3A, 1MHz, DDR Memory Termination Supply
MAX1809
V
IN
1400
1200
OPERATING FREQUENCY (kHz)
1000
V
OUT
= 2.5V
800
600
400
200
SYNCHRONOUS BUCK MODE
(SOURCING CURRENT)
I
SOURCE
V
OUT
V
OUT
= 1.25V
0
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
V
IN
(V)
V
IN
Figure 4. Maximum Recommended Operating Frequency vs.
Input Voltage
I
SINK
V
SOURCE
> V
OUT
SYNCHRONOUS BOOST MODE
(SINKING CURRENT)
Figure 3. Sourcing and Sinking Capabilities of the MAX1809
V
PMOS
= the voltage drop across the internal
PMOS power switch
|
I
OUT
✕
R
PMOS
|
V
NMOS
= the voltage drop across the internal
NMOS synchronous-rectifier switch
|
I
OUT
✕
R
NMOS
|
f
SW
= switching frequency
Make sure that t
ON
and t
OFF
are greater than 400ns
when sourcing current. Select R
TOFF
according to the
formula:
R
TOFF
= (t
OFF
- 0.07µs)
✕
(117kΩ /1.00µs)
Recommended values for R
TOFF
range from 36kΩ to
430kΩ for off-times of 0.4µs to 4µs.
When sinking current, the switching frequency increas-
es due to the on-resistances of the internal switches
adding to the voltage across the inductor, reducing the
on-time. Calculate t
ON
when sinking current using the
equation:
V
OUT
−
V
NMOS
t
ON
=
t
OFF
V
IN
−
V
OUT
+
V
PMOS
Check that t
ON
in the current sinking mode is greater
than 350ns.
Programming the Switching Frequency
and Off-Time and On-Time
The MAX1809 features a programmable PWM-mode
switching frequency, which is set by the input and out-
put voltage and the value of R
TOFF
, connected from
TOFF to GND. R
TOFF
sets the PMOS power switch off-
time in PWM mode. Use the following equation to select
the off-time while sourcing current according to the
desired switching frequency in PWM mode:
t
OFF
=
(
V
IN
−
V
OUT
−
V
PMOS
)
f
SW
(
V
IN
−
V
PMOS
+
V
NMOS
)
where:
t
OFF
= the programmed off-time
V
IN
= the input voltage
V
OUT
= the output voltage
Inductor Selection
The key inductor parameters must be specified: inductor
value (L) and peak current (I
PEAK
). The following equa-
tion includes a constant, denoted as LIR, which is the
ratio of peak-to-peak inductor AC current (ripple current)
9
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