3A, 1MHz, DDR Memory Termination Supply
MAX1809
where C = 2.5nF and f
SW
is the switching frequency.
Resistive losses in the two power switches are approxi-
mated by:
P
D(RES)
= I
OUT
2
✕
R
PMOS
where R
PMOS
is the on-resistance of the PMOS switch.
The junction-to-ambient thermal resistance required to
dissipate this amount of power is calculated by:
θ
JA
= (T
J,MAX
- T
A,MAX
) / (P
D(CAP)
+ P
D(RES)
)
where:
θ
JA
= junction-to-ambient thermal resistance
T
J,MAX
= maximum junction temperature
T
A,MAX
= maximum ambient temperature
Setting the Output Voltage
The output voltage of the MAX1809 is set by an external
voltage applied to the EXTREF pin. This can come directly
from another voltage source or external reference.
As an active termination supply in DDR applications
(see Active Bus Termination in the
Applications
Information
section), the output of the MAX1809 is reg-
ulated at half the DDR supply voltage. In mobile sys-
tems, the DDR supply voltage is 2.5V, and the
termination voltage is 1.25V ±40mV. To regulate to
1.25V, an external divide-by-2 resistor network is
placed across the DDR supply voltage to generate
1.25V. This 1.25V is connected to EXTREF, which sets
the output voltage of the MAX1809. When FB is directly
tied to the output (Figure 5), the output voltage range is
limited by the external reference’s input voltage limits
(see EC table). External reference may not be set within
1.7V of the minimum supply voltage. V
EXTREF
should
be limited to less than 1.4V for 3.3V input voltage.
Failure to comply can cause the part to operate abnor-
mally and may cause part damage.
Alternatively, the output can be adjusted up to V
IN
by
connecting FB to a resistor-divider between the output
voltage and ground (Figure 6). Use 50kΩ for R1. R2 is
given by:
V
R2
=
R1
OUT
−
1
V
EXTREF
Design Procedure
For typical applications, use the recommended compo-
nent values in Figure 1. For other applications, take the
following steps:
1) Select the desired PWM-mode switching frequency.
See Figure 4 for maximum operating frequency.
2) Select the constant off-time as a function of input
voltage, output voltage, and switching frequency.
3) Select R
TOFF
as a function of off-time.
4) Select the inductor as a function of output voltage,
off-time, and peak-to-peak inductor current.
0.01µF
SS
V
IN
2.2µF
V
CC
FB
IN
V
IN
(3.0V TO 5.5V)
C
IN
CERAMIC
MAX1809
CURRENT
SENSE
EXTREF
PWM LOGIC
AND
DRIVERS
LX
L
SHDN
REF
1µF
GND
R
TOFF
PGND
C
OUT
REF
TIMER
Figure 2. Functional Diagram
8
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