MAX668 Maxim, MAX668 Datasheet - Page 16

• Manufacturer Part Number: MAX668
• Description: 1.8V to 28V Input / PWM Step-Up Controllers in MAX
• Manufacturer: Maxim

larger or smaller than the output voltage, such as when
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 N-channel MOSFET (Q1) must be selected to with-
stand a drain-to-source voltage (V
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.
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
Figure 6. MAX668 in SEPIC Configuration
16
1 F
______________________________________________________________________________________
D1: MBR5340T3, 3A, 40V SCHOTTKY DIODE
R4: WSL-2512-R020F, 0.02
C3: AVX TPSZ686M020R0150, 68 F, 150m ESR
100k
R3
0.22 F
1
2
4
5
DS
LDO
FREQ
REF
FB
) greater than the
9
V CC
GND
3
MAX668
10
SHDN
22 F x 3
@ 35V
PGND
7
CS+
EXT
8
6
4.9 H
CTX5-4
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.
3V to 25V
L1
V
IN
R4
0.02
Q1
30V
FDS6680
10 F @ 35V
C2
Isolated +5V to +5V Power Supply
520pF
C4
40V
D1
R1
75k
R2
25k
C3
68 F x 3
V
5V @ 1A
OUT