LT3570 1.5A Buck Converter, 1.5A Boost Converter and LDO Controller DESCRIPTION FEATURES n n n n n n n n The LT®3570 is a buck and boost converter with internal power switches and LDO controller. Each converter is designed with a 1.5A current limit and an input range from 2.5V to 36V, making the LT3570 ideal for a wide variety of applications. Switching frequencies up to 2.1MHz are programmed with an external timing resistor and the oscillator can be synchronized to an external clock up to 2.75MHz. 2.
LT3570 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN1, VIN2, VIN3, VBIAS Voltage ..................................40V BOOST Voltage .........................................................60V BOOST Pin Above SW2 .............................................25V NPN_DRV Voltage .......................................................8V SW1 Voltage .............................................................40V SHDN1, SHDN2, SHDN3 Voltage ..............................40V SYNC, RT Voltage .........................
LT3570 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN1,2,3 = 12V, VSHDN1,2,3 = 12V unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS 2.1 2.5 V 2.1 2.5 V 0 1.5 μA Minimum Operating Voltage (VIN1) (Note 3) l Minimum Operating Voltage (VIN2) (Note 3) l Shutdown Current (Note 4) VSHDN1,2,3 = 0V VIN1 Quiescent Current VSHDN1 = 12V, VSHDN2,3 = 0V, VC1 = 0.
LT3570 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN1,2,3 = 12V, VSHDN1,2,3 = 12V unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS SW2 Error Amplifier 2 Transconductance 195 μMho Error Amplifier 2 Voltage Gain 100 V/V VC2 Pin Switching Threshold 700 mV VC2 to SW2 Current Gain 5.4 SW2 Current Limit (Note 7) 1.5 2.
LT3570 TYPICAL PERFORMANCE CHARACTERISTICS VIN1 Quiescent Current vs Temperature VIN2 Quiescent Current vs Temperature 3.5 3.0 4.0 900 3.5 800 700 CURRENT (mA) 2.0 1.5 1.0 0.5 0 –50 –25 0 CURRENT (μA) 3.0 2.5 CURRENT (mA) VIN3 Quiescent Current vs Temperature 2.5 2.0 1.5 200 0.5 100 0 1.25 25 50 75 100 125 150 TEMPERATURE (°C) 75 100 125 150 1.00 0.75 0.50 0 –50 –25 SHDN Pin Current vs Voltage 40 30 20 10 0.
LT3570 TYPICAL PERFORMANCE CHARACTERISTICS SW2 Saturation Voltage vs SW2 Current 3.0 350 2.5 300 250 2.0 VOLTAGE (mV) CURRENT (A) SW2 Current Limit vs Duty Cycle 1.5 1.0 200 150 100 TJ = 125°C TJ = 25°C TJ = –40°C 0.5 0 0 TJ = 125°C TJ = 25°C TJ = –40°C 50 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.
LT3570 PIN FUNCTIONS (QFN/TSSOP) VIN2 (Pins 1,2/Pin 14): Input Voltage for the Buck Regulator. This pin also supplies the current to the internal circuitry of the LT3570. This pin must be locally bypassed with a capacitor. SHDN3 (Pin13/Pin 4): Shutdown Pin. Tie to 1.5V or more to enable the NPN LDO. Tie all SHDN pins to 0.3V or less to shutdown the part. SW2 (Pin 3/Pin15): Switch Node. This pin connects to the emitter of an internal NPN power switch.
C2 R2B R1B D3 L2 C6B D2 C5 R4 FB2 SS2 SW2 Q2 BOOST RT SYNC R5 VIN2 R 788mV 4.5μA A8 Q S + + – – + A5 A7 A6 – + Figure 1. Block Diagram C4B C4A R3A R3B – + A3 SHDN2 VC1 A9 OSCILLATOR A10 REGULATOR SHDN1 VC2 BIAS Q A4 R S 788mV SHDN3 – + A1 A2 A11 + + – – + GND 788mV FB1 4.
LT3570 OPERATION The LT3570 is a constant frequency, current mode, buck converter and boost converter with an NPN LDO regulator. Operation can be best understood by referring to the Block Diagram. If all of the SHDN pins are held low, the LT3570 is shut down and draws zero quiescent current. When any of the pins exceed 1.4V the internal bias circuits turn on. Each regulator will only begin regulating when its corresponding SHDN pin is pulled high.
LT3570 APPLICATIONS INFORMATION FB Resistor Network The output voltage is programmed with a resistor divider (refer to the Block Diagram) between the output and the FB pin. Choose the resistors according to: ⎞ ⎛ V R1= R2 ⎜ OUT – 1⎟ ⎝ 788mV ⎠ Buck Inductor Selection and Maximum Output Current A good first choice for the inductor value is L= VOUT2 + VF for SW2 0.75 • f where VF is the voltage drop of the catch diode (~0.4V) and f is the switching frequency.
LT3570 APPLICATIONS INFORMATION to 1.2A at DC2 = 0.8. The maximum output current is a function of the chosen inductor value: IOUT2(MAX) =ILIM2 – ΔIL2 2 = 1.5 • (1– 0.25 • DC2) – ΔIL2 2 Choosing an inductor value so that the ripple current is small will allow a maximum output current near the switch current limit. One approach to choosing the inductor is to start with the simple rule given above, look at the available inductors and choose one to meet cost or space goals.
