LTC3863 60V Low IQ Inverting DC/DC Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n Wide Operating VIN Range: 3.5V to 60V Wide Negative VOUT Range: –0.
LTC3863 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage (VIN).......................... –0.3V to 65V VIN-VSENSE Voltage....................................... –0.3V to 6V VIN-VCAP Voltage......................................... –0.3V to 10V RUN Voltage............................................... –0.3V to 65V VFBN, PLLIN/MODE Voltages........................ –0.3V to 6V SS, ITH, FREQ, VFB Voltages......................... –0.
LTC3863 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, unless otherwise noted. (Note 4) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Input Supply VIN Input Voltage Operating Range VUVLO Undervoltage Lockout IQ Input DC Supply Current 3.5 (VIN-VCAP) Ramping Up Threshold (VIN-VCAP) Ramping Down Threshold Hysteresis l l 3.25 3.00 60 V 3.50 3.25 0.25 3.8 3.
LTC3863 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, unless otherwise noted. (Note 4) SYMBOL PARAMETER CONDITIONS VCAP Gate Bias LDO Output Voltage (VIN-VCAP) IGATE = 0mA VCAPDROP Gate Bias LDO Dropout Voltage VIN = 5V, IGATE = 15mA MIN TYP MAX UNITS 7.6 8.0 8.5 V 0.2 0.5 V 0.002 0.
LTC3863 TYPICAL PERFORMANCE CHARACTERISTICS Pulse-Skipping Mode Operation Waveforms TA = 25°C, unless otherwise noted.
LTC3863 TYPICAL PERFORMANCE CHARACTERISTICS Overcurrent Protection VIN Line Transient Behavior Short-Circuit Protection SHORTCIRCUIT TRIGGER ILOAD 1A/DIV VOUT 500mV/DIV SOFT-START FOLDBACK COUT DISCHARGE 500µs/DIV VIN = 12V VOUT = –5V ILOAD2 = 1A TO SHORT-CIRCUIT FIGURE 7 CIRCUIT 3863 G10 1100 VIN = 12V VOUT = –5V ILOAD = 0A FIGURE 7 CIRCUIT 135 VOUT 50mV/DIV 3863 G11 10ms/DIV VIN = 12V, SURGE TO 48V VOUT = –5V ILOAD = 500mA FIGURE 7 CIRCUIT Pulse-Skipping Mode Input Current Over Input Voltage
LTC3863 TYPICAL PERFORMANCE CHARACTERISTICS Free Running Frequency Over Input Voltage Free Running Frequency Over Temperature FREQ = OPEN 550 500 f (kHz) 500 f (kHz) 120 FREQUENCY FOLDBACK (%) FREQ = OPEN 550 450 450 400 400 FREQ = 0V 350 0 20 10 30 VIN (V) FREQ = 0V 350 40 50 300 –75 60 25 75 125 TEMPERATURE (°C) –25 GATE Bias LDO (VIN - VCAP) Load Regulation 0.1 –0.3 –0.4 –3.0 0 5 10 IGATE (mA) 15 –0.
LTC3863 PIN FUNCTIONS PLLIN/MODE (Pin 1): External Reference Clock Input and Burst Mode Enable/Disable. When an external clock is applied to this pin, the internal phase-locked loop will synchronize the turn-on edge of the gate drive signal with the rising edge of the external clock. When no external clock is applied, this input determines the operation during light loading. Floating this pin selects low IQ (40μA) Burst Mode operation. Pulling to ground selects pulse-skipping mode operation.
LTC3863 FUNCTIONAL DIAGRAM VIN UVLO + – 0.4µA RUN 1.26V CIN VIN 3.25V RSENSE SENSE RUN + – – LOGIC CONTROL DRV GATE MP D1 Q R PLLIN/MODE 20µA FREQ MODE/CLOCK DETECT + PLL SYSTEM – VCO RFREQ ICMP OUT VIN – 8V O.425V – CAP + + 10µA + + – SGND SLOPE COMPENSATION OV + EA (Gm = 1.8mS) 0.88V – PGND L LDO SS 0.8V EN CSS 0.5µA ITH RFB1 VFBN + CLOCK VOUT COUT CCAP IN Burst Mode OPERATION – S CFB2 RFB2 VFB 3863 FD RITH CITH1 3863f For more information www.
