Datasheet

MAX1809
3A, 1MHz, DDR Memory Termination Supply
_______________________________________________________________________________________ 9
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:
where:
t
OFF
= the programmed off-time
V
IN
= the input voltage
V
OUT
= the output voltage
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:
Check that t
ON
in the current sinking mode is greater
than 350ns.
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)
tt
VV
VV V
ON OFF
OUT NMOS
IN OUT PMOS
=
+






−
−
t
VV V
fVV V
OFF
IN OUT PMOS
SW IN PMOS NMOS
=
()
+
()
−−
−
Figure 3. Sourcing and Sinking Capabilities of the MAX1809
SYNCHRONOUS BUCK MODE
(SOURCING CURRENT)
V
IN
V
OUT
I
SOURCE
SYNCHRONOUS BOOST MODE
(SINKING CURRENT)
V
IN
V
SOURCE
> V
OUT
I
SINK
Figure 4. Maximum Recommended Operating Frequency vs.
Input Voltage
0
400
200
800
600
1200
1000
1400
2.7 3.1 3.5 4.3 4.7 5.1 5.53.9
V
IN
(V)
OPERATING FREQUENCY (kHz)
V
OUT
= 2.5V
V
OUT
= 1.25V