DESIGN FEATURES LT1619 Boost Controller Provides Efficient Solutions for by Bing Fong Ma and Low Voltage Inputs Kurk Mathews Introduction As logic voltages continue to fall, it is increasingly common to find a high current, low voltage (3.3V or less) supply satisfying a large portion of a circuit’s power requirement. While this, in itself, is not a problem, generating subsequent voltages at even moderate currents from such a low voltage source can be challenging. Selecting a topology such as the boost, SEPIC or flyback converter is the easy part. Unfortunately, finding a switching regulator controller or MOSFET that works well at low voltages has been difficult until now. The new LT1619 provides a complete solution for low voltage and other applications requiring low-side MOS power transistors. The LT1619 is a 300kHz (externally synchronizable to frequencies as high as 500kHz) current mode PWM controller capable of operating from inputs ranging from 1.9V to 18V. Its features include a rail-to-rail 1A MOSFET driver capable of driving an external MOSFET gate to within 350mV of the supply rail and to within 100mV of ground. A separate driver supply pin (DRV) allows the gate voltage to be bootstrapped above the input voltage. A 50mV low-side current limit threshold reduces the sense resistor’s power dissipation, further improving efficiency. At light loads, the controller automatically switches to Burst Mode™ operation to conserve power. In shutdown, the LT1619 requires only 15µA of quiescent current. The LT1619 is available in 8-lead SO and MSOP packages. 3.3V to 5V Converters Figure 1 shows a 3.3V to 5V/2.2A boost supply using the LT1619. Low parts count, small size and high effiLinear Technology Magazine • September 2000 press switching transients caused by diode reverse recovery or parasitic circuit elements. ciency make it a perfect solution when a moderate amount of 5V power is required in a predominately 3.3V system. The output voltage is returned to the DRV pin, further enhancing M1. In Figure 2, the same basic circuit’s output is increased to 40W (5V/8A) by substituting higher current components. The highlighted loop is kept tight on the PC board to reduce switching transients produced by high pulsating currents. Efficiency remains above 86% for output currents between 0.1A and 5A (83% at 8A). The LT1619 operates smoothly by not entering current limit with 16A peak current through the 0.002Ω sense resistor. The gate charging current tends to produce spikes across the sense resistor at switch turn-on. The internal current sense amplifier is blanked for 280ns to prevent these spurious switching spikes from causing PWM jitter. Although this blanking sets a minimum switch on-time, the controller is capable of skipping cycles at light load with Burst Mode operation disabled. In situations where the internal leading-edge blanking is not long enough, a lowpass filter can be used on SENSE pin to further sup- R2 12.4k R1 37.4k 1 2 3 220pF RC 75k CG 15nF 4 S/S VIN DRV FB LT1619 VC GND GATE SENSE (714) 373-7334 C1: PANASONIC EEFCDOK220R (408) 986-0424 COUT: KEMET T495X227K010AS × 2 D1: ON SEMICONDUCTOR MBRD835L (602) 244-6600 Choosing the MOSFET The LT1619 is designed to drive an N-channel MOSFET with up to 60nC of total gate charge (Qg). Significant advances have been made in low voltage (<30V) power MOSFETs recently. 30mΩ, low voltage, low threshold FETs with less than 60nC of gate charge are readily available. Besides meeting voltage, current, gate drive and RDS(ON) requirements, choosing a transistor with Qg < 60nC will allow direct gate drive from the controller, resulting in a simpler and lower cost design. For transistors with Qg between 60nC and 80nC, first try driving the transistor from the controller before using an external driver. An external driver is recommended for MOSFETs with higher than 80nC of total gate charge. 