MAX1901 Datasheet, PDF (26/33 Pages) Maxim Integrated Products – 500kHz Multi-Output, Low-Noise Power-Supply Controllers for Notebook Computers

500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
Lossless-Inductor Current-Sensing
The DC resistance (DCR) of the inductor can be used
to sense inductor current to improve the efficiency and
to reduce the cost by eliminating the sense resistor.
Figure 7 shows the sense circuit, where L is the induc-
tance, R_L is the inductor DCR, R_S, and C_S form an RC
low-pass sense network. If the time constant of the
inductor is equal to that of the sense network, i.e.:
L
R_L
= R_SC_S
then the voltage across C_S becomes
V_S = R_L × I_L
where I_L is the inductor current.
Determine the required sense-resistor value using the
equation given in the Current-Sense Resistor Value
section. Choose an inductor with DCR equal or greater
than the sense resistor value. If the DCR is greater than
the sense-resistor value, use a divider to scale down
the voltage. Use the maximum inductance and mini-
mum DCR to get the maximum possible inductor time
constant. Select R_S and C_S so that the maximum sense
network time constant is equal or greater than the maxi-
mum inductor time constant.
Reduced Output Capacitance Application
In applications where higher output ripple is accept-
able, lower output capacitance or higher ESR output
capacitors can be used. In such cases, cycle-by-cycle
stability is maintained by adding feedforward compen-
sation to offset for the increased output ESR. Figure 8
shows the addition of the feedforward compensation
circuit. C_FB provides noise filtering, R_FF is the feedfor-
ward resistor and C_LX provides DC blocking. Use
100pF for C_FB and C_LX. Select R_FF according to the
equation below:
R_FF ≤
4 × R3 × L × f
ESR
Set the value for R_FF close to the calculation. Do not
make R_FF too small as that will introduce too much
feedforward, possibly causing an overvoltage to be
seen at the feedback pin, and changing the mode of
operation to a voltage mode.
PC Board Layout Considerations
Good PC board layout is required in order to achieve
specified noise, efficiency, and stability performance.
The PC board layout artist must be given explicit
instructions, preferably a pencil sketch showing the
placement of power-switching components and high-
current routing. A ground plane is essential for optimum
DH_
LX_
MAX1901
MAX1902 _{CSH_}
MAX1904
CSL_
DL_
V_IN
C_IN
INDUCTOR
L
R_L
V_OUT
R_S
C_S
C_OUT
DH_
CSL_
CSH_
LX_
MAX1901
MAX1902
MAX1904 FB_
DL_
V_IN
C_IN
L
C_LX R_FF
R_SENSE
C_FB
V_OUT
R3
C_OUT
R4
Figure 7. Lossless Inductor Current Sensing
Figure 8. Adding Feedforward Compensation
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