DESIGN FEATURES Versatile New Switching Regulator by Craig Varga Fits in SO-8 Introduction Linear Technology recently introduced the LTC1530 synchronous buck regulator controller. Although packaged in an 8-pin SO, it has proven to be remarkably capable and versatile. The part is loosely based on the popular LTC1430, but with numerous enhancements. Features include current limiting that senses the voltage across the RDS(ON) of the high-side MOSFET (no sense resistor required), built in soft-start, 1% accurate reference, gate drivers capable of handling large MOSFETs, and micropower shutdown. The error amplifier transconductance is higher than that of previous generation parts and is trimmed for accuracy and stabilized over temperature. The IMAX current, which programs current limit, has a positive temperature coefficient to Instead of the traditional current sense resistor, the LTC1530 relies on the RDS(ON) of the high-side MOSFET as its source of load current information. This saves the space, cost and the power dissipation of an additional resistor in the power path. The programming current (IMAX) has a positive temperature coefficient that approximates the positive TC of a MOSFET’s RDS(ON). This tends to flatten the current-limit trip point as a function of temperature. In a slight overload, the LTC1530 provides “square current limiting.” In other words, the regulator starts to look like a current source. In the event of a significant overload, should the output fall to less than one-half of the nominal output voltage, the soft-start capacitor will be discharged very quickly. This forces help cancel the positive temperature coefficient of the MOSFET’s RDS(ON). This allows for more consistent current limit over temperature. Although intended primarily for buck regulator designs, the part has been successfully designed into boost, positive-tonegative and negative-to-positive converters. A Quick Look at the Insides Figure 1 is the basic block diagram. The LTC1530 is a voltage mode control, synchronous buck regulator controller. An on-chip oscillator generates a 300kHz ramp waveform. The output of the error amplifier is compared to this ramp by the PWM comparator. So far, nothing extraordinary. Current-limit circuitry, however, is a little more unusual. DISDR LOGIC AND THERMAL SHUTDOWN INTERNAL OSCILLATOR 1 PVCC POWER DOWN 8 G1 – ICOMP PWM 7 G2 + COMP 4 ISS MSS CSS Ω gm = 2m ERR + MAX MIN – – + + – FB VREF VREF – 3% FB – 6 IFB VSENSE 3 VOUT FOR FIXED VOLTAGE VERSIONS CC + 3 VREF + 3% 5 IMAX IMAX MHCL HCL* MONO + LVC – VREF /2 VREF VREF – 3% VREF + 3% VREF /2 VREF 1530 BD *HCL = HARD CURRENT LIMIT Figure 1. LTC1530 block diagram 14 Linear Technology Magazine • February 1999 DESIGN FEATURES OPTIONAL, INSTALL IF NO 12V D2 BAT54S 12V C1 1µF 16V VIN 5V C4 1µF 16V + C11 4.7µF, 16V KEMET Ta + C12 0.22µF R4 750Ω C2–C3 330µF, 6.3V KEMET Ta ×2 C5 68pF 1 4 C6 1800pF 2 R5 10k Q3 2N7002 3 LTC1530S8 PVCC IMAX COMP G1 VFB IFB GND G2 5 8 6 Q1 IRF7805 7 R3 100Ω L1 3.5µH ETQP6F3R5SFA Q2 IRF7805 D1 MBRS130T3 R10 10k C7 270pF R1 16.5k 1% VREF = 1.233V R6 R2 R8 R9 1.0Ω R7 75k 68.1k 20.5k 11.3k + 1% 1% 1% 1% 3.3V 2.5V 1.5V 1.8V C13 2200pF JP1 JP2 JP3 VOUT 1.5V, 1.8V, 2.5V OR 3.3V AT 6A C8–C10 330µF 6.3V KEMET Ta ×3 ON/OFF Figure 2. 6A buck regulator; output voltage is jumper selectable for 1.5V, 1.8V, 2.5V or 3.3V. the regulator into shutdown for a period of time, typically a few milliseconds. After the time delay, the supply attempts to restart. If the overload still exists, the hiccup mode operation will continue. Once the short is removed, the regulator will start normally. Unlike its predecessor, the LTC1530’s soft-start capacitor is internal. The start-up rise time was chosen to satisfy the vast majority of application requirements. Turn on is clean, well controlled and monotonic. Since dynamic performance is of extreme importance in many of today’s systems, the LTC1530 incorporates several features to provide improved response times to load transients. First are the min/max comparators. Figure 3. Output voltage at turn-on for Figure 2’s circuit Linear Technology Magazine • February 1999 These are a pair of comparators that continuously monitor the output voltage. If the output is more than 3% on either side of nominal, the appropriate comparator forces the duty factor to maximum or zero in an attempt to restore the output to the correct level as quickly as possible. Eventually, the