 1775 F12 + LTC1775 U TYPICAL APPLICATIO S 3.3V/5A Fixed Output VIN 5V TO 28V RF 1Ω LTC1775 16 VIN EXTVCC 2 15 SYNC TK 3 14 SW RUN/SS CC1 2.2nF 4 CSS RC INTVCC 0.1µF 10k 5 CC2 220pF 6 7 8 TG FCB BOOST ITH INTVCC SGND VOSENSE VPROG BG PGND CIN 22µF 30V ×2 + CF 0.1µF 1 M1 Si4412DY L1 10µH 13 VOUT 3.3V 5A 12 DB CMDSH-3 11 CB 0.1µF 10 + 9 D1 MBRS140T3 + M2 Si4412DY CVCC 4.7µF COUT 100µF 10V 0.065Ω ×4 1775 TA01 CIN: SANYO 30SC22M COUT: AVX TPSD107M010R0065 L1: SUMIDA CDRH-125 5V/20A Fixed Output RF 1Ω 6V 1 2 3 CSS 0.1µF CC1 2.2nF INTVCC RC 10k CC2 220pF 4 5 6 7 8 LTC1775 EXTVCC VIN SYNC TK RUN/SS FCB ITH SGND VOSENSE VPROG SW TG BOOST INTVCC BG PGND CF 0.1µF 16 + 15 M1 IRL3803 14 Q2 FMMT720 13 Q1 FMMT619 12 11 + 10 CVCC 4.7µF 9 L1 3.7µH CB 0.47µF DB CMDSH-3 Q3 FMMT619 Q4 FMMT720 VIN 6V TO 28V CIN 22µF 30V ×8 VOUT 5V 20A D1 MBRD835L + M2 IRL3803 COUT 680µF 6.3V ×2 1775 TA02 CIN: SANYO 30SC22M COUT: SANYO 6SP680M L1: MAGNETICS 55206-A2, 3.7µH, 6 TURNS, 13 GAUGE 21 LTC1775 U TYPICAL APPLICATIO S –5V/2.5A Positive to Negative Converter RF 1Ω VIN 5V TO 10V CF 0.1µF 1 2 EXTVCC VIN SYNC TK 16 M1 Si4412DY 15 3 CC1 2.2nF CSS RC 10k 0.1µF CC2 220pF 14 SW RUN/SS LTC1775 4 13 TG FCB 5 12 BOOST ITH 6 7 8 INTVCC SGND VOSENSE VPROG BG DB CMDSH-3 D1 MBR140T3 10 + COUT 680µF 6.3V M2 Si4412DY + PGND CIN 220µF 16V L1 12µH CB 0.22µF 11 + CVCC 4.7µF VOUT –5V 2.5A 9 1775 TA04 CIN: SANYO 16SV220M COUT: SANYO 6SP680M L1: 12µH, 5A 12V Output, Single Inductor, Buck/Boost Converter RF 1Ω 1 CSS 0.1µF 2 EXTVCC VIN 15 SYNC TK 14 RUN/SS SW LTC1775 13 RC OPEN 4 FCB TG 20k 12 5 ITH BOOST CC2 220pF 6 11 SGND INTVCC M1 Si4410DY L1 13µH 3 CC1 2.2nF 7 R1 11k 8 R2 100k VOSENSE VPROG BG PGND DB CMDSH-3 10 9 CIN: UNITED CHEMICON THCR70E1H226ZT-CBU COUT: SANYO 165A-150M L1: 13µH/11A 22 VIN 6V TO 18V CF 0.1µF 16 CVCC CB 4.7µF 0.33µF + D2 MBRD835L M3 Si4410DY + M2 Si4410DY D1 MBRS340 CIN 22µF 50V ×2 VOUT 12V + COUT 150µF 16V ×2 VIN 6.0 12 18 IOUT 2.6 4.0 4.8 1775 TA05 LTC1775 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. GN Package 16-Lead Plastic SSOP (Narrow 0.150) (LTC DWG # 05-08-1641) 0.189 – 0.196* (4.801 – 4.978) 0.009 (0.229) REF 16 15 14 13 12 11 10 9 0.229 – 0.244 (5.817 – 6.198) 0.150 – 0.157** (3.810 – 3.988) 1 0.015 ± 0.004 × 45° (0.38 ± 0.10) 0.007 – 0.0098 (0.178 – 0.249) 2 3 4 5 6 7 0.053 – 0.068 (1.351 – 1.727) 8 0.004 – 0.0098 (0.102 – 0.249) 0° – 8° TYP 0.016 – 0.050 (0.406 – 1.270) 0.0250 (0.635) BSC 0.008 – 0.012 (0.203 – 0.305) * DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE GN16 (SSOP) 1098 S Package 