Design procedure – Rainbow Electronics MAX1760 User Manual
Page 9
Reference
The MAX1760 has an internal 1.250V ±1% reference.
Connect a 0.22µF ceramic bypass capacitor from REF
to GND within 0.2in (5mm) of the REF pin. REF can
source up to 50µA of external load current.
Design Procedure
Setting the Output Voltages
For a fixed 3.3V output, connect FB to GND. To set
other output voltages between 2.5V and 5.5V, connect
a resistor voltage-divider to FB from OUT to GND
(Figure 3). The input bias current into FB is <20nA,
allowing large-value divider resistors without sacrificing
accuracy. Connect the resistor voltage-divider as close
to the IC as possible, within 0.2in (5mm) of FB. Choose
R2 of 270k
Ω or less, then calculate R1 using:
where V
FB
, the boost-regulator feedback set point, is
1.24V.
Setting the Switch Current Limit
and Soft-Start
The ISET pin adjusts the inductor current limit and
implements soft-start. With ISET connected to REF, the
inductor current limits at 1.25A. With ISET connected to
a resistive divider set from REF to GND, the current limit
is reduced according to:
Implement soft-start by placing a resistor from ISET to
REF and a capacitor from ISET to GND. In shutdown,
ISET is discharged to GND through an on-chip 100k
Ω
resistor. At power-up, ISET is 0V and the LX current is
zero. As the capacitor voltage rises, the current
increases and the output voltage rises. The soft-start
I
1.25A
V
1.25V
LIM
ISET
=
R1 R2
V
V
1
OUT
FB
=
−
MAX1760
0.8A, Low-Noise, 1MHz,
Step-Up DC-DC Converter
_______________________________________________________________________________________
9
Figure 3. Connecting Resistors for External Feedback
ISET
REF
MAX1760
R
SS
C
SS
0.22
µF
t
SS
=
R
SS
C
SS
R
SS
≥ 470k
I
LIM
= 1.25A
Figure 4. Soft-Start with Maximum Switch Limit Current
ISET
REF
MAX1760
R
SS1
C
SS
0.22
µF
R
SS2
t
SS
= (
R
SS1
II
R
SS2
)
C
SS
I
LIM
= 1.25A
R
SS1
+
R
SS2
R
SS2
( )
R
SS1
≥ 470k
Figure 5. Soft-Start with Reduced Switch Current Limit
FB
PGND GND
POUT
LX
ISET
OUT
REF
CLK/SEL
V
IN
= 2.4V
V
OUT
ON
MAX1760
100
µF
3.3
µH
0.68
µF
4.7
Ω
33
µF
0.22
µF
R1
R2
-
1
R1 = R2
( )
VOUT
VFB
V
FB
= 1.24V