DESIGN IDEAS Wide-Input-Range, Low Voltage by Kurk Mathews Flyback Regulator This constant-frequency current mode controller includes a high-side differential current sense amplifier and a floating high current N-Channel MOSFET driver. In the buck mode, an external bootstrap capacitor between the BOOST and SW pins works in conjunction with the internal 5.6V regulator and diode to provide a regulated supply for a high-side driver. In the boost, buck-boost or flyback mode, the SW pin is grounded, providing drive for a low-side MOSFET. An example of a wide-input-range flyback is shown in Figure 2. The Many new switching regulators are designed with a specific application or topology in mind. If your requirements happen to fall within these parameters, all is well. Unfortunately, when faced with unusual requirements, the designer is often forced to choose bare-bones, universal regulators. The LTC1624 overcomes these issues by providing a full featured regulator that can operate in the step-down (buck), step-up (boost), buck-boost or flyback mode. The functional diagram in Figure 1 reveals the flexibility of this device. circuit provides ±50V at 75mA from a 4.75 to 24V source. The sum of line-, load- and cross-regulation is better than ±5%. The SW pin voltage is controlled by the internal 5.6V regulator, allowing the input voltage to be above Q1’s 16V maximum gate-tosource voltage rating. 200kHz fixed frequency operation minimizes the size of T1. The R-C snubber formed by C1 and R1 in combination with T1’s low leakage inductance keeps Q1’s drain voltage well below its 100V rating. To improve cross-regulation, Q2, R2 and R3 were included to disable continued on page 31 VIN + RSENSE CIN SENSE – VIN 1 8 1.19V REF 2.5µA 4k – I1 – 8k + VIN I2 4k 5.6V INTVCC REG INTVCC DB + BOOST SLOPE COMP 1.19V 7 3µA 180k 3µA CB + RUN – FLOATING DRIVER – 0.8V ST TG 6 + 8k 30k 1.19V 1.5V + gm = 1m Ω 2 VOUT + COUT 1.19V R EA Q – S CC DROPOUT DET 200kHz 1.28V + VFB OV – 3 SLOPE COMP OSC VFB R1 5 SWITCH LOGIC D1 + RC L1 SW B – ITH /RUN N-CHANNEL MOSFET R2 COSC 200kHz INTVCC 1-SHOT 400ns N-CHANNEL MOSFET GND 4 1624 FD Figure 1. LTC1624 function diagram Linear Technology Magazine • May 1998 27 DESIGN IDEAS Experimental Results 10 0 Figure 7, curve A, shows the amplitude response of the filter hardware illustrated in Figure 8. No attempt was made to adjust any component. Both notches are fully resolved, but due to the tolerances of the components and the finite bandwidth of the active circuitry, the stopband attenuation, although impressive, is 2dB above the theoretical value. Subsequently, the value of RQ1 was lowered to 16.2k (curve B) to better define the –10 GAIN (dB) –20 –30 –40 –50 –60 –70 –90 B A –80 70 80 90 100 250 200 FREQUENCY (kHz) Figure 7. Amplitude response of Figure 7’s filter before (A) and after (B) RQ1 was lowered to 16.2k to better define the notch. first notch. The filter reaches attenuation levels beyond 85dB all the way up to 0.5MHz input frequencies. The measured attenuation at 1MHz was still better than 78dB. The dynamic range of the circuit is quite impressive: the measured wideband noise was 40µVRMS and the THD for 1VRMS and 50kHz input signal was better than –80dB. 1 Hauser, Max. “Universal Continuous-Time Filter Challenges Discrete Designs.” Linear Technology VIII:1 (February 1998), p.1. RFF2, 15k reduced to 3.97 for reasons mentioned above and for improving the transient response of the circuit. See Figure 7 for the amplitude response; note the slight rolloff at the cutoff frequency. Figure 8 shows the complete hardware realization using all four sections of an LTC1562 continuous-time quad Operational Filter IC. The algorithm outlined above was followed to calculate the values of the external passive components. The circuit occupies as much real estate as a U.S. dime. This is quite significant considering the cumbersome alternative of a fully discrete realization with op amps, Rs and Cs. CIN1, 68pF RIN2, 7.68k 1 VIN RIN1, 34k RQ1, 17.4k 2 R21,10k 3 4 5V 0.1µF 6 RIN3, 102k 7 RQ3, 35.7k 8 INV C BP B BP C LP B LP C V– V + LTC1562 5 R23, 8.25k INV B SHDN AGND LP A LP D BP A BP D INV A CIN3, 39pF INV D 16 15 RQ2, 11k 14 R22, 15k 13 –5V 0.1µF 12 11 10 R24, 26.1k 9 RQ4, 8.87k RIN4, 9.09k VOUT RFF4, 26.1k 1562 TA03 ALL RESISTORS = 1% METAL FILM 603 SURFACE MOUNT ALL CAPACITORS = 5% 805 SURFACE MOUNT Figure 8. Hardware realization of the filter in Figure 6, using all four sections of an LTC1562. 220pF Flyback, continued from page 27 47Ω T1 12 50V/75mA MURS120T3 18k 1 VIN 4.75V–24V 330µF 35V SANYO MV-GX 0.02Ω + R3 220Ω 1µF 8 1 SENSE– VIN BOOST LTC1624 TG SW ITH/RUN GND 2 4 7 6 5 1µF C1 220pF Q2 MPS2222A 15k 11 10 4 2 8 9 5 10Ω 1µF MURS120T3 18k 7 Q1 IRL540N VFB 3 1k 0.01µF 1µF R1 47Ω 3 1µF 6 –50V/75mA 620k R2 43k T1: COILTRONICS VP3-0138, 1:1:1:1:1:1 (SIX WINDINGS, EACH 11.2µH) Figure 2. Wide-input-range flyback regulator provides ±50V at 75mA. Burst Mode™ operation (a feature that improves efficiency at light load conditions by skipping switching cycles). The LTC1624’s 95% maxiLinear Technology Magazine • May 1998 mum duty cycle accommodates the 5to-1 input voltage range. Finally, by reconfiguring T1’s secondaries, a variety of output configurations, such as 24V out (four windings in parallel), single 50V/150mA or a single 100V output, are possible with this same basic circuit. 31