16-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) 0.386 – 0.394* (9.804 – 10.008) 16 15 14 13 12 11 10 9 0.150 – 0.157** (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) 1 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 2 3 4 5 6 0.053 – 0.069 (1.346 – 1.752) 0.014 – 0.019 (0.355 – 0.483) TYP 8 0.004 – 0.010 (0.101 – 0.254) 0° – 8° TYP 0.016 – 0.050 (0.406 – 1.270) 7 0.050 (1.270) BSC S16 1098 *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 23 LTC1775 U TYPICAL APPLICATIO S 2.5V/5A Adjustable Output RF 1Ω LTC1775 16 VIN EXTVCC 15 2 SYNC TK 14 3 SW RUN/SS CC2 220pF 4 5 6 7 OPEN 8 TG FCB ITH SGND BOOST INTVCC VOSENSE VPROG BG PGND M1 1/2 FDS8936A L1 6.1µH 13 12 D1 MBRS140 R2 11k 1% M2 1/2 FDS8936A R1 10k 1% DB CB CMDSH-3 0.22µF 11 10 9 CIN 15µF 35V ×3 + CF 0.1µF 1 CC1 2.2nF CSS RC INTVCC 0.1µF 10k VIN 5V TO 25V + CVCC 4.7µF VOUT 2.5V 5A + COUT 680µF 4V ×2 1775 TA03 CIN: KEMET T495X156M035AS COUT: KEMET T510X687K004AS L1: SUMIDA CDRH127-6R1 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1339 High Power Synchronous DC/DC Controller 60V Maximum Input Voltage LTC1436A-PLL High Efficiency Low Noise Synchronous Step-Down Controller PLL Synchronization and Auxiliary Linear Regulator LTC1438 Dual High Efficiency Step-Down Controller Power-On Reset and Low-Battery Comparator LTC1530 High Power Synchronous Step-Down Controller SO-8 with Current Limit, No RSENSE Saves Space, Fixed Frequency Ideal for 5V to 3.3V LTC1538-AUX Dual High Efficiency Step-Down Controller 5V Standby Output and Auxiliary Linear Regulator LTC1625 No RSENSE Current Mode Synchronous Step-Down Controller Burst Mode Operation, 16-Lead GN Package LTC1628 Dual High Efficiency 2-Phase Step-Down Controller Antiphase Drive, Standby 5V and 3.3V LDO, Fault Protection, VIN Up to 36V LTC1629 PolyPhaseTM High Efficiency Step-Down Controller Current Mode Ensures Accurate Current Sharing, Expandable Up to 200A, VIN Up to 36V LTC1649 3.3V Input High Power Step-Down Controller 2.7V to 5V Input, 90% Efficiency, Ideal for 3.3V to 1.xV – 2.xV Up to 20A LTC1702 Dual 2-Phase High Frequency, High Efficiency Voltage Mode Synchronous Step-Down Controller 500kHz; VIN Up to 7V; VOUT1, VOUT2 as Low as 2.x, 1.x, IOUT1, IOUT2 as High as 20A; No Sense Resistor Required LTC1735 High Efficiency Synchronous Step-Down Controller Output Fault Protection, 16-Pin GN Package, 4V ≤ VIN ≤ 36V PolyPhase is a trademark of Linear Technology Corporation. 24 Linear Technology Corporation 1775f LT/TP 0500 4K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com LINEAR TECHNOLOGY CORPORATION 1999