advertisement Triple Output 3.3V, 5V, and 12V High Efficiency Notebook Power Supply – Design Note 78 Randy G. Flatness The operating current levels for both regulator sections are user programmable, via external current sense resistors, to set current limit. A wide input voltage range for the LTC1142 allows operation from 4V to 16V. The LTC1142HV extends this voltage range to 20V, permitting operation with up to 12-cell battery packs. Both regulator blocks in the LTC1142 and LTC1142HV use a constant off-time current mode architectures with Burst Mode operation. This results in a power supply that has very high efficiency over a wide load current range, fast transient response, and very low dropout. The LTC1142 is ideal for applications requiring 5V and 3.3V output voltages with high conversion efficiencies over a wide load current range in a small amount of board space. The application circuit in Figure 2 is configured to provide output voltages of 3.3V, 5V, and 12V. The current capability of both the 3.3V and 5V outputs is 2A (2.5A peak). The logic controlled 12V output can provide 150mA (200mA peak), which is ideal for flash memory applications. The operating efficiency shown in Figure 1 exceeds 90% for both the 3.3V and 5V sections. 100 LTC1142 VIN = 8V 5V SECTION 95 90 EFFICIENCY (%) The new LTC®1142 is a dual 5V and 3.3V synchronous step-down switching regulator controller featuring automatic Burst Mode® operation to maintain high efficiencies at low output currents. Two independent regulator sections, each driving a pair of complementary MOSFETs, may be shut down separately to less than 20μA /output. This feature is an absolute necessity to maximize battery life in portable applications. Additionally, the input voltage to each regulator section can be individually connected to different potentials (20V maximum) allowing a wide range of novel applications. 85 LTC1142 VIN = 8V 3.3V SECTION 80 75 70 65 60 1 10 100 1000 2500 OUTPUT CURRENT (mA) %/t' Figure 1. LTC1142 Efficiency The 3.3V section of the circuit in Figure 2 is comprised of the main switch Q4, synchronous switch Q5, inductor L1, and current shunt RSENSE3. The current sense resistor RSENSE monitors the inductor current and is used to set the output current according to the formula IOUT = 100mV/RSENSE. Advantages of current control include excellent line and load transient rejection, inherent short-circuit protection and controlled start-up currents. Peak inductor currents for L1 and T1 of the circuit in Figure 2 are limited to 150mV/RSENSE or 3.0A and 3.75A respectively. When the output current for either regulator section drops below approximately 15mV/RSENSE, that section automatically enters Burst Mode operation to reduce switching losses. In this mode the LTC1142 holds both MOSFETs off and sleeps at 160μA supply current while the output capacitor supports the load. When the output capacitor discharges 50mV, the LTC1142 briefly turns this section back on, or “bursts” to recharge the output capacitor. The timing capacitor pins, which go to 0V during the sleep interval, can be monitored with an oscilloscope to observe burst action. As the load current is decreased the circuit will burst less and less frequently. L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 01/94/78_conv The timing capacitors CT3 and CT5 set the off-time according to the formula tOFF = 1.3 × 104 × CT. The constant off-time architecture maintains a constant ripple current while the operating frequency varies with input voltage. The 3.3V section has an off-time of approximately 5μs resulting in a operating frequency of 120kHz at 8V input voltage. The 5V section has an off-time of 3.5μs and a switching frequency of 107kHz at 8V input voltage. The LTC1142 synchronous switch removes the normal limitation that power must be drawn from the primary 5V inductor winding in order to extract power from the auxiliary winding. With synchronous switching the auxiliary 12V output may be loaded without regard to the 5V primary output load providing that the loop remains in continuous mode operation. When the 12V output is activated by a TTL high (6V maximum) on the 12V enable line, the 5V section of the LTC1142 is forced into continuous mode. A resistor divider composed of R1, R5 and switch Q1 forces an offset subtracting from the internal 25mV offset at pin 14. When this external offset cancels the built-in 25mV offset Burst Mode operation is inhibited. The operation of the 5V section is identical to the 3.3V section with inductor L1 replaced by transformer T1. The 12V output voltage is derived from an auxiliary winding on the 5V inductor T1. The output from this additional winding is rectified by diode D3 and applied to the input of an LT1121 regulator. The 12V output voltage is set by resistors R3 and R4. A turns ratio of 1:1.8 is used for T1 to ensure that the input voltage to the LT1121 is high enough to keep the regulator out of dropout while maximizing efficiency. 22μF VIN (6.5V TO 35V × 2 18V) + + 24 VIN3 VOUT3 3.3V/2A 23 L1 33μH RSENSE3 0.05Ω 1 2 P-DRIVE 5 SENSE + 3 SENSE + 5 + SENSE SENSE –5 N-DRIVE 5 P-GND3 S-GND3 CT3 4 3 25 ITH3 ITH5 27 510Ω 13 T1 CT5 S-GND5 P-GND5 17 11 18 1.8 T 30μH RSENSE5 0.04Ω R2 100< R1 100Ω VOUT5 5V/2A 15 1000pF –3 N-DRIVE 3 6 Q5 Si9410DY 220μF 10V × 2 Q2 Si9430DY 9 LTC1142HV 28 D1 MBRS140 10 VIN5 SHUTDOWN 5 P-DRIVE 3 2000pF 22μF 35V × 2 1μF 16 SHUTDOWN 3 + + 0V = NORMAL >1.5V = SHUTDOWN 1μF Q4 Si9430DY For additional high efficiency circuits see Application Note 54. 14 D2 MBRS140 20 Q3 Si9410DY Q1 VN7002 R5 18k 510Ω + 220μF 10V × 2 CT3 3300pF 3300pF CT5 270pF 390pF 12V ENABLE 0V = 12V OFF >3V = 12V ON (6V MAX) 12V/150mA RSENSE3: RSENSE5: L1: T1: KRL SL-1R050J KRL SL-1R040J COILTRONICS CTX-33-4 PRIMARY = 30RH PRIMARY: SECONDARY = 1:1.8 DALE LPE-6562-A026 22μF 25V + + 20pF VOUT R3 660k SHUTDOWN ADJ R4 300k LT1121 D3 MBRS140 22μF 35V 22Ω 1000pF VIN DN78 • F02 GND Figure 2. LTC1142 Triple Output High Efficiency Power Supply Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 361 dn78f_conv LT/TP 0194 190K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1994