ADP1853 Datasheet, PDF (21/28 Pages) Analog Devices – Synchronous, Step-Down DC-to-DC Controller

Data Sheet
Because C_HF << C_I, calculate C_HF as follows:
C _HF
=
1
πf _SW R_Z
(9)
Next, calculate the feedforward capacitor, C_FF, assuming R_FF <<
R_TOP:
R _FF
=
1
πC _FF f _SW
(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
R_RAMP
=
7 ×10⁶ × L
A_CS × R_CS
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:
V_RAMP
=
100
V_IN
pF ×
− 0.2 V
f_SW × R_RAMP
where 100 pF is the effective capacitance of the internal ramp
capacitor, C_RAMP, with ±4% tolerance over the temperature and
V_IN range.
ADP1853
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:
10 μA ≤ ^{VIN − 0.2 V} ≤ 160 μA
R_RAMP
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.
V_IN
R_RAMP
RAMP
ADP1853
DH
CS
ILIM
R_ILIM
DL
R_CSG
Figure 28. Slope Compensation and CS Gain Connection
Rev. 0 | Page 21 of 28