Rainbow Electronics MAX1627 User Manual
Page 9
least 100 times FB’s input bias current. Capacitor C
R2
is used to compensate the MAX1627 for even switch-
ing. Values between 0pF and 330pF work for many
applications. See the
Stability and MAX1627 Feedback
Compensation section for details.
Current-Sense-Resistor Selection
The current-sense comparator limits the peak switching
current to V
CS
/R
SENSE
, where R
SENSE
is the value of
the current-sense resistor and V
CS
is the current-sense
threshold. V
CS
is typically 100mV, but can range from
85mV to 115mV. Minimizing the peak switching current
will increase efficiency and reduce the size and cost of
external components. However, since available output
current is a function of the peak switching current, the
peak current limit must not be set too low.
Set the peak current limit above 1.3 times the maximum
load current by setting the current-sense resistor to:
Alternatively, select the current-sense resistor for 5V
and 3.3V output applications using the current-sense
resistor graphs in Figures 6a and 6b. The current-sense
resistor’s power rating should be 20% higher than:
Standard wire-wound resistors have an inductance
high enough to degrade performance, and are not rec-
ommended. Surface-mount (chip) resistors have very
little inductance and are well suited for use as current-
sense resistors. Power metal-strip resistors feature
1/2W and 1W power dissipation, 1% tolerance, and
inductance below 5nH. Resistance values between
10m
Ω
and 500m
Ω
are available.
Inductor Selection
The essential parameters for inductor selection are
inductance and current rating. The MAX1626/MAX1627
operate with a wide range of inductance values. In many
applications, values between 10µH and 68µH take best
advantage of the controller’s high switching frequency.
Calculate the minimum inductance value as follows:
where 2µs is the minimum on-time. Inductor values
between two and six times L
(MIN)
are recommended.
L
=
V+
- V
(MIN)
(MAX)
OUT
(
)
(
)
x
s
V
R
CS MIN
CS
2
µ
R
=
V
R
POWER RATING (W)
2
CS
CS MAX
(
)
R
=
V
1.3 x I
CS
CS(MIN)
OUT(MAX)
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
_______________________________________________________________________________________
9
R3
C
R2
R2
FROM
OUTPUT
TO FB
Figure 5. Adjustable-Output Operation Using the MAX1627
4.5
5.5
5.0
6.0
12
10
14
16
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (A)
3.0
3.5
2.5
2.0
1.5
1.0
0
0.5
V
OUT
= 5V
R
SENSE
= 0.03
Ω
R
SENSE
= 0.04
Ω
R
SENSE
= 0.05
Ω
R
SENSE
= 0.1
Ω
Figure 6a. MAX1626 5V-Operation Current-Sense Resistor
Graph
3.0
4.0
3.5
4.5
12
10
14
16
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (A)
3.0
3.5
2.5
2.0
1.5
1.0
0
0.5
V
OUT
= 3.3V
R
SENSE
= 0.03
Ω
R
SENSE
= 0.04
Ω
R
SENSE
= 0.05
Ω
R
SENSE
= 0.1
Ω
Figure 6b. MAX1626 3.3V-Operation Current-Sense Resistor
Graph