error amplifier and main feedback loop will catch up and force the output to settle nicely. The error amplifier is also an improvement over earlier designs. The transconductance and output impedance have both been increased substantially from the LTC1430 values. This has the effect of raising the DC open-loop gain of the amplifier, resulting in better line and load regulation. Transconductance is also trimmed to ensure accuracy. The result is more predictable and repeatable loop response. The amplifier gm is temperature compensated so loop gain stays nearly constant over temperature extremes. The LTC1530 also has a low power shutdown mode. If the Comp pin is pulled to ground with an open collector or open drain transistor, the LTC1530’s quiescent current will drop to approximately 45µA. Virtually all integrated circuits have some quirks that will get you in trouble if you don’t pay attention. The LTC1530 is no exception. Care must be taken in choosing the power MOSFETs used in circuits that depend on a charge pump to supply gate-drive power. It is essential to select a FET for the upper device that will be almost fully enhanced before the PVCC supply voltage reaches 8V with whatever main input voltage happens to be available. Failure to heed this requirement can lead to a circuit that may not start up properly at all times. Standard logic-level FETs work fine. Be sure VTH is less than 2V in the worst case. The cause of this start-up phenomenon is related to the way the current limit circuit behaves. Below a PVCC level of 8V, current limit is disabled. Assume for the sake of this discussion that the main input supply is derived from 5V. At turn on, as the charge pump gradually pushes the PVCC supply upward, the currentlimit circuit wakes up at 8V on PVCC. If the 5V supply is exactly 5V, the gate drive available for the FET is only 3V (8V – 5V). If the FET’s RDS(ON) is very high relative to its nominal value at this point, the current-limit circuit may activate in a misguided attempt to maintain control of the output current. If, at the same time, the output voltage has come up to less than onehalf of its final value, the LTC1530 will respond by discharging the soft15 DESIGN FEATURES D1 BAT54S + C12 1µF 16V C11 4.7µF 16V C11 0.1µF C2, C3, C8–C10: PANASONIC SP TYPE (201) 348-7522 L1: GOWANDA 50-324 OR DALE IHLP2525 (SEE TEXT) VIN 5V + C4 1µF 10V (716) 532-2234 (605) 665-1627 R4 750Ω C2–C3 47µF, 6.3V ×2 1 4 C5 4700pF C7 68pF 3 2 LTC1530S8 PVCC COMP VFB GND IMAX G1 IFB G2 5 8 6 Q1A Si4936DY 7 R5 100Ω R2 10k L1 4.7µH VOUT 3.3V/3A C6 470pF Q1B Si4936DY VREF = 1.233V R1 16.9k 1% + R7 10k 1% C8–C10 56µF 4V ×3 Figure 4. 3.3V/3A regulator start capacitor and trying to initiate a restart. As long as the output voltage has reached a level of greater than onehalf of its final value before the PVCC voltage reaches 8V, the output will continue to rise in current limit. If the output is below this level, start-up is not ensured. If the PVCC supply is derived from a 12V source instead of charge pumped from the 5V supply, this problem cannot occur. A Few Circuit Examples The LTC1530 turns out to be a rather versatile device. Although intended as a buck regulator, the part has been successfully used in boost and buckboost designs. Figure 2 is a classic buck topology. The circuit was designed to handle approximately 6A while maintaining a low profile. Input and output capacitors are tantalum devices. The inductor is a very low DC resistance design for high efficiency. The input is 5V, while the output voltage can be jumper selected for 3.3V, 2.5V, 1.8V or 1.5V. The photo in Figure 3 shows the output voltage rise at turn on. A clean, monotonic rise is evident. Figure 4 is a 3A design that has a total height of less than 2.4mm. The inductor is a Gowanda part #50-324, which mounts through a hole in the PCB for a total height above the board of approximately 1.5mm. Output ripple voltage is approximately 10mVP-P at a 3A load with the specified Panasonic SP series output capacitors. There are several options for the main inductor. The overall smallest size available is an IHLP-2525 by Dale Electronics. It’s 3mm tall but only 6.4mm on a side. Output ripple is about 50% higher with this inductor. Figure 5 is an example of a synchronous boost regulator. The input is 3.3V and the output is 5V. The circuit is rated for a maximum output current of 6A. Since the output curL1* 2.5µH VIN 3.3V + C3 1µF 16V C4 1µF X7R 1 2 3 LTC1517-5 5 VIN C1– C1–C2 470µF 16V ×2 C6 0.22µF 4 C1+ VOUT C9 1µF X7R 4 C15 100pF C14 0.022µF 3 2 D3 FMM914 + C12 10µF 16V C13 1µF 16V C5 0.22µF D1 MBR0530 5 GND D2 MBR0530 RS1 (OPTIONAL, SEE TEXT) LTC1530S8 IMAX PVCC COMP G2 VFB IFB GND G1 L2** 10µH 1 7 6 8 R1 12k Q2 IRF7801 VREF = 1.233V R2 71.5k 1% + + R3 23.2k 1% *L1: PULSE PE-53681 (619) 674-8100 **L2: COILCRAFT DS3316P-102 (847) 639-6400 VOUT 5V/6A Q1 IRF7801 C10 470µF 6.3V C7, C8, C11 470µF 6.3V ×3 Figure 5. 