5V to –48V Supply The LT1619 is not limited to low output voltage supplies. As the demand for networking equipment grows, the need arises for a –48V supply capable of powering telecommunication lines. VIN 3.3V 8 7 6 + 0.1µF C1 22µF L1 5.6µH 5A VOUT 5V 2.2A 0.1µF M1 D1 + 5 COUT 440µF RSENSE 0.01Ω L1: COILCRAFT DO5022P-562 M1: SILICONIX Si9804 (847) 639-6400 (800) 554-5565 Figure 1. High efficiency 3.3V to 5V DC/DC converter 11 DESIGN FEATURES 90 1 2 3 150pF RC 75k CG 15nF 4 VIN S/S FB DRV LT1619 GATE VC GND SENSE 8 + C1 300µF 1µF 7 89 87 VOUT 5V/8A 0.1µF 6 M1 VIN = 5.25V 88 L1 1µH EFFICIENCY (%) R2 12.4k R1 37.4k VIN 3.3V D1 + 5 RSENSE 0.002Ω 1W COUT 220µF ×4 86 85 VIN = 4.75V 84 83 VIN = 5.0V 82 81 80 C1: COUT: L1: D1: M1: SANYO POSCAP 6TPB150M ×2 SANYO POSCAP 10TPB220M ×4 SUMIDA CEPH149-1R0 ON SEMICONDUCTOR MBR1454CD FAIRCHILD FDS6680A 0 (847) 956-0667 (602) 244-6600 (408) 822-2126 200 300 400 LOAD CURRENT (mA) 500 600 Figure 4. Efficiency of Figure 3’s circuit Figure 2. 3.3V to 5V/8A DC/DC converter The circuit shown in Figure 3 is capable of delivering 24 watts at –48V from a 5V input. Although high current 5V sources are commonly available in many systems, lower input voltages generally mean higher input currents and lower efficiency. Fortunately, with a relatively simple topology and a 5V input, the circuit shown achieves well over 80% efficiency (see Figure 4). 100 THICK TRACES = HIGH CURRENT (SEE TEXT) (619) 661-6835 T1 stores energy during the ontime of Q1, which is transferred to two stacked 24V outputs to make –48V. C6 charges to a DC value equal to 29V (VIN + 24V), clamping T1’s leakage inductance spike and providing a path for input current during Q1’s off time. This results in continuous input current, reducing capacitor ripple current requirements. Reduced input ripple current (characteristic of this topol- ogy) demands sensing of switch current instead of input current. A number of other features improve the efficiency and performance of the circuit. D3 and R9 provide undervoltage lockout. Q2 and Q3 translate the –48V output to 1.2V required by the feedback pin (VFB) to regulate output voltage. The LT1619’s fixed frequency, current mode operation with internal slope compensation permits high duty cycle operation required in this application. T1 VOUT –48V + (561) 752-5000 (619) 661-6835 (602) 244-6600 (516) 435-1110 (800) 554-5565 C4 4.7µF FILM C5 470µF 35V D2 VIN 5V LT1619 7 8 C2 1500µF 6.3V R1 15Ω + T1: COILTRONICS CTX02-14261 (EFD20-3F3 6 WINDINGS, EACH 12µH) C1, C2, C5: SANYO MV-GX D1, D2: ON SEMICONDUCTOR MBRS340T3 D3: CENTRAL SEMICONDUCTOR CMPZ5229B Q1: SILICONIX SUD45N05-20L C1 470µF 35V D1 1nF + D3 1 4 DRV VIN S/S GND C R5 1M Q2 2N5210 R9 1.1k R8 36k C10 22nF COM C7 220pF R3 12k FB 2 V 3 COM C9 10µF R2 30Ω Q1 C6 4.7µF FILM GATE 6 SENSE 5 C8 2.2nF Q3 2N5210 R7 0.007Ω R6 10.5k 1% 432k 1% Figure 3. 24W, 4.75V to 5.25V in, –48V/5A out supply 12 Linear Technology Magazine • September 2000 DESIGN FEATURES T1: PHILIPS EFD20-3F3-A-100-S CORE SET (0.013" GAP, AL = 100nH/T2) (914) 246-2811 W4 6 TURNS TRIFILAR 28AWG 2mil POLYESTER FILM W3 24 TURNS 28AWG W2 24 TURNS 28AWG T1 R3 1k 1W C4 0.22µF W1 6 TURNS TRIFILAR 28AWG VIN 12V + R1 43Ω W1 W4 C1 150µF 20V D2 OUT COM C3 330pF R2 43Ω W3 W2 C5 330pF C2 2.2µF 40V D1 C6 1µF 50V C7 2.2µF 40V D3 IN COM R5 30k U1 LT1619 R10 510k 7 8 D4 1 4 DRV VIN S/S GND GATE 6 SENSE 5 FB 2 VC 3 10k VOUT1 –32.5V VOUT2 –65V C1: SANYO OSCON 20SV150M D1–D3: ON SEMICONDUCTOR MBRS1100T3 D4: CENTRAL SEMICONDUCTOR CMPZ5237B Q1 D6: CENTRAL SEMICONDUCTOR CMPZ5234B Q1: INTERNATIONAL RECTIFIER IRLRO24N Q2, Q3: 2N2222 ISO1: SEIMENS CNY17-3 –32.5V R17 51k (619) 661-6835 (602) 244-6600 (516) 435-1110 (310) 332-3331 (108) 257-7910 R6 10k Q2 R16 51k R8 62k ISO1 Q3 R15 51k C10 0.1µF R9 100Ω C11 1µF R11 0.008Ω C9 220pF R13 120Ω C8 470pF R12 470Ω U2 LT1431CZ R7 100Ω D6 6.2V R14 2.49k –65V Figure 5. Isolated SLIC flyback supply; VIN = 12V; VOUT = –32V and –65V (20W maximum) –32V and –65V Isolated Local SLIC Supply with UVLO Subscriber line interface circuit (SLIC) devices are used to provide telephone interface functions; they require negative power supplies for interface and ringing. Figure 5 satisfies these requirements by providing isolated –32.5V and –65V supplies from a 12V source. The supply is configured as a flyback converter. T1’s secondary turns ratio is 1:1. U2, ISO1 and associated circuitry provide feedback to U1, maintaining 32.5V across each secondary winding. The two secondaries are stacked to provide –65V. C6 is added to improve cross-regulation, even when most of the power is drawn from one winding. An additional benefit of the stacked windings is a lower voltage stress on output diodes and capacitors. Other output voltages can be realized by adjusting T1 and the feedback components. Linear Technology Magazine • September 2000 The value of primary current sense resistor, R11, is chosen to provide approximately 20 watts out with a 12V input. Power can be drawn from the –32.5V or –65V winding as required by the SLIC. Full load efficiency is 82% D4, R5, R10, R15–R17, Q2 and Q3 provide undervoltage lockout to ensure adequate gate voltage to Q1. The LT1619 has an internal undervoltage lockout (UVLO) threshold of 1.85V. Although the threshold is ideal for low voltage boost converters, it is too low when operating from a higher voltage power source. The shutdown/ synchronization pin (S/S) is used to modify the UVLO threshold. Shutdown is active low and, for normal operation, the S/S pin is tied to the input. The hysteretic UVLO circuit in Figure 5 has thresholds of 10V and 8.4V and operates on supply voltages as low as 0.9V. With VIN rising but below the upper threshold, Q2 is off and Q3 saturates. The S/S pin is pulled to the ground and the controller is shut off. As VIN crosses the upper threshold, Q2 turns on, Q3 turns off and the controller starts switching. The lower threshold is the VIN voltage that causes Q2 to switch off. Resistors R15–R17 and the Zener diode set the trip voltages. The collector voltage of Q3 is made 1.4V (above the maximum shutdown threshold at the S/S pin) at the lower UVLO threshold. With the addition of a capacitor on the VIN pin and a resistor in the path between the VIN pin and the input voltage, trickle-start from high voltage input sources (such as a 36V–72V telecom bus) is accommodated with the same basic circuit shown in Figure␣ 5. 13 DESIGN FEATURES C4, C5: VITRAMON VJ1825Y155MBX (1825/X7R) C6: TAIYO YUDEN LMK325BJ106MN (1210/X7R) C8: TAIYO YUDEN EMK316BJ105ML (1206X7R) D1: ON SEMICONDUCTOR MBRS340T3 D2: ON SEMICONDUCTOR MBRS0530T1 D3: 1N4687 4.3V LOW LEVEL (IZT = 50µA) Q1: ZETEX FMMT3904 Q2: ON SEMICONDUCTOR MMFT3055VL T1: COILTRONICS VP1-0190 (ER11/5, 6 WINDINGS EACH 12.2µH) VIN+ 4V TO 28V C4 1.5µF VIN– R3 5.6k (203) 268-6261 (408) 573-4150 7 10 12 2 1 5 4 3 8 6 11 (516) 543-7100 (800) 282-9855 (561) 752-5000 D2 8 1 D3 4 C8 1µF DRV VIN S/S GND D1 C5 1.5µF LT1619 7 9 (602) 244-6600 VOUT Q1 T1 GATE 6 SENSE 5 R7 30Ω FB 2 V 3 C7 220pF C VOUT+ Q2 R6 3.74k 1% C6 10µF VOUT– C1 0.022µF R9 2.2k C9 2.2nF R5 100Ω R8 0.015Ω R10 1.24k 1% Figure 6. 2.5W, 4V to 28V in, 5V/0.5A out supply 12V to 5V Automotive Supply Modifying Burst Mode Operation Figure 6 show a 5V, 0.5A SEPIC (single-ended primary inductance converter) supply designed to operate from a 12V battery. Once started, D2 provides voltage to the LT1619 and Q2, allowing the input voltage to drop as low as 4V. Q1 and D3 limit the start-up voltage to the LT1619 and, along with Q2 (60V), allow operation to 28V. C5 provides a path for continuous input current and directs T1’s leakage energy to the output. The result is increased efficiency and reduced input capacitor ripple current requirements. The LT1619’s 300kHz operating frequency allows for smaller magnetics (T1 is approximately 0.5in2) and smaller capacitors. In some applications, the high output ripple voltage or audible noise of Burst Mode operation is undesirable. Due to the unique design of the current sense amplifier, the LT1619 can be easily modified so that it does not burst at light load. In Figure 7, the input bias current of the currentsense amplifier is used to develop an offset voltage across an external resistor, ROS. This offset voltage makes the switch current appear higher to the sense amplifier, with the effect that the VC operating range is shifted upwards. The peak switch current before entering Burst Mode operation is greatly reduced. CURRENT SENSE AMPLIFIER – 14 The LT1619 solves many of the problems associated with low input voltage source DC to DC converters. Its numerous features make it an ideal choice for a wide range of applications requiring low-side MOS power transistors. ID + for the latest information on LTC products, visit www.linear-tech.com Conclusion IBIAS = 120µA IBIAS = 120µA 5 SENSE ROS RSENSE 4 GND Figure 7. Lowering Burst Mode operation current limit Authors can be contacted at (408) 432-1900 Linear Technology Magazine • September 2000