Datasheet
MAX1809
3A, 1MHz, DDR Memory Termination Supply
______________________________________________________________________________________ 13
the dissipation in the termination resistor is halved com-
pared to a termination scheme that connects the resistive
terminators to ground.
The V
TT
supply requires that it regulates to half the
memory voltage (V
DDQ
), tracks the changes of the
memory voltage, and is able to source and sink current
depending on the state of the bus. These requirements
are met in the MAX1809.
Discharging the Output in Shutdown
When SHDN is brought low after the controller has
been on for a while, the output may remain high if there
is no leakage or discharge path to bring the output
down. For DDR memory systems, keeping V
TT
at 1.25V
when V
DDQ
(2.5V) is shut down violates the DDR speci-
fications. This can result in the bus latching if the sys-
tem is subsequently turned on or possibly damaging
the memory subsystem.
When using the MAX1809 to generate the V
TT
output of
1.25V, several circuits are recommended to discharge
the output when the MAX1809 is shut down. These are
shown in Figure 9. Solution (a) is a diode added from
V
TT
to V
DDQ
so that V
TT
is discharged when VDDQ
goes low. Alternatively, solution (b) uses a small signal
transistor to discharge V
TT
when the MAX1809 is shut
down.
Startup in Sinking Mode
The MAX1809 will not startup until the feedback voltage
is made less than the external reference voltage when
power is applied or when the part is exiting shutdown.
In applications that cannot guarantee V
FB
< V
EXTREF
Figure 10. Starting the MAX1809 in Sinking Mode with V
OUT
>V
EXTREF
MAX1809
IN
C
IN
V
CC
INPUT VOLTAGE
(3V TO 5.5V)
SHDN
SS
LX
L
PGND
GND
FB
SHDN
V
TT
=V
DDQ
/2
10Ω
100Ω
C
OUT
2.2µF
0.01µF
Figure 9. Discharging the Output of the MAX1809 in Shutdown
MAX1809
IN
C
IN
V
CC
INPUT VOLTAGE
(3V TO 5.5V)
SHDN
EXTREF
LX
L
(a)
(b)
PGND
GND
FB
SHDN
SHDN
2N7002
V
TT
= V
DDQ
/2
10Ω
10kΩ
10kΩ
V
DDQ
(2.5V)
V
SSQ
C
OUT
0.1µF
2.2µF
0.1µF
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
SHDN LX
PGND
LX
PGND
V
CC
GND
REF
GND
TOP VIEW
MAX1809
QSOP
IN
LX
EXTREF
IN
SS
TOFF
FB
Pin Configurations (continued)