Rainbow Electronics MAX1771 - Manual (Page 14)

MAX1771
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 t
rr 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.
12V or Adjustable, High-Efficiency,
Low I
Q , Step-Up DC-DC Controller
14 ______________________________________________________________________________________
MAX1771
SHDN
R1
0.1
Ω
REF
AGND
R2
200k Ω
1%
R3
28k Ω
1%
GND
4
ON
OFF
5
FB
NOTE: HIGH-
CURRENT GND
Q1**
*SEE TEXT FOR FURTHER
COMPONENT INFORMATION
**Q1 = MOTOROLA MMFT3055ELT1
L1 + L2 = ONE COILTRONICS CTX20-4
3
6
EXT
CS
1
L1
20
µH
8
2
D1
1N5819
D2*
1N4148
C3
100
µF
16V
C1
33 µF
16V
C2*
1 µF
C5
0.1 µF
V+
V IN
4.5V TO 15V
V OUT
12V
250mA
L2*
20
µH
7
C4
100
µF
16V
NOTE: KEEP ALL TRACES CONNECTED
TO PIN 3 AS SHORT AS POSSIBLE
Figure 6. 12V Buck/Boost from a 4.5V to 15V Input