advertisement Floating Input Extends Regulator Capabilities – Design Note 21 Brian Huffman however, if the secondary windings are isolated from one another, a low dropout positive voltage regulator can be used for negative regulation (Figure 1). Many applications require circuit performance that is unachievable with conventional regulator design. This results in added complexity to the circuit. However, some problems can easily be solved by floating the input to the regulator. A floating input can either be a battery, or a secondary winding that is galvanically isolated from all other windings. With this method high efficiency negative voltage regulation, high voltage regulation, and low saturation loss positive buck switching regulator can all be achieved easily. In this circuit the LT®1086 servos the voltage between the output and the adjust pin to 1.25V. The positive regulation is accomplished by conventional regulator design. Negative voltage regulation is achieved by connecting the output of the positive voltage regulator to ground. The VIN pin floats to 1.5V or greater, above ground. This technique can be used with any positive voltage regulator, although highest efficiency occurs with low dropout types. Low dropout negative voltage regulators are not currently available. This would seem to preclude high efficiency negative linear regulators. Such regulation is frequently desired in switching supply post regulators; L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. FEEDBACK PATH MUR410 5V OUTPUT (TYPICAL) + 470μF MUR410 VIN LT1086 ADJ + 470μF +VIN 12V 1.5A VOUT 124Ω* + 10μF + 10μF 1N4002 10μF 1N4002 1.07k* MUR410 VIN SWITCHING REGULATOR LT1086 VOUT ADJ + 470μF 124Ω* + 10μF + 1.07k* * = 1% FILM RESISTORS DN021 F01 Figure 1. High Efficiency Negative Voltage Regulation 04/89/21_conv –12V 1.5A Another example where floating a linear regulator can be useful is shown in Figure 2. In this case high voltage regulation can be handled if split secondary windings are available. This allows the regulators to be connected in series. Neither regulator exceeds its maximum differential voltage even under short circuit conditions. High current positive buck switching regulators can have excessive saturation losses since most switches are Darlingtons. As much as 2V can be dropped across a Darlington or composite PNP switching transistor. However, efficiency can be increased and power dissipation requirements greatly reduced if the input is allowed to float (Figure 3). ~ 18VAC + VIN LT1086 R1 100Ω 1000μF 115VAC ~ – ~ + VOUT 5V–45V VOUT ADJ + MDA201 The circuit in Figure 3 uses an LT1070 to perform a buck conversion. The LT1070 is a current mode switching regulator. The VSW pin output is a collector of a common emitter NPN, so current flows through it when it is low. The 40kHz repetition rate is set by the LT1070’s internal oscillator. When the VSW pins “on,” current flows through the load, the inductor, and into the VSW pin. During this time a magnetic field is built up in the inductor. When the switch is turned “off,” the magnetic field collapses dumping energy into the load through D1. The input of the switching regulator floats to a potential set by the output. R2 2k R3 2k 1W + 10μF 1N4002 10μF 1N4002 STANCOR P-8685 18VAC VIN VOUT ADJ + MDA201 LT1086 + 1000μF ~ – 360Ω 5W R4 2k 1W VOUT = 2[1.25V(R2/R1 + 2)] IF R3 = R4, IADJ = 0 DN021 F02 Figure 2. High Voltage Regulation ~ 115VAC MDA801 ~ VOUT 5V 4A + VIN 12V (10V-40V) – VIN RL 4700μF 3.9k* VSW + 4700μF * = 1% FILM RESISTORS MBR360 = MOTOROLA L1 = PULSE ENGINEERING #PE-92113 + D1 MBR745 L1 170μH LT1070 2N5401 VFB GND VC 1k 1.1k* 1μF DN021 F03 Figure 3. Floating Input Low Saturation Loss Buck Regulator Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn21f_conv IM/GP 0389 165K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1989