Datasheet
MAX1763
The output capacitance can be determined by the
function:
C
OUT
≥ [ (V
REF
/ V
OUT
)
✕
G
M
✕
G
FS
✕
C
G
✕
(R
G
✕
2) ]
and
C
OUT
≥ 10
✕
[ (V
REF
/ [V
OUT
✕
GBP])
✕
G
M
✕
G
FS
✕
R
G
]
where V
REF
is the 0.983V reference voltage, G
M
is the
10mS internal amplifier transconductance, G
FS
is the
external MOSFET transconductance, R
G
is the gate-
source resistor, and GBP is the gain-bandwidth prod-
uct of the internal gain block, 63Mrad/s.
__________________
_
Design Procedure
Setting the Output Voltage
For a fixed 3.3V output, connect FB to GND. To set the
output voltage between 2.5V and 5.5V, connect a resis-
tor voltage-divider to FB from OUT to GND (Figure 6).
The input bias current into FB is less than 100nA, allow-
ing 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 30kΩ or less, then calculate R1 using:
R1 = R2((V
OUT
/ V
FB
) - 1)
where V
FB
, the boost-regulator feedback set point, is
1.245V.
Setting the Switch Current Limit
and Soft-Start
The ISET pin adjusts the inductor peak current and can
also be used to implement soft-start. With ISET con-
nected to REF, the inductor current limits at 2.5A. With
ISET connected to a resistive divider set from REF to
GND, the current limit is reduced according to:
I
LIM
= 2.5(V
ISET
/ 1.25) [A]
Implement soft-start by placing a resistor from ISET to
REF (>300kΩ) and a capacitor from ISET to GND. In
shutdown, ISET is discharged to GND through an inter-
nal 100kΩ resistor. As the capacitor voltage rises, the
output current is allowed to increase, and the output
voltage rises. The speed at which the output rises is
determined by the soft-start time constant:
t
SS
= R
SS
C
SS
where R
SS
≥ 300k.
Both features may be implemented simultaneously by
placing a capacitor across the lower resistor of the cur-
rent-limiting resistive divider (Figures 7 and 8).
Package Selection
The MAX1763 is available in two packages, a 16-pin
QSOP and a 16-pin TSSOP-EP. Since the MAX1763
has excellent efficiency, most applications are well
served by the QSOP package. If the application
requires high power dissipation, or operation in a high
ambient temperature, choose the TSSOP-EP package.
The TSSOP-EP is equipped with an exposed metal pad
on its underside for soldering to grounded circuit board
copper. This reduces the junction-to-case thermal
resistance of the package from +115°C/W for QSOP to
+53°C/W for the TSSOP-EP.
1.5A, Low-Noise, 1MHz, Step-Up
DC-DC Converter
12 ______________________________________________________________________________________
MAX1763
OUT
FB
R2
R1
R1 = R2
(
- 1
)
V
OUT
V
FB
, V
FB
= 1.245V, R2 ≤ 30k
MAX1763
REF
ISET
R
SS
C
SS
0.22μF
I
LIM
= 2.5A
t
SS
= R
SS
C
SS
MAX1763
REF
ISET
R
SS1
C
SS
0.22μF
I
LIM
= 2.5A
(
)
R
SS2
R
SS1
+ R
SS2
t
SS
= (R
SS1
R
SS2
)C
SS
R
SS2
Figure 7. Soft-Start with Maximum Switch Limit Current
Figure 8. Soft-Start with Reduced Switch Limit Current
Figure 6. Connecting Resistors for External Feedback