Datasheet MAX1771 (Maxim) - 14

MAX1771
12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller
tries to use internal feedback and looks to V+ for its feedback signal. However, since V+ may be greater VIN 4.5V TO 15V than the internally set feedback (12V for the MAX1771), the IC may think the output is sufficiently high and not C1 start. D2 ensures start-up by pulling FB above ground 33µF and forcing the external feedback mode. In a normal L1 2 16V 20µH (not AC-coupled) boost circuit, D2 isn’t needed, since OFF V 4 V+ OUT the output and FB rise as soon as input power is C2* D1 SHDN 12V 1µF 1N5819 250mA applied. ON MAX1771
Transformerless -48V to +5V at 300mA
1 Q1** EXT The circuit in Figure 7 uses a transformerless design to 8 L2* supply 5V at 300mA from a -30V to -75V input supply. CS 20 C3 µH C4 6 100µF 100µF The MAX1771 is biased such that its ground connec- AGND R1 16V 16V tions are made to the -48V input. The IC’s supply volt- 5 7 0.1Ω REF GND age (at V+) is set to about 9.4V (with respect to -48V) C5 FB 0.1 NOTE: HIGH- µF by a zener-biased emitter follower (Q2). An N-channel R3 3 CURRENT GND FET (Q1) is driven in a boost configuration. Output reg- 28kΩ 1% R2 ulation is achieved by a transistor (Q3), which level 200kΩ shifts a feedback signal from the 5V output to the IC’s D2* 1% 1N4148 FB input. Conversion efficiency is typically 82%. *SEE TEXT FOR FURTHER COMPONENT INFORMATION When selecting components, be sure that D1, Q1, Q2, NOTE: KEEP ALL TRACES CONNECTED **Q1 = MOTOROLA MMFT3055ELT1 TO PIN 3 AS SHORT AS POSSIBLE Q3, and C6 are rated for the full input voltage plus a L1 + L2 = ONE COILTRONICS CTX20-4 reasonable safety margin. Also, if D1 is substituted, it should be a fast-recovery type with a trr less than 30ns. R7, R9, C8, and D3 are optional and may be used to Figure 6. 12V Buck/Boost from a 4.5V to 15V Input soft start the circuit to prevent excessive current surges carefully observe the component voltage ratings, since at power-up. some components must withstand the sum of the input
Battery-Powered LCD Bias Supply
and output voltage (27V in this case). The circuit oper- The circuit in Figure 8 boosts two cells (2V min) to 24V ates as an AC-coupled boost converter, and does not for LCD bias or other positive output applications. change operating modes when crossing from buck to Output power is boosted from the battery input, while boost. There is no instability around a 12V input. V+ voltage for the MAX1771 is supplied by a 5V or 3.3V Efficiency ranges from 85% at medium loads to about logic supply. 82% at full load. Also, when shutdown is activated (SHDN high) the output goes to 0V and sources no cur-
5V, 1A Boost Converter
rent. A 1µF ceramic capacitor is used for C2. A larger The circuit in Figure 9 boosts a 2.7V to 5.5V input to a capacitor value improves efficiency by about 1% to 3%. regulated 5V, 1A output for logic, RF power, or PCMCIA D2 ensures start-up for this AC-coupled configuration applications. Efficiency vs. load current is shown in the by overriding the MAX1771’s Dual-Mode feature, which adjacent graph. allows the use of preset internal or user-set external feedback. When operating in Dual-Mode, the IC first 14 Maxim Integrated