LTC3863 OPERATION LTC3863 Main Control Loop The LTC3863 is a nonsynchronous inverting PMOS controller, where an inverting amplifier is used to sense the negative output voltage below ground. The LTC3863 uses a peak current mode control architecture to regulate the output. A feedback resistor, RFB1, is placed between VOUT and VFBN and a second resistor, RFB2, is placed between VFBN and and VFB. The LTC3863 has a trimmed internal reference, VREF, that is equal to (VFB – VFBN).
LTC3863 OPERATION The start-up of the output voltage VOUT is controlled by the voltage on the SS pin. When the voltage on the SS pin is less than the 0.8V internal reference, the VFB pin is regulated to the voltage on the SS pin. This allows the SS pin to be used to program a soft-start by connecting an external capacitor from the SS pin to signal ground. An internal 10µA pull-up current charges this capacitor, creating a voltage ramp on the SS pin. As the SS voltage rises from 0V to 0.
LTC3863 OPERATION The oscillator’s default frequency is based on the operating frequency set by the FREQ pin. If the oscillator’s default frequency is near the external clock frequency, only slight adjustments are needed for the PLL to synchronize the external P-channel MOSFET’s turn-on edge to the rising edge of the external clock. This allows the PLL to lock rapidly without deviating far from the desired frequency. The PLL is guaranteed from 75kHz to 750kHz.
LTC3863 APPLICATIONS INFORMATION The LTC3863 is a nonsynchronous inverting, current mode, constant frequency PWM controller. It drives an external P-channel power MOSFET which connects to a Schottky power diode acting as the commutating catch diode. The input range extends from 3.5V to 60V. The output range has no theoretical minimum or maximum, but the duty factor and external components practically limit the output to one-tenth and ten times the input voltage.
LTC3863 APPLICATIONS INFORMATION The LTC3863 can free-run at a user programmed switching frequency, or it can synchronize with an external clock to run at the clock frequency. When the LTC3863 is synchronized, the GATE pin will synchronize in phase with the rising edge of the applied clock in order to turn the external P-channel MOSFET on. The switching frequency of the LTC3863 is programmed with the FREQ pin, and the external clock is applied at the PLLIN/MODE pin.
LTC3863 APPLICATIONS INFORMATION The duty factor increases with increasing VOUT and decreasing VIN. For a given VOUT, the maximum duty factor occurs at minimum VIN. A typical starting point for selecting an inductor is to choose the inductance such that the maximum peak-to-peak inductor ripple current, ∆IL(MAX), is set to 40% ~ 50% of the inductor average current, IL(AVG), at maximum load current.
LTC3863 APPLICATIONS INFORMATION The LTC3863 has internal filtering of the current sense voltage which should be adequate in most applications. However, adding a provision for an external filter offers added flexibility and noise immunity, should it be necessary. The filter can be created by placing a resistor from the RSENSE resistor to the SENSE pin and a capacitor across the VIN and SENSE pins.
LTC3863 APPLICATIONS INFORMATION voltage rises across the resistor load. The Miller charge (the increase in coulombs on the horizontal axis from a to b while the curve is flat) is specified for a given VSD test voltage, but can be adjusted for different VSD voltages by multiplying by the ratio of the adjusted VSD to the curve specified VSD value.
LTC3863 APPLICATIONS INFORMATION The formula shows that the RMS current is greater than the maximum IOUT when VOUT is greater than VIN. Choose capacitors with higher RMS rating with sufficient margin. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life, which makes it advisable to derate the capacitor. The selection of COUT is primarily determined by the ESR required to minimize voltage ripple and load step transients.
LTC3863 APPLICATIONS INFORMATION Soft-start is enabled by connecting a capacitor from the SS pin to ground. An internal 10µA current source charges the capacitor, providing a linear ramping voltage at the SS pin that causes VOUT to rise smoothly from 0V to its final regulated value. The total soft-start time will be approximately: 0.8V 10µA When the LTC3863 is configured to track another supply, a voltage divider can be used from the tracking supply to the SS pin to scale the ramp rate appropriately.
LTC3863 APPLICATIONS INFORMATION VIN ––VOUT VOLTAGE frequency, f, will fold back to a minimum value of 0.18 • f when VFB reaches 0V. Both current limit and frequency foldback are active in all modes of operation. In a shortcircuit fault condition, the output current is first limited by current limit and then further reduced by folding back the operating frequency as the short becomes more severe. The worst-case fault condition occurs when VOUT is shorted to ground.
LTC3863 APPLICATIONS INFORMATION If the duty cycle falls below what can be accommodated by the minimum on-time, the controller will skip cycles. However, the output voltage will continue to regulate. Efficiency Considerations 2. Transition Loss: Transition loss of the P-channel MOSFET becomes significant only when operating at high input voltages (typically 20V or greater.
LTC3863 APPLICATIONS INFORMATION OPTI-LOOP® Compensation OPTI-LOOP compensation, through the availability of the ITH pin, allows the transient response to be optimized for a wide range of loads and output capacitors. The ITH pin not only allows optimization of the control loop behavior but also provides a test point for the regulator ’s DC-coupled and AC-filtered closed-loop response. The DC step, rise time and settling at this test point truly reflects the closedloop response.
LTC3863 APPLICATIONS INFORMATION the error amplifier output drive current on ITH of 100µA. The effect causes ITH to appear clamped in response to a transient load current step which causes excessive output droop. An RITH greater than 20k allows ITH to swing 1.5V with margin for temperature and part to part variation and should never have this issue. In applications with less severe transient load step requirements, RITH can safely be set as low as 10k. We do not recommend less than 10k in any application.
LTC3863 APPLICATIONS INFORMATION The boundary output current for continuous/discontinuous mode is calculated: IOUT(CDB) = 55V 2 • 5V 2 •12µH• 320kHz • ( 55V + 5V ) 2 = 0.55A The maximum inductor peak current occurs at minimum VIN of 4.5V and full load of 1.8A where LTC3863 operates in continuous mode is: 1.8A • ( 4.5V + 5V + 0.5V ) ∆IL + 5V 2 0.644A = 3.6A + ≈ 4.25A 2 IL(PEAK _ MAX) = Next, set the RSENSE resistor value to ensure that the converter can deliver the maximum peak inductor current of 4.
LTC3863 APPLICATIONS INFORMATION VR = 80V, θJA = 55°C/W) for this application. The power dissipation and junction temperature at TA = 70° and full load = 1.8A can be calculated as: PDIODE = 1.8A • 0.45V = 0.81W TJ = 70°C + 0.81W • 55°C/W = 114°C These power dissipation calculations show that careful attention to heat sinking will be necessary. For the input bypass capacitors, choose low ESR ceramic capacitors that can handle the maximum RMS current at the minimum VIN of 4.5V: ICIN(RMS) ≈ 1.8A • | –5V| = 1.
LTC3863 APPLICATIONS INFORMATION CINB VIN CCAP RUN CSS CAP CIN VIN CPITH SENSE SS GATE CITH CSF PLLIN/MODE RITH RFREQ RSF RGATE + RSENSE – Q1 D1 VOUT COUT LTC3863 L1 ITH RFB1 FREQ SGND VFBN RFB2 CFB2 GROUND PLANE TO PGND PGND VFB 3863 F08 Figure 8: LTC3863 Generic Application Schematic with Optional Current Sense Filter and Series Gate Resistor CINB TO Q1 GATE RGATE GATE CSF TO RSENSE+ VIN SENSE CAP CCAP RSF TO RSENSE– 3863 F09 Figure 9: LTC3863 Recommended Gate Driver PC B
LTC3863 APPLICATIONS INFORMATION 3. Place CIN, sense resistor, P-channel MOSFET, inductor, and primary COUT capacitors close together in one compact area. The junction connecting the drain of the P‑channel MOSFET, cathode of the Schottky, and (+) terminal of the inductor (this junction is commonly referred to as switch or phase node) should be compact but be large enough to handle the inductor currents without large copper losses.
LTC3863 APPLICATIONS INFORMATION Table 2 FAILURE MODE VOUT IOUT IVIN f None –5V 1A 453mA 350kHz RECOVERY WHEN FAULT IS REMOVED? BEHAVIOR N/A Normal Operation. Pin Open Open Pin 1 (PLLIN/MODE) –5V 1A 453mA 350kHz OK Pin already left open in normal application, so no difference. Open Pin 2 (FREQ) –5V 1A 453mA 535kHz OK Frequency jumps to default open value. Open Pin 3 (GND) –5V 1A 453mA 350kHz OK Exposed pad still provides GND connection to device.
LTC3863 TYPICAL APPLICATIONS 0.47µF CAP VIN 25mΩ RUN + COUT4 100µF 6.3V 0.47µF MOD 320kHz SS CAP RUN 220pF 16mΩ PLLIN/MODE 0.1µF SENSE SS 220pF D2 ITH PGOOD *VOUT FOLLOWS VIN WHEN 3.5V < VIN < 5.2V Q1 D1 GATE FREQ 15k 52.3k 10nF 15nF 10k 52.
LTC3863 TYPICAL APPLICATIONS CAP RUN VIN 25mΩ PLLIN/MODE 0.1µF 4.7nF 50pF SENSE SS GATE 36.5k 100k LTC3863 Q1 D1 L1 15µH ITH COUT1 10µF 25V ×2 191k FREQ SGND VFBN 47pF PGND VFB 3863 F12a PULSE-SKIPPING MODE Burst Mode OPERATION VIN = 12V VOUT = –18V 50 60 8 50 7 40 6 5 EFFICIENCY 4 40 90 VIN = 12V VOUT = –18V 75 60 PHASE 30 45 20 30 10 15 GAIN 30 3 20 2 –10 –15 1 –20 –30 0 –30 10 0 0.001 POWER LOSS 0.01 0.
LTC3863 TYPICAL APPLICATIONS 0.47µF CAP RUN VIN 82mΩ PLLIN/MODE 0.1µF 3.3nF 82pF SENSE SS GATE 5k 61.9k LTC3863 Q1 D1 L1 15µH ITH COUT1 10µF 25V ×2 95.3k FREQ SGND VFBN 10pF VFB PGND 3863 F13a 191k Gain/Phase 0.4 0.2 10 0.1 POWER LOSS 0 0.2 60 PHASE 30 GAIN (dB) EFFICIENCY 20 40 45 20 30 10 15 GAIN 0 0 –10 –15 –20 –30 –30 1 10 FREQUENCY (kHz) PHASE (DEG) 0.3 0.02 LOAD CURRENT (A) 90 VIN = 12V VOUT = –0.4V 75 50 30 0 0.002 60 0.
LTC3863 TYPICAL APPLICATIONS 0.47µF 440kHz RUN CAP VIN 27mΩ PLLIN/MODE + VIN CIN1 12V TO 42V 100µF 50V SENSE SS 1M 64.9k GATE LTC3863 Q1 D1 L1 10µH ITH COUT1 4.7µF 100V ×2 196k FREQ SGND VFBN 100pF PGND VFB 3863 F14a PULSE-SKIPPING MODE Burst Mode OPERATION VIN = 24V VOUT = –48V 50 70 8 60 7 50 75 40 60 6 EFFICIENCY 5 105 VIN = 24V VOUT = –48V 90 PHASE 30 45 30 20 40 4 30 3 20 2 –10 –15 1 –20 –30 0 –30 10 0 0.001 POWER LOSS 0.01 LOAD CURRENT (A) 0.
LTC3863 TYPICAL APPLICATIONS 0.47µF RUN CAP VIN 16mΩ PLLIN/MODE 0.1µF 27nF 390pF 61.9k VIN CIN1 4.5V TO 16V 100µF 20V SENSE SS Q1 D1 GATE 14.7k + LTC3863 L1 10µH ITH RFBO 698k 511k FREQ SGND VFBN 68pF VFB PGND 3863 F15a COUT1 33µF 16V ×2 + 350kHz CIN2 10µF 25V ×2 CIN1: PANASONIC 20SVP100M C : TDK C3225X7R1E106M 80.
LTC3863 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DE/UE Package 12-Lead Plastic DFN (4mm × 3mm) (Reference LTC DWG # 05-08-1695 Rev D) 0.70 ±0.05 3.30 ±0.05 3.60 ±0.05 2.20 ±0.05 1.70 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 2.50 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ±0.10 (2 SIDES) 7 R = 0.115 TYP 0.40 ±0.10 12 R = 0.05 TYP PIN 1 TOP MARK (NOTE 6) 0.
LTC3863 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 12-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1666 Rev F) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ±0.102 (.112 ±.004) 5.23 (.206) MIN 2.845 ±0.102 (.112 ±.004) 0.889 ±0.127 (.035 ±.005) 6 1 1.651 ±0.102 (.065 ±.004) 1.651 ±0.102 3.20 – 3.45 (.065 ±.004) (.126 – .136) 12 0.65 0.42 ±0.038 (.0256) (.0165 ±.
LTC3863 TYPICAL APPLICATION Efficiency 90 9 70 7 RUN CAP VIN 39mΩ PLLIN/MODE 180pF SS 52.3k LTC3863 Q1 D1 1M 1M FREQ SGND VFBN 3.3pF 40 VFB 3863 F16a COUT1 1µF 250V ×2 30 3 2 POWER LOSS 0.01 LOAD CURRENT (A) 1 0.1 0 3863 F16b Gain/Phase 40 CIN1: CDE AFK686M63G24T-F C : MURATA GRM32ER71H106H 10.
