Datasheet

Data Sheet ADP1851
Rev. 0 | Page 19 of 24
Because C
HF
<< C
I
, calculate C
HF
as follows:
Z
SW
HF
Rf
C
π
=
1
(11)
Next, calculate the feedforward capacitor, C
FF
, assuming
R
FF
<< R
TOP
.
SW
FF
FF
fC
R
π
=
1
(12)
Check that the calculated component values are reasonable.
For example, capacitors smaller than about 10 pF should be
avoided. In addition, R
Z
values less than 3 kΩ and C
I
values
greater than 10 nF should be avoided. If necessary, recalculate
the compensation network with a different starting value for
R
TOP
. If R
Z
is too small or C
I
is too big, start with a larger value
for R
TOP
. This compensation technique should yield a good
working solution.
When precise compensation is needed, use the ADIsimPower
design tool.
LOOP COMPENSATION—CURRENT MODE
Compensate the ADP1851 error voltage loop in current mode
using Type II compensation.
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 RAMP pin. To set the effective
slope compensation, connect a resistor (R
RAMP
) between the
RAMP pin and the input voltage (V
IN
). R
RAMP
is calculated by
CSCS
RAMP
RA
L
R
×
××
=
6
107
where:
L is the inductor value measured in µH.
R
CS
(mΩ) is the resistance of the current sense element between
SW and PGND (R
DSON_MAX
is the low-side MOSFET maximum
on resistance).
A
CS
is the current sense amplifier gain and is 3 V / V, 6 V / V,
or 12 V / V.
Thus, the voltage ramp amplitude, V
RAMP
, is:
RAMPSW
IN
RAMP
Rf
V
V
××
−
=
pF100
V2.0
where 100 pF is the effective capacitance of the internal ramp
capacitor, C
RAMP
, with ±4% tolerance over the temperature and
V
IN
range.
The voltage at the RAMP pin is fixed at 0.2 V, and the current
going into RAMP should be between 10 µA and 160 µA. Make
sure that the following condition is satisfied:
μA160
V2.0
μA10 ≤
−
≤
RAMP
IN
R
V
For example, with an input voltage of 12 V, R
RAMP
should not
exceed 1.1 MΩ. If the calculated R
RAMP
value produces less than
10 µA, then select an R
RAMP
value that produces between 10 µA
and 15 µA.
Figure 27 illustrates the connection of the slope compensation
resistor, R
RAMP
, and the current sense gain resistor, R
CSG
.
Figure 27. 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 3 V/V, 6 V/V, or 12 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. Note that V
CSMIN
or V
CSMAX
is not the same as V
COMP
, which has a range of 0.9 V to
2.2 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
L
CSMIN
ARIV ××−=
_
2
1
V75.0
CSMAXDSON
L
LOADMAXCSMAX
ARIIV ××−+=
_
)
2
1
(V75.0
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.
I
L
is the peak-to-peak ripple current in the inductor.
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.
V
CSMAX
is the maximum amplified voltage of the internal current
sense amplifier at the maximum output current.
I
LOADMAX
is the maximum output dc load current.
R
DSON_MAX
is the low-side MOSFET maximum on resistance.
R
CSG
R
RAMP
V
IN
RAMP
ADP1851
DH
DL
SW
ILIM
R
ILIM
10595-027