5V/6A synchronous boost regulator 16 Linear Technology Magazine • February 1999 DESIGN FEATURES rent waveform is discontinuous, the output ripple is inherently large in any boost regulator. The second stage LC filter is added to clean things up a bit. The feedback divider connects to the output before the LC filter for a reason. If the divider is connected after the LC filter, the extra 180° of phase shift above the LC corner frequency will make the regulator’s feedback loop unstable. The DC resistance of the inductor is small, so the effect on load regulation is minimal. The LTC1517 charge pump is used to generate a sufficiently high voltage for the LTC1530 to function correctly and also to ensure adequate gate drive for the power MOSFETs. It runs from the 3.3V input and delivers a regulated 5V output. Once the main output comes into regulation, charge pump power is derived through D2. This causes the LTC1517’s regulated output voltage set-point to be exceeded and D3 back biases, shutting the LTC1517 down. Note that current limit is disabled in this design by grounding IMAX and connecting IFB to VIN. Since in the boost topology there is a direct DC path from input to output, there is no point in using the current limit feature except to protect against inductor saturation. It is also worth mentioning that the FET RDS(ON) cannot be used as the current sense resistor in this application because FET Q2’s drain is not common to VIN. If inductor saturation protection is desirable, it is possible to install a small value current sense resistor between C2 and L1. Install an appropriate value resistor (RS1) between C2 and L1; connect the IMAX pin to the C2 side of RS1 (instead of ground) and connect IFB directly to the input side of L1. Just don’t expect the circuit to limit current in the event of a short circuit. Figure 6 is a positive input to negative 5V output design. Since the LTC1530 needs to be referenced to the –5V output, the design requires external gate-drive circuitry for both the main and synchronous FETs. The absolute maximum voltage rating of the LTC1530’s gate drive would be exceeded if the high-side gate were driven directly. Q3 and the associated parts at the input to the LTC1693 gate driver provide the required levelshift function. The synchronous FET is driven by the other half of the LTC1693. The driver is only required at this location to match the propagation delay of the high-side drive. Failure to pay attention to these details will result in severely degraded efficiency. Output currents of up to 4A can be obtained from this circuit. Like the boost regulator, the output currents are discontinuous, so ripple on the output is somewhat high. A small, second-stage LC filter can easily remedy this if desired. Conclusion The LTC1530 is a small, versatile controller that is usable in numerous topologies and over a wide range of power levels. In the basic buck applications for which it was designed, the LTC1530 permits the designer to realize very simple, low parts count designs that require minimal real estate. The part provides clean turnon and current-limit characteristics. With a little ingenuity, it is possible to develop circuits different than those that the part’s designers intended, but which give excellent performance nonetheless. VIN 5V R8 470Ω 1/4W C7 4.7µF + R1 2.7k R3 1.3k C3 10µF C4 0.1µF LTC1530-ADJ 5 PVCC IMAX 4 8 COMP G1 3 6 VSENSE IFB 2 7 GND G2 R4 2.0k RC 4.7k Q1 Q3 D2 2N7002 MBR0530T1 1 C1 1000pF C2 27µF 5 CIN 4 1/2 LTC1693-2 R9 1k Q4 2N3906 + 3 + Q2 R7 3.3Ω L1 2.5µH C8 4.7µF + CC 0.22µF 1 8 7 COUT VOUT –5V/5A + + 6 D1 MBR0530T1 C6 0.1µF R10 47Ω 2 1/2 LTC1693-2 R5 2.96k D4 1N4148 D3 1N4148 + R6 1k C5 6.8µF R2 100Ω L1: PANASONIC ETQP6F2R5FA (201) 348-7522 CIN: 3× SANYO 10MV1200GX COUT: 4× SANYO 6MV1500GX (619) 661-6835 Q1, Q2: SUD50N03-10 Figure 6. 5V to –5V/4A synchronous switching, inverting polarity converter Linear Technology Magazine • February 1999 17