?2003 Microchip Technology Inc.
DS20091B-page 39
MCP18480
4.0    APPLICATIONS INFORMATION
The MCP18480 can be programmed to have the PWR-
GOOD signal be either active-high or active-low via the
I
SET
pin and the connection of the external R
ISET
resis-
tor (see Section 6.8.8, Bias Block). If the R
ISET
resis-
tor   is   connected   between   I
SET
  and   V
NEG
,   the
PWRGOOD output pin is an active-high signal. If the
R
ISET
resistor is connected between I
SET
and V
REFIN
,
the PWRGOOD output pin is an active-low signal.
For systems using an active-low-enabled DC/DC con-
verter module, the MCP18480 should be programmed
for a high-active PWRGOOD output. Tying the R
ISET
resistor to the V
NEG
pin configures the PWRGOOD to
be an active-high signal. The active-high PWRGOOD
switches on the external NPN and the collector of the
external NPN (labeled as GOODPWR) is pulled to
V
NEG
, enabling a low-active GOODPWR and resulting
in enabling the DC/DC module.
For   active-high   DC/DC   converter   modules,   the
MCP18480 should be programmed for a low active
PWRGOOD output. Connecting R
ISET
to the V
REFIN
pin will enable an active-low PWRGOOD output. Refer
to Figure 4-1 and Figure 4-2 for schematics.
Figure 4-1 shows a typical telecom application circuit
where the DC/DC module is active-high. Figure 4-2
shows a typical telecom application circuit where the
DC/DC module is active-low. The polarity of the
MCP18480s PWRGOOD pin (active-high or active-
low) is dependant on the state of the I
SET
pin.
FIGURE 4-1:
Typical Operating Circuit for Telecom Applications with Active-High power Module -
foldback current limit enabled.
+
R
UV1
R
OV2
R
UV2
C
UVD
R
ISET
Ctimer
R
DISCH
R
UVHYS
R
POS
C
BYP2
R
SENSE
R
DRAIN1
R
DRAIN2
R
FB1
R
FB2
C
GD
R
GD
R
G1
C
G1
Q
PG2
Q
PG3
R
PG2
R
PG6
M
1
C
BYPL
R
BYPL
V
NEG
R
PG1
R
PG5
4 k&
2 礔
10 nF
1.74 M&
453 k&
280 k&
59 k&
30.9 k&
800 nF
124 k&
680 nF
1.6 M&
0.01&
7.5 k&
110 k&
MPSA43
100 nF
10&
3.3 nF
18 k&
680&
36 k&
36 k&
1500&
100 礔
51 k&
2N5400
NTE261
124 k&
115 k&
1.74 M&
1.6 M&
NTE2388
R
Z
24.9 k&
5V
R
PG3
R
PG4
Q
PG1
RESTART
ENABLE
PWRGOOD
V
POS
OV
TH
UV
TH
UV
HYS
UV
D
V
REFIN
V
REFOUT
CL
I
SET
TIMER
OV
O
DRAIN
TH
V
FB
GATE
SENSE
V
NEG
R
DISCH
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
DC/DC
Converter
Module
V
IN+
V
IN-
V
OUT+
V
OUT-
ON/OFF
100 V
78V
Fuse 10A
GND
Transorb
C
BYP1
R
OV1
GOODPWR
MCP18480
S
RS
S
EN