Core voltage positioning and loop compensation, Nb smps current sense – Rainbow Electronics MAX17480 User Manual
Page 32
MAX17480
AMD 2-/3-Output Mobile Serial
VID Controller
32
______________________________________________________________________________________
where L
ESL
is the equivalent series inductance of the
current-sense resistor, R
SENSE
is current-sense resis-
tance value, and C
SENSE
and R
EQ
are the time-con-
stant matching components.
Using the DC resistance (R
DCR
) of the output inductor
allows higher efficiency. In this configuration, the initial
tolerance and temperature coefficient of the inductor’s
DCR must be accounted for in the output-voltage
droop-error budget and power monitor. This current-
sense method uses an RC filtering network to extract
the current information from the output inductor (see
Figure 7). The time constant of the RC network should
match the inductor’s time constant (L/R
DCR
):
where C
SENSE
and R
EQ
are the time-constant matching
components. To minimize the current-sense error due to
the current-sense inputs’ bias current (I
CSP
and I
CSN
),
choose R
EQ
less than 2k
Ω and use the above equation
to determine the sense capacitance (C
SENSE
). Choose
capacitors with 5% tolerance and resistors with 1% tol-
erance specifications. Temperature compensation is
recommended for this current-sense method. See the
Core Voltage Positioning and Loop Compensation
sec-
tion for detailed information.
Additional R
LX
and C
LX
are always added between the
LX_ and CSP_ pins if DCR sensing is used, and they
provide additional overdrive to the current-sense signal
to improve the noise immunity; otherwise, there might
be too much jitter or the system could be unstable.
NB SMPS Current Sense
The NB current sense is achieved by sensing the volt-
age across the high-side internal MOSFET during the
on-time. The current information is computed by dividing
the sensed voltage by the MOSFET’s on-resistance,
R
ON(NH3)
.
L
R
R
C
DCR
EQ SENSE
=
L
R
R
C
ESL
SENSE
EQ SENSE
=
MAX17480
N
L
D
L
LX_
DL_
DH_
CSP_
CSN_
N
H
C
IN
INPUT (V
IN
)
SENSE RESISTOR
A) OUTPUT SERIES RESISTOR SENSING
L
C
OUT
L
ESL
R
SENSE
C
EQ
R
EQ
=
L
ESL
R
SENSE
R
EQ
C
EQ
MAX17480
N
L
D
L
LX_
DL_
DH_
CSP_
CSN_
N
H
C
IN
INPUT (V
IN
)
INDUCTOR
B) LOSSLESS INDUCTOR DCR SENSING
FOR THERMAL COMPENSATION:
R2 SHOULD CONSIST OF AN NTC RESISTOR
IN SERIES WITH A STANDARD THIN-FILM
RESISTOR.
C
OUT
L
R
DCR
R
CS
=
× R
DCR
R2
R1 + R2
R
LX
R2
R1
C
LX
C
EQ
R
DCR
=
+
×
L
C
EQ
1
R1
1
R2
Figure 7. Current-Sense Configurations