1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
larger or smaller than the output voltage, such as when
Isolated +5V to +5V Power Supply
converting four NiMH, NiCd, or Alkaline cells to a 5V
output. The SEPIC configuration is often a good choice
for combined step-up/step-down applications.
The circuit of Figure 7 provides a 5V isolated output at
400mA from a 5V input power supply. Transformer T1
provides electrical isolation for the forward path of the
converter, while the TLV431 shunt regulator and
MOC211 opto-isolator provide an isolated feedback
error voltage for the converter. The output voltage is set
by resistors R2 and R3 such that the mid-point of the
divider is 1.24V (threshold of TLV431). Output voltage
can be adjusted from 1.24V to 6V by selecting the
proper ratio for R2 and R3. For output voltages greater
than 6V, substitute the TL431 for the TLV431, and use
2.5V as the voltage at the midpoint of the voltage-
divider.
The N-channel MOSFET (Q1) must be selected to with-
stand a drain-to-source voltage (V ) greater than the
DS
sum of the input and output voltages. The coupling
capacitor (C2) must be a low-ESR type to achieve max-
imum efficiency. C2 must also be able to handle high
ripple currents; ordinary tantalum capacitors should not
be used for high-current designs.
The circuit in Figure 6 provides greater than 1A output
current at 5V when operating with an input voltage from
3V to 25V. Efficiency will typically be between 70% and
85%, depending upon the input voltage and output cur-
rent.
8/MAX69
V
IN
3V to 25V
22µF x 3
@ 35V
L1
4.9µH
CTX5-4
D1
40V
9
10
V
OUT
5V @ 1A
V
LDO
SHDN
CC
C2
1
2
10µF @ 35V
C3
68µF x 3
Q1
30V
FDS6680
FREQ MAX668
REF
8
6
EXT
CS+
1µF
4
5
R3
100k
0.22µF
R4
0.02Ω
FB
GND
3
R1
75k
PGND
7
C4
520pF
R2
25k
D1: MBR5340T3, 3A, 40V SCHOTTKY DIODE
R4: WSL-2512-R020F, 0.02Ω
C3: AVX TPSZ686M020R0150, 68µF, 150mΩ ESR
Figure 6. MAX668 in SEPIC Configuration
16 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
