MAX1809 Просмотр технического описания (PDF) - Maxim Integrated
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MAX1809 Datasheet PDF : 17 Pages
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3A, 1MHz, DDR Memory Termination Supply
VIN
ISOURCE
VOUT
SYNCHRONOUS BUCK MODE
(SOURCING CURRENT)
VIN
1400
1200
1000
VOUT = 2.5V
800
600
VOUT = 1.25V
400
200
0
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
VIN (V)
ISINK
VSOURCE > VOUT
SYNCHRONOUS BOOST MODE
(SINKING CURRENT)
Figure 3. Sourcing and Sinking Capabilities of the MAX1809
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 RTOFF, connected from
TOFF to GND. RTOFF 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:
(( ) ) tOFF
=
VIN − VOUT − VPMOS
fSW VIN − VPMOS + VNMOS
where:
tOFF = the programmed off-time
VIN = the input voltage
VOUT = the output voltage
Figure 4. Maximum Recommended Operating Frequency vs.
Input Voltage
VPMOS = the voltage drop across the internal
PMOS power switch |IOUT ✕ RPMOS|
VNMOS = the voltage drop across the internal
NMOS synchronous-rectifier switch |IOUT ✕
RNMOS|
fSW = switching frequency
Make sure that tON and tOFF are greater than 400ns
when sourcing current. Select RTOFF according to the
formula:
RTOFF = (tOFF - 0.07µs) ✕ (117kΩ /1.00µs)
Recommended values for RTOFF 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 tON when sinking current using the
equation:
tON
=
tOFF
VOUT − VNMOS
VIN − VOUT + VPMOS
Check that tON in the current sinking mode is greater
than 350ns.
Inductor Selection
The key inductor parameters must be specified: inductor
value (L) and peak current (IPEAK). The following equa-
tion includes a constant, denoted as LIR, which is the
ratio of peak-to-peak inductor AC current (ripple current)
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