Datasheet

ADP1877 Data Sheet
Rev. D | Page 18 of 32
ACCURATE CURRENT-LIMIT SENSING
R
DSON
of the MOSFET can vary by more than 50% over the
temperature range. Accurate current limit sensing can be
achieved by adding a current sense resistor from the source of
the low-side MOSFET to PGND. Make sure that the power
rating of the current sense resistor is adequate for the application.
Apply the above equation and calculate R
ILIM
by replacing
R
DSON_MAX
with R
SENSE
. The Figure 31 illustrates the
implementation of this accurate current limit sensing.
VIN
R
ILIM
DHx
DLx
SWx
ILIMx
R
SENSE
ADP1877
08299-008
Figure 31. Accurate Current Limit Sensing
SETTING THE SLOPE COMPENSATION
In a current-mode control topology, slope compensation is
needed to prevent subharmonic oscillations in the inductor
current and to maintain a stable output. The external slope
compensation is implemented by summing the amplified sense
signal and a scaled voltage at the RAMPx pin. To implement the
slope compensation, connect a resistor between RAMPx and
the input voltage. The resistor, R
RAMP
, is calculated by
MAXDSONCS
RAMP
RA
L
R
_
10
106.3
×
×
=
where:
3.6 × 10
10
is an internal parameter.
L is the inductance of the inductor.
R
DSON_MAX
is the the low-side MOSFET maximum on resistance.
A
CS
is the gain, either 3 V/V, 6 V/V, 12 V/V, or 24 V/V, of the
current sense amplifier (see the Setting the Current Sense Gain
section for more details).
Keep in mind that R
DSON
is temperature dependent and can vary
as much as 0.4%/
o
C. Choose R
DSON
at the maximum operating
temperature. The voltage at RAMPx is fixed at 0.2 V, and the
current going into RAMPx should be in between 6 µA and
200 µA. Make sure that the following condition is satisfied:
A200
V2.0
A6 ≤
−
≤
RAMP
IN
R
V
For instance, with an input voltage of 12 V, R
RAMP
should not
exceed 1.9 MΩ. If the calculated R
RAMP
produces less than 6 µA,
then select a R
RAMP
value that produces between 6 µA and 20 µA.
Figure 32 illustrates the connection of the slope compensation
resistor R
RAMP
and the current sense gain resistor R
CSG
.
VIN
R
ILIM
DHx
RAMP
DLx
SWx
ILIMx
R
CSG
ADP1877
R
RAMP
08299-009
Figure 32. Slope Compensation and CS Gain Connection
SETTING THE CURRENT SENSE GAIN
The voltage drop across the external low-side MOSFET is
sensed by a current sense amplifier by multiplying the peak
inductor current and the R
DSON
of the MOSFET. The result is
then amplified by a gain factor of either 3 V/V, 6 V/V, 12 V/V,
or 24 V/V, which is programmable by an external resistor, R
CSG
,
connected to the DL pin. This gain is sensed only during
power-up and not during normal operation. The amplified
voltage is summed with the slope compensation ramp voltage
and fed into the PWM controller for a stable regulation voltage.
The voltage range of the internal node, V
CS
, is between 0.4 V
and 2.2 V. Select the current sense gain such that the internal
minimum amplified voltage (V
CSMIN
) is above 0.4 V and the
maximum amplified voltage (V
CSMAX
) is 2.1 V. Do not set V
CSMAX
above 2.1 V to account for temperature and part-to-part
variations. Note that V
CSMIN
or V
CSMAX
is not the same as V
COMP
,
which has a range of 0.75 V to 2.25 V. The following are
equations for V
CSMIN
and V
CSMAX
:
CSMINDSON
LPP
CSMIN
ARIVV ××−=
_
2
1
75.0
CSMAXDSON
LPP
LOADMAXCSMAX
ARIIVV ××−+=
_
)
2
1
(75.0
where:
V
CSMIN
is the minimum amplified voltage of the internal current
sense amplifier at zero output current.
V
CSMAX
is the maximum amplified voltage of the internal current
sense amplifier at maximum output current.
R
DSON_MIN
is the the low-side MOSFET minimum on resistance.
The zero-current level voltage of the current sense amplifier is
0.75 V.
I
LPP
is the peak-to-peak ripple current in the inductor.
I
LOADMAX
is the maximum output DC load current.
Table 6 shows the appropriate current sense gain settings for a
given R
DSON
maximum load current and a 33% inductor current
ripple. Because of the variation in R
DSON
of the power MOSFETs
(part-to-part variation and overtemperature) and the variation
of the inductors, the users must verify that V
COMP
does not
exceed 2.2 V at the maximum output load current.