MAX1771 Просмотр технического описания (PDF) - Maxim Integrated

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MAX1771

12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller

Maxim Integrated 

12V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controller

VIN

4.5V TO 15V

OFF

ON

4

SHDN

C1

33µF

2 16V

V+

L1†

20µH

C2* D1

1µF 1N5819

VOUT

12V

250mA

MAX1771

EXT 1

Q1**

C5

0.1µF

5

REF

R3

28k

1%

8

CS

AGND 6

GND 7

FB

3

D2*

L2*

20µH C3 C4

R1

0.1Ω

100µF 100µF

16V 16V

NOTE: HIGH-

CURRENT GND

R2

200k

1%

1N4148

*SEE TEXT FOR FURTHER

NOTE: KEEP ALL TRACES CONNECTED

TO PIN 3 AS SHORT AS POSSIBLE

COMPONENT INFORMATION

**Q1 = MOTOROLA MMFT3055ELT1

† L1 + L2 = ONE COILTRONICS CTX20-4

Figure 6. 12V Buck/Boost from a 4.5V to 15V Input

carefully observe the component voltage ratings, since

some components must withstand the sum of the input

and output voltage (27V in this case). The circuit oper-

ates as an AC-coupled boost converter, and does not

change operating modes when crossing from buck to

boost. There is no instability around a 12V input.

Efficiency ranges from 85% at medium loads to about

82% at full load. Also, when shutdown is activated

(SHDN high) the output goes to 0V and sources no cur-

rent. A 1µF ceramic capacitor is used for C2. A larger

capacitor value improves efficiency by about 1% to 3%.

D2 ensures start-up for this AC-coupled configuration

by overriding the MAX1771’s Dual-Mode feature, which

allows the use of preset internal or user-set external

feedback. When operating in Dual-Mode, the IC first

tries to use internal feedback and looks to V+ for its

feedback signal. However, since V+ may be greater

than the internally set feedback (12V for the MAX1771),

the IC may think the output is sufficiently high and not

start. D2 ensures start-up by pulling FB above ground

and forcing the external feedback mode. In a normal

(not AC-coupled) boost circuit, D2 isn’t needed, since

the output and FB rise as soon as input power is

applied.

Transformerless -48V to +5V at 300mA

The circuit in Figure 7 uses a transformerless design to

supply 5V at 300mA from a -30V to -75V input supply.

The MAX1771 is biased such that its ground connec-

tions are made to the -48V input. The IC’s supply volt-

age (at V+) is set to about 9.4V (with respect to -48V)

by a zener-biased emitter follower (Q2). An N-channel

FET (Q1) is driven in a boost configuration. Output reg-

ulation is achieved by a transistor (Q3), which level

shifts a feedback signal from the 5V output to the IC’s

FB input. Conversion efficiency is typically 82%.

When selecting components, be sure that D1, Q1, Q2,

Q3, and C6 are rated for the full input voltage plus a

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

soft start the circuit to prevent excessive current surges

at power-up.

Battery-Powered LCD Bias Supply

The circuit in Figure 8 boosts two cells (2V min) to 24V

for LCD bias or other positive output applications.

Output power is boosted from the battery input, while

V+ voltage for the MAX1771 is supplied by a 5V or 3.3V

logic supply.

5V, 1A Boost Converter

The circuit in Figure 9 boosts a 2.7V to 5.5V input to a

regulated 5V, 1A output for logic, RF power, or PCMCIA

applications. Efficiency vs. load current is shown in the

adjacent graph.

14 ______________________________________________________________________________________

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