Datasheet

Data Sheet ADP1850
Rev. A | Page 17 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 is achieved
by adding a current sense resistor from the source of the low-
side MOSFET to PGNDx. Make sure that the power rating of the
current sense resistor is adequate for the application. Apply the
previous equation and calculate R
ILIM
by replacing R
DSON_MAX
with R
SENSE
. Figure 30 illustrates the implementation of accurate
current limit sensing.
V
IN
ADP1850
DHx
SWx
ILIMx
DLx
R
ILIM
R
SENSE
09440-031
Figure 30. 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
_
9
107
×
×
=
where:
7 × 10
9
is an internal parameter.
L is the inductance (with units in H) of the inductor.
R
DSON_MAX
is 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).
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 between 10 µA and 160 µA. Make sure
that the following condition is satisfied:
A160
V2.0
A10 μ≤
−
≤μ
RAMP
IN
R
V
For instance, with an input voltage of 12 V, R
RAMP
should not
exceed 1.1 MΩ. If the calculated R
RAMP
produces less than 10 µA,
then select an R
RAMP
value that produces between 10 µA and 15 µA.
Figure 31 illustrates the connection of the slope compensation
resistor, R
RAMP
, and the current sense gain resistor, R
CSG
.
V
IN
ADP1850
DHx
SWx
ILIMx
DLx
R
ILIM
R
CSG
RAMP
R
RAMP
09440-032
Figure 31. 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 DLx 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. Note that V
CSMIN
or V
CSMAX
is not the same as V
COMP
, which has a range of 0.85 V
to 2.25 V. Make sure that the maximum V
COMP
(V
COMPMAX
) does
not exceed 2.2 V to account for temperature and part-to-part
variations. See the following equations for V
CSMIN
, V
CSMAX
, and
V
COMPMAX
:
CSMINDSON
LPP
CSMIN
ARIV ××−=
_
2
1
V75.0
CSMAXDSON
LPP
LOADMAXCSMAX
ARIIVV ××−+=
_
)
2
1
(75.0
CSMAX
RAMP
ON
IN
COMPMAX
V
R
tV
V +
×
−
=
pF100
)V2.0(
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 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.
V
COMPMAX
is the maximum voltage at the COMP pin.
100 pF is an internal parameter.
t
ON
is the high-side driver (DH) on time.