ADP1621-EVAL(RevB) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
ADP1621-EVAL Datasheet PDF : 32 Pages
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Data Sheet
ADP1621
frequency occurs well below the frequency of the RHP zero. The
location of the RHP zero is determined by the following equation:
f Z,RHP
= (1− D)2
×
RLOAD
2π× L
(25)
where fZ,RHP is the RHP zero frequency, and RLOAD is the equivalent
load resistance or the output voltage divided by the load current.
To stabilize the regulator, ensure that the regulator crossover
frequency is less than or equal to one-fifth of the RHP zero
frequency and less than or equal to one-fifteenth of the switching
frequency. For an initial practical design, choose the crossover
frequency fC to be the lower of
fC
=
f SW
15
(26)
and
fC =
fZ ,RHP
5
(27)
where fC is the crossover frequency, and fSW is the switching
frequency.
The regulator loop gain is
AVL
=
VFB
VOUT
× (1 − D)× g m × | ZCOMP
|× 1
n × RCS
× | ZOUT
|
(28)
where AVL is the loop gain, VFB is the feedback regulation
voltage (typically 1.215 V), VOUT is the regulated output voltage,
D is the duty cycle, gm is the error amplifier transconductance
gain (typically 300 µS), ZCOMP is the impedance of the RC network
from COMP to GND, n is the current-sense amplifier gain
(typically 9.5), RCS is the current-sense resistance, and ZOUT is
the impedance of the load and output capacitor. In the case of
lossless current sensing, as shown in Figure 28, RCS is equal to the
on resistance, RDSON, of the external power MOSFET. Otherwise,
RCS represents the external current-sense resistor, as shown in
Figure 29.
To determine the crossover frequency, it is important to note
that at that frequency the compensation impedance, ZCOMP, is
dominated by Resistor RCOMP, and the output impedance, ZOUT,
is dominated by the impedance of the output capacitor, COUT.
When solving for the crossover frequency, the equation is
simplified to
| AVL | =
V FB
VOUT
×(1− D)×
gm
× RCOMP
×
1
n× RCS
×
2π×
1
f C ×COUT
=1
(29)
where fC is the crossover frequency, RCOMP is the compensation
resistor, and COUT is the output capacitance.
Solving for RCOMP gives
RCOMP
=
2π ×
f C × COUT × n × RCS ×VOUT
VFB × (1 − D)× g m
(30)
Once the compensation resistor, RCOMP, is known, