LTC3608 18V, 8A Monolithic Synchronous Step-Down DC/DC Converter Description Features n n n n n n n n n n n n n n n n n 8A Output Current Wide VIN Range = 4V to 18V Internal N-Channel MOSFETs True Current Mode Control Optimized for High Step-Down Ratios tON(MIN) ≤ 100nsec Extremely Fast Transient Response Stable with Ceramic COUT ±1% 0.
LTC3608 41 SW 42 SW 43 SW 44 SW 45 SW 46 SW 47 SW 48 PVIN 49 PVIN TOP VIEW 50 PVIN Input Supply Voltage (SVIN, PVIN, ION)........ 20V to –0.3V Boosted Topside Driver Supply Voltage (BOOST)................................................. 26V to –0.3V SW Voltage............................................. 20V to –0.3V INTVCC, EXTVCC, (BOOST – SW), RUN/SS, PGOOD Voltages....................................... 7V to –0.3V FCB, VON, VRNG Voltages............. INTVCC + 0.3V to –0.
LTC3608 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 15V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Main Control Loop SVIN Operating Input Voltage Range IQ Input DC Supply Current Normal Shutdown Supply Current VFB Feedback Reference Voltage 4 ITH = 1.2V, –40°C to 85°C (Note 3) ITH = 1.2V, –40°C to 125°C (Note 3) l 0.594 0.
LTC3608 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 15V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 5 5.5 V –0.
LTC3608 Typical Performance Characteristics Efficiency vs Load Current Efficiency vs Input Voltage 100 100 FCB = 5V FIGURE 6 CIRCUIT 90 80 30 20 10 VIN = 12V FREQ = 550kHz 0 0.01 0.1 1 LOAD CURRENT (A) VOUT = 5V VOUT = 3.3V VOUT = 2.5V VOUT = 2.5V VOUT = 1.8V VOUT = 1.2V VOUT = 1V 90 ILOAD = 1A 85 80 10 600 0.60 CONTINUOUS MODE DISCONTINUOUS MODE ILOAD = 1A 400 20 10 15 INPUT VOLTAGE (V) 5 10 15 INPUT VOLTAGE (V) 3608 G05 2.
LTC3608 Typical Performance Characteristics 200 150 100 50 0 –50 –25 25 75 0 50 TEMPERATURE (°C) 100 15 10 0.7 0.8 VRNG VOLTAGE (V) 0.9 MAXIMUM VALLEY CURRENT (A) 15 10 5 0 50 75 25 TEMPERATURE (°C) 100 3608 G14 3 3608 G15 Maximum Valley Current Limit in Foldback 125 16 14 12 10 8 6 4 4 8 3608 G16 20 12 16 INPUT VOLTAGE (V) 2.0 0.62 10 5 0 0 0.1 0.2 3608 G17 0.3 VFB (V) 0.4 0.5 0.6 3608 G18 Error Amplifier gm vs Temperature 1.8 0.
LTC3608 Typical Performance Characteristics Input and Shutdown Currents vs Input Voltage 0.20 25 800 20 SHUTDOWN 600 15 400 10 EXTVCC = 5V 0 5 10 15 INPUT VOLTAGE (V) 20 14 0 –0.30 0 –0.40 FCB PIN CURRENT (µA) 4 2 2 0 40 10 20 30 INTVCC LOAD CURRENT (mA) 100 125 –0.25 2 –0.50 –0.75 –1.00 3608 G24 25 75 0 50 TEMPERATURE (°C) 100 UNDERVOLTAGE LOCKOUT THRESHOLD (V) RUN/SS PIN CURRENT (µA) 4.5 LATCHOFF ENABLE 4.
LTC3608 Pin Functions PVIN (Pins 1, 2, 3, 4, 5, 6, 7, 48, 49, 50, 51, 52, 53): Main Input Supply. Decouple this pin to power PGND with the input capacitance, CIN SW (Pins 8, 33, 41, 42, 43, 44, 45, 46, 47, 55): Switch Node Connection to the Inductor. The (–) terminal of the bootstrap capacitor, CB, also connects here. This pin swings from a diode voltage drop below ground up to VIN. SGND (Pins 10, 14, 15, 20, 26, 27, 54): Signal Ground.
LTC3608 Functional Diagram RON VON ION 13 22 FCB EXTVCC 19 29 SVIN 30 4.7V 0.7V 2.4V + 1µA PVIN – 1, 2, 3, 4, 5, 6, 7, 48, 49, 50, 51, 52, 53 0.6V REF 0.6V CIN 5V REG INTVCC + – 31, 32 F 11 VVON tON = (10pF) IION R S Q FCNT SW + ICMP – L1 DB VOUT 8, 33, 41, 42, 43, 44, 45, 46, 47, 55 SWITCH LOGIC IREV – SHDN 1.4V COUT OV M2 + CVCC 17 PGND × (0.5 TO 2) 0.7V 34, 35, 36, 37, 38, 39, 40 16 PGOOD 1 240k + 1V Q2 Q4 – Q6 ITHB R2 0.
LTC3608 Operation Main Control Loop The LTC3608 is a high efficiency monolithic synchronous, step-down DC/DC converter utilizing a constant on-time, current mode architecture. It operates from an input voltage range of 4V to 18V (20V maximum) and provides a regulated output voltage at up to 8A of output current. The internal synchronous power switch increases efficiency and eliminates the need for an external Schottky diode.
LTC3608 Applications Information The basic LTC3608 application circuit is shown on the front page of this data sheet. External component selection is primarily determined by the maximum load current. The LTC3608 uses the on-resistance of the synchronous power MOSFET for determining the inductor current. The desired amount of ripple current and operating frequency also determines the inductor value.
LTC3608 Applications Information Because the voltage at the ION pin is about 0.7V, the current into this pin is not exactly inversely proportional to VIN, especially in applications with lower input voltages. To correct for this error, an additional resistor, RON2, connected from the ION pin to the 5V INTVCC supply will further stabilize the frequency. RON2 = 5V R 0.7V ON Changes in the load current magnitude will also cause frequency shift.
LTC3608 Applications Information 2.0 CVON 0.01µF RVON2 100k RC VON LTC3608 ITH CC (2a) VOUT INTVCC RVON1 3k 10k CVON 0.01µF RVON2 10k RC Q1 2N5087 CC VON LTC3608 ITH DROPOUT REGION 1.0 0.5 0 0.25 0.50 0.75 DUTY CYCLE (VOUT/VIN) 1.0 3608 F03 Figure 3. Maximum Switching Frequency vs Duty Cycle 3608 F02 Figure 2.
LTC3608 Applications Information 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. A variety of inductors designed for high current, low voltage applications are available from manufacturers such as Sumida, Panasonic, Coiltronics, Coilcraft and Toko. CIN and COUT Selection The input capacitance, CIN, is required to filter the square wave current at the drain of the top MOSFET.
LTC3608 Applications Information Fault Conditions: Current Limit and Foldback threshold forces continuous synchronous operation, allowing current to reverse at light loads and maintaining high frequency operation. The LTC3608 has a current mode controller which inherently limits the cycle-by-cycle inductor current not only in steady state operation but also in transient. To further limit current in the event of a short circuit to ground, the LTC3608 includes foldback current limiting.
LTC3608 Applications Information The EXTVCC pin can be used to provide MOSFET gate drive and control power from the output or another external source during normal operation. Whenever the EXTVCC pin is above 4.7V the internal 5V regulator is shut off and an internal 50mA P-channel switch connects the EXTVCC pin to INTVCC. INTVCC power is supplied from EXTVCC until this pin drops below 4.5V. Do not apply more than 7V to the EXTVCC pin and ensure that EXTVCC ≤ VIN.
LTC3608 Applications Information INTVCC RSS* VIN 3.3V OR 5V D1 RUN/SS RSS* CSS D2* RUN/SS 2N7002 CSS 3608 F05 *OPTIONAL TO OVERRIDE OVERCURRENT LATCHOFF (5a) (5b) Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%.
LTC3608 Applications Information Design Example Next, set up VRNG voltage and check the ILIMIT. Tying VRNG to 0.5V will set the typical current limit to 11A, and tying VRNG to GND will result in a typical current around 16A. CIN is chosen for an RMS current rating of about 5A at 85°C. The output capacitors are chosen for a low ESR of 0.002Ω to minimize output voltage changes due to inductor ripple current and load steps.
LTC3608 Applications Information How to Reduce SW Ringing As with any switching regulator, there will be voltage ringing on the SW node, especially for high input voltages. The ringing amplitude and duration is dependent on the switching speed (gate drive), layout (parasitic inductance) and MOSFET output capacitance. This ringing contributes to the overall EMI, noise and high frequency ripple. One way to reduce ringing is to optimize layout. A good layout minimizes parasitic inductance.
LTC3608 Applications Information CVCC SW VIN NC SGND SVIN EXTVCC INTVCC SW INTVCC PGND PGND PGND PGND PGND SW SGND PVIN FCB PVIN ITH PVIN VRNG PVIN PGOOD PVIN SGND SGND 52 NC LTC3608 1 2 3 4 5 6 7 8 9 10 11 12 13 14 VON 51 SW RUN/SS 50 CIN ION BOOST 49 SW SGND 48 VFB NC 47 SW SW 46 NC PVIN VOUT SW PVIN 45 NC PVIN 44 SW PVIN 43 SGND PVIN COUT SW PVIN 42 PVIN 41 PGND PGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 R1 R2
LTC3608 Typical Applications 3.6V Input to 1.5V/8A at 750kHz VIN2 = 5V INTVCC CVCC 4.7µF 6.3V PGND EXTVCC C4 0.01µF CF 0.
LTC3608 Typical Applications 5V to 18V Input to 1.2V/8A at 550kHz RF1 1Ω INTVCC CVCC 4.7µF 6.3V PGND VIN2 EXTVCC C4 0.01µF CF 0.
LTC3608 Typical Applications 5V to 18V Input to 1.8V/8A All Ceramic 1MHz RF1 1Ω INTVCC EXTVCC C4 0.01µF CF 0.1µF 25V CVCC 4.7µF 6.
LTC3608 Package Description WKG Package 52-Lead QFN Multipad (7mm × 8mm) (Reference LTC DWG # 05-08-1768 Rev Ø) SEATING PLANE A 7.00 BSC 0.00 – 0.05 2.625 REF 41 B PAD 1 CORNER 4 2.90 REF 0.50 BSC 40 bbb M C A B 7 8.00 BSC PIN 1 ID 52 1 3.40 REF 3.20 ± 0.10 2.025 ± 0.10 2.925 ± 0.10 3.90 ± 0.10 3.40 REF 33 8 32 9 1.00 REF 10 aaa C 2x NX b TOP VIEW 0.90 ± 0.10 NX 0.08 C // ccc C 8 7.50 ± 0.05 2.90 REF 0.50 BSC 1.35 ± 0.10 1.775 REF 15 0.25 ± 0.
LTC3608 Revision History (Revision history begins at Rev C) REV DATE DESCRIPTION C 06/10 Updated SW voltage range in Absolute Maximum Ratings. PAGE NUMBER 2 Note 4 updated. 4 3608fc 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.
LTC3608 Typical Application 14V to 18V Input to 12V/5A at 500kHz CVCC 4.7µF, 6.3V PGND INTVCC SW SGND 42 43 44 45 46 47 NC SGND SVIN EXTVCC INTVCC SW PGND PGND PGND PGND PGND VFB SW ION SW NC LTC3608 SW 48 SGND PVIN CIN: TAIYO YUDEN TMK432BJ106MM COUT: SANYO 16SVP180MX L1: CDEP85NP-4R3MC-88 PVIN PGOOD PVIN SGND 2 3 4 5 6 7 8 9 10 11 12 13 14 25 24 DB CMDSH-3 R1 3.16k 1% C1 22 21 CON 0.01µF 20 R5 24.9k 18 17 16 15 90.
