ADP1621 Data Sheet
Rev. C | Page 18 of 32
For low ESR output capacitors, such as ceramic capacitors, C2
is small, generally in the range of 10 pF to 400 pF. Because of the
parasitic inductance, resistance, and capacitance of the PCB layout,
the RCOMP, CCOMP, and C2 values might need to be adjusted by
observing the load transient response of the ADP1621 to establish a
stable operating system and achieve optimal transient performance.
For most applications, RCOMP is in the range of 5 kΩ to 100 kΩ,
and CCOMP is in the range of 100 pF to 30 nF.
COMP
C
COMP
R
COMP
C2
REF g
m
23
06090-030
Figure 31. Compensation Components
SLOPE COMPENSATION
The ADP1621 includes a circuit that allows adjustable slope
compensation. Slope compensation is required by current-
mode regulators to stabilize the current-control loop when
operating in continuous conduction and the switching duty
cycle is greater than 50%.
Slope compensation is achieved by internally forcing a ramping
current source out of the CS current-sense pin. By placing a resistor
between the CS pin and the current sensing device (the drain of
the external MOSFET in the case of lossless current sensing or
the source of the MOSFET if a current-sense resistor is used), a
voltage is developed across the resistor that is proportional to
the slope-compensation current.
To ensure stability of the current-mode control loop, use a
compensation voltage slope that is equal to or greater than one-
half of the current-sense representation of the inductor current
downslope. Therefore, it follows that
L
VVV
R
ft
fI
RIND
OUT
CS
SWMINOFF,
SWSC,PK
S
1
2 (33)
where:
RS is the slope-compensation resistor.
ISC,PK is the peak slope-compensation current.
fSW is the switching frequency.
RCS is the current-sense resistor.
VOUT is the regulated output voltage.
VD is the forward-voltage drop of the diode.
VIN is the input voltage.
tOFF,MIN is the minimum off time.
L is the power-stage inductor. In the case of lossless current
sensing, RCS is equal to the on resistance, RDSON, of the external
power MOSFET. Otherwise, RCS represents the external current-
sense resistor.
Solving for RS gives the following slope-compensation criterion:
LfI
ftVVVR
R
SWPKSC
SWMINOFF
IND
OUT
CS
S
,
,
2
1 (34)
Keep in mind that the above inequality is a function of both
ADP1621 parameters and off-chip components, the values of
which vary from device to device and with temperature. Select
RS to ensure current-loop stability for all possible variations.
After accounting for parameter variations, use values of RS that
are as close to the calculated limit as possible because excessive
slope compensation reduces the benefits of current-mode control
and increases the “softness” of the current limit, as discussed in the
Current Limit section. Given a typical peak slope-compensation
current of 70 μA, RS must not exceed 1.6 kΩ because the voltage at
the CS pin is typically clamped at 116 mV. It is also recommended
that RS be greater than 20 Ω. If the calculated RS is greater than
1.6 kΩ, the parameters in Equation 34, such as RCS, fSW, and L,
can be adjusted such that RS is less than 1.6 kΩ.
In conclusion, the value of RS should be 20 Ω ≤ RS ≤ 1.6 kΩ.
CURRENT LIMIT
The current limit in the ADP1621 limits the peak inductor
current and is achieved by the COMP voltage clamp. The peak
inductor current, IL,PK, is given by
CS
SWMINOFF
SPKSCZCTCOMPCLAMPCOMP
PKL R
ft
DRI
n
VV
I
,
,,,
,
1 (35)
where:
VCOMP,CLAMP is the COMP clamp voltage (typically 2.0 V).
VCOMP,ZCT is the COMP zero-current threshold (typically 1.0 V).
n is the current-sense amplifier gain (typically 9.5).
ISC,PK is the peak slope-compensation current (typically 70 μA).
RS is the slope-compensation resistor.
D is the duty cycle.
fSW is the switching frequency.
tOFF,MIN is the minimum off time (typically 190 ns).
RCS is the current-sense resistor. In the case of lossless current
sensing, RCS is equal to the on resistance, RDSON, of the external
power MOSFET. Otherwise, RCS represents the external current-
sense resistor.
The current limit in the ADP1621 is a soft current limit. When
the inductor current reaches the IL,PK limit given in Equation 35,
the duty cycle decreases, and the output voltage drops below the
desired voltage. The IL,PK limit in Equation 35 then increases in
response to the smaller duty cycle, D. The larger the slope-
compensation resistor, RS, the larger the effect on IL,PK for an
incremental decrease in D. This behavior results in a soft current
limit for the ADP1621. Use values of RS that are as close as possible
to the calculated limit derived from Equation 34. If high-precision
current limiting is required, consider inserting a fuse in series
with the inductor.