Datasheet

Data Sheet ADP1853
Rev. 0 | Page 21 of 28
Because C
HF
<< C
I
, calculate C
HF
as follows:
Z
SW
HF
Rf
C
π
=
1
(9)
Next, calculate the feedforward capacitor, C
FF
, assuming R
FF
<<
R
TOP
:
SW
FF
FF
fC
R
π
=
1
(10)
Check that the calculated component values are reasonable.
For instance, 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 ADP1853 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 resistance of the current sense element between
CS and PGND (for instance, 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 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 28 illustrates the connection of the slope compensation
resistor, R
RAMP
, and the current sense gain resistor, R
CSG
.
Figure 28. Slope Compensation and CS Gain Connection
ADP1853
DH
CS
ILIM
DL
R
ILIM
R
CSG
RAMP
R
RAMP
V
IN
10594-034