MIC2584/2585
Micrel
Q1
IRF7822
(SO-8)
RSENSE1
0.012Ω
5%
VOUT1
5V@2.5A
VIN1
5V
1
2
3
4
CLOAD1
220µF
*D1
(8V)
C1
1µF
Q2
IRF7822
(SO-8)
RSENSE2
0.012Ω
5%
VOUT2
3.3V@2.5A
VIN2
3.3V
1
2
3
4
CLOAD2
220µF
*D2
(8V)
C2
1µF
R5
24
23
1
2
20.5kΩ
1%
VCC1
SENSE1 VCC2
SENSE2
GATE2
3
7
R1
C4
0.022µF
33kΩ
R3
FB2
39.2kΩ
1%
R6
8.66kΩ
1%
8
ON
5
OUT2
R2
47kΩ
R9
3.6kΩ
Q3
ZTX788A
R8
1.5kΩ
6
MIC2585-1
11
DIS2
CFILTER
CDLY
C5
0.01µF
Q4
TCR22-4
C7
0.033µF
R10
22
360Ω
GATE1
C3
0.022µF
12
R7
680Ω
C6
0.1µF
19
20
18
DIS1
OUT1
FB1
GND
13
TRK
9
R4
15.8kΩ
1%
Undervoltage (OUT1) = 4.4V
Undervoltage (OUT2) = 2.85V
Circuit Breaker Response Time = 5ms
Sequenced Output Delay (Turn-On) = 20ms
*Dual package Diode is AZ23C8V2
Resistors are 5% unless specified otherwise
Additional pins omitted for clarity
Figure 8. First On—Last Off Application Circuit
Output Undervoltage Detection
2) Next, determine R12 using the output “good”
voltage of 10.5V and the following equation:
Foroutputundervoltagedetection,thefirstconsiderationisto
establish the output voltage level that indicates “power is
good.” For this example, the output value for which a 12V
supply will signal “good” is 10.5V. Next, consider the toler-
R12 +R13
(
)
V
= V
FB1(MAX)
(10)
OUT1(Good)
R13
ances of the input supply and FB threshold (V ). For this
Using some basic algebra and simplifying Equation 10 to
isolate R12, yields:
FB
example, givena12V±5%supplyforChannel1, theresulting
output voltage may be as low as 11.4V and as high as 12.6V.
Additionally, the FB1 threshold has ±50mV tolerance and
may be as low as 1.19V and as high as 1.29V. Thus, to
determine the values of the resistive divider network (R12
and R13) at the FB1 pin, shown in the typical application
circuit on page 1, use the following iterative design proce-
dure.
VOUT1(Good)
R12 = R13
where V
–1
(10.1)
VFB1(MAX)
= 1.29V, V
= 10.5V, and R13 is
FB1(MAX)
OUT1(Good)
14.7kΩ. Substituting these values into Equation 10.1 now
yields R12 = 104.95kΩ. A standard 105kΩ ± 1% is selected.
Now, consider the 11.4V minimum output voltage, the lower
tolerance for R13 and higher tolerance for R12, 14.55kΩ and
106.05kΩ, respectively. With only 11.4V available, the voltage
1) Choose R13 so as to limit the current through
the divider to approximately 100µA or less.
V
1.29V
FB1(MAX)
sensed at the FB1 pin exceeds V
, thus the /POR and
R13
12.9kΩ
.
FB1(MAX)
100µA
100µA
PG1 (MIC2585) signals will transition from LOW to HIGH,
indicating “power is good” given the worse case tolerances of
thisexample. AsimilarapproachshouldbeusedforChannel2.
R13 is chosen as 14.7kΩ ± 1%.
MIC2584/2585
20
March 2005
MICREL [ MICREL SEMICONDUCTOR ]
