SC1544 Ver la hoja de datos (PDF) - Semtech Corporation
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SC1544 Datasheet PDF : 15 Pages
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SC1544
POWER MANAGEMENT
Applications Information (Cont.)
PRELIMINARY
Linear Regulator Stability
.or the 2.5V Dual Memory linear regulator:
When extremely low ESR capacitors (Oscons, Polymers,
high value ceramics) are used on outputs requiring fast
transients (3.3V/2.5V memory), the linear regulators may
exhibit some overshoot and/or transient instability.
External compensation may be necessary (5VSB to 3.3V
Dual Memory shown):
3.3V DUAL MEM
R1 1k
Q6
IRLR3103
5VSB
C1
22nF
U1
1
G5
2
3.3VDM
3
G6
4
G7
5
AGP
6
TYPEDET
7
PWR_OK
8
EN
9
/S3
10
/S5
11
USB
12
PCI
SC1544-3.3
24
G8
23
1.8V
22
G4
21
3.3VD
20
GND
19
G2/3
18
G1
17
5VD
16
5VSB
15
FC
14
+CAP
13
-CAP
R2 ≤
2.5
≤ 71 Ω
350µA •100
2.5V < VOUT (AJUSTED) < 3.3V
C6
GND
100uF/6.3V
R1 I SENSE
U1
1
G5
VOUT (FIXED)
R2
2
2.5VDM
3
G6
4
G7
5
AGP
6
TYPEDET
7
PWR_OK
8
EN
9
/S3
10
/S5
11
USB
12
PCI
SC1544-2.5
24
G8
23
1.8V
22
G4
21
3.3VD
20
GND
19
G2/3
18
G1
17
5VD
16
5VSB
15
FC
14
+CAP
13
-CAP
Another possibility is to use dual capacitor types for the
load capacitor - a ceramic capacitor will provide high
frequency decoupling for the load while an electrolytic
capacitor (high ESR) will tame the Q to prevent instability:
3.3V DUAL MEM
C1 +
Electrolytic - bulk
C2
Ceramic - high frequency decoupling
Adjusting the Output Voltage of the Linear Regulators
The output voltage can only be adjusted upwards from
the fixed output voltage, and can be calculated using the
following equation:
VOUT( ADJUSTED )
=
VOUT(FIXED)
•
1 +
R1
R2
+
R1• ISENSE
Volts
Therefore to set the 2.5V Dual Memory linear regulator
to 2.6V, for example, R1 = 2.8Ω and R2 = 69.8Ω. The
maximum voltage to which an output can be set using
this method is limited by the input voltage to the .ET(s)
and the RDS(ON) of the .ET(s).
It is possible to adjust the output voltage of the linear
regulators (1.8V, 2.5V Dual Memory and AGP) by
applying an external resistor divider to the sense pin (see
below). Since the sense pins sink a nominal 100µA (1.8V
LDO, 150µA for the 1.5V AGP LDO, 325µA for the 3.3V
AGP LDO and 350µA for the 2.5V Dual Memory LDO),
the resistor values should be selected to allow 100x that
current to flow through the divider. This will ensure that
variations in the sense current will have negligible affect
on the output voltage regulation. Thus a target value for
R2 (maximum) can be calculated:
R2 ≤ VOUT(FIXED)
Ω
ISENSE • 100
Please note that this technique cannot be used for the
3.3V Dual Memory, 3.3V Dual and 5V Dual outputs since
the output is switched via a pass device (i.e. not
regulated) when not in S0.
.ault Protection Hints
Loss of AC Power: if it is possible during brownouts or
momentary loss of AC power to the computer silver box
that PWR_OK can assert low while S3 and S5 remain
high then the over current protection may trigger. This is
because the state machine will ignore this illegal
transition and monitor all outputs as if they are still in
S0, despite the fact that the inputs are going away. If
5VSB decays slowly, S3 and S5 remain high, and one
output drops below the OCP threshold for long enough
2002 Semtech Corp.
13
www.semtech.com
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