LT3570 APPLICATIONS INFORMATION and small circuit size, are therefore an option. You can estimate output ripple with the following equations: VRIPPLE = ΔIL2 for ceramic capacitors 8 • f • COUT and VRIPPLE = ΔIL2 • ESR for electrolytic capacitors (tantalum and aluminum) The RMS content of this ripple is very low so the RMS current rating of the output capacitor is usually not of concern. It can be estimated with the formula: IC(RMS) = ΔIL2 Table 2.
LT3570 APPLICATIONS INFORMATION into a tight local loop, minimizing EMI. The input capacitor must have low impedance at the switching frequency to do this effectively and it must have an adequate ripple current rating. The RMS input current is: IIN2(RMS) =IOUT2 • VOUT2 ( VIN2 – VOUT2 ) VIN2 < IOUT2 2 and is largest when VIN2 = 2 • VOUT2 (50% duty cycle). Considering that the maximum load current is ~1.5A, RMS ripple current will always be less than 0.75A.
LT3570 APPLICATIONS INFORMATION Boost Diode Selection D3 A Schottky diode is recommended for use with the LT3570 inverter/boost regulator. The Microsemi UPS120 is a very good choice. Where the input to output voltage differential exceeds 20V, use the UPS140 (a 40V diode). These diodes are rated to handle an average forward current of 1A. For applications where the average forward current of the diode is less than 0.5A, use an ON Semiconductor MBR0520L diode.
LT3570 APPLICATIONS INFORMATION compensation resistor, RC, is usually in the range of 5k to 50k. A good technique to compensate a new application is to use a 50k potentiometer in place of RC, and use a 1nF capacitor for CC. By adjusting the potentiometer while observing the transient response, the optimum value for RC can be found. Figures 3a to 3c illustrate this process for the circuit of Figure 1 with load current stepped from 100mA to 500mA for the buck converter.
LT3570 APPLICATIONS INFORMATION 25 The free-running frequency is set through a resistor from the RT pin to ground. The oscillator frequency vs RT can be seen in Figure 4. The oscillator can be synchronized with an external clock applied to the SYNC pin. When synchronizing the oscillator, the free running frequency must be set approximately 10% lower than the desired synchronized frequency.
LT3570 APPLICATIONS INFORMATION will be enabled again going through a soft-start cycle. Note: Overtemperature protection is intended to protect the device during momentary overload conditions. Continuous operation above the specified maximum operating junction temperature may result in device degradation or failure. to system ground at one location. Additionally, keep the SW and BOOST nodes as small as possible.
LT3570 TYPICAL APPLICATIONS DSL Modem VIN 8V TO 28V C9 10μF VIN1 VIN2 VIN3 BIAS SHDN1 SHDN2 SHDN3 SHDN1 SHDN2 SHDN3 D3 BOOST SW2 SW1 C1 10μF L2 10μH D2 L1 D1 4.7μH VOUT1 8V 250mA C8 100nF R1 105k LT3570 FB1 SS1 VC1 R2 11.5k FB2 SS2 VC2 VOUT2 5V R3 118k C2 22μF C5 10nF R8 25k R4 22.1k C7 1nF R7 25k NPN_DRV C6 1nF Q1 10nF VOUT3 3.3V 500mA R5 34.0k RT FB3 GND SYNC C3 2.2μF R6 10.7k R9 20.0k 3570 TA02 “Dying Gasp” System VIN 12V C9 10μF C10 0.
LT3570 PACKAGE DESCRIPTION UF Package 24-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1697) BOTTOM VIEW—EXPOSED PAD 4.00 ± 0.10 (4 SIDES) 0.70 ±0.05 R = 0.115 TYP 0.75 ± 0.05 PIN 1 TOP MARK (NOTE 6) PIN 1 NOTCH R = 0.20 TYP OR 0.35 × 45° CHAMFER 23 24 0.40 ± 0.10 1 2 4.50 ± 0.05 2.45 ± 0.05 3.10 ± 0.05 (4 SIDES) 2.45 ± 0.10 (4-SIDES) PACKAGE OUTLINE (UF24) QFN 0105 0.200 REF 0.25 ±0.05 0.50 BSC 0.25 ± 0.05 0.00 – 0.05 0.50 BSC NOTE: 1.
LT3570 TYPICAL APPLICATION PDA Core VIN 4V TO 12V C9 10μF VIN1 VIN2 VIN3 BIAS SHDN1 SHDN2 SHDN3 D1 VOUT1 15V 200mA SHDN1 SHDN2 SHDN3 C8 100nF SW2 R1 191k LT3570 FB1 SS1 VC1 R2 10.7k FB2 SS2 VC2 VOUT2 3.3V 500mA R3 34k C2 22μF R8 25k C5 10nF R4 10.7k C7 1nF R7 25k NPN_DRV C6 1nF C4 10nF L2 8.2μH D2 L1 12μH SW1 C1 10μF D3 BOOST Q1 R5 13.7k SYNC RT FB3 GND R6 10.7k R9 20k VOUT3 1.8V 500mA C3 4.7μF 3570 TA04 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1767 1.5A, 1.
