Setting the current limit, 1] where v, 0v. choose r9 to be 30k ω or less – Rainbow Electronics MAX1774 User Manual
Page 17: Figure 1). the value of r, Can be calculated with the following equa- tion: r, Where v, 80mv is the current-sense threshold, and i
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
______________________________________________________________________________________
17
voltage-divider to FBC (Figure 1). Resistor values can
be calculated with the following equation:
R8 = R9
✕
[(V
OUTC
/ V
FBC
) - 1]
where V
FBC
= +1.0V. Choose R9 to be 30k
Ω
or less.
Setting the Current Limit
The main regulator current limit is set externally through
a small current-sense resistor, R
CS
(Figure 1). The
value of R
CS
can be calculated with the following equa-
tion:
R
CS
= V
CLM
/ (1.3
✕
I
OUT
)
where V
CLM
= 80mV is the current-sense threshold,
and I
OUT
is the current delivered to the output. The
core and backup converter current limits are set inter-
nally and cannot be modified.
Careful layout of the current-sense signal traces is
imperative. Place R
CS
as close to the MAX1774 as pos-
sible. The two traces should have matching length and
width, be as far as possible from noisy switching sig-
PGNDC
FBC
LXC
INC
FBM
CS-
CS+
PGND
ACI
PDRV
NDRV
CVH
MAIN
C5
1
µ
F
P2
N1
L1
5
µ
H
C
MAIN
47
µ
F
C6
10
µ
F
R
CS
CORE
C
CORE
22
µ
F
R10
R11
R8
R9
L2
5.4
µ
H
C7
1
µ
F
R5
1M
Ω
R6
1M
Ω
R7
1M
Ω
GND
REF
CVL
LXB
BIN
IN
DBI
LBI
BACKUP
BATTERY
MAIN
BATTERY
V
IN_AC
R1
C1
10
µ
F
C2
10
µ
F
C4
0.22
µ
F
C3
1
µ
F
D2
L3
22
µ
H
P1
R4
R2
R3
D1
BKOFF
BKUP
LBO
ACO
SHDNC
SHDNM
MDRV
MAX1774
ON
OFF
ON
OFF
INS
NDS356AP
NSD03A10
2.7V
TO
20V
EP05
Q03L
LXB2 (QFN ONLY)
0.9V
TO
5.5V
1.0V
TO
5.5V
2.6V
TO
5.5V
FDS8928A
2.7V TO 28V
1M
Ω
Figure 4. Typical Application Circuit (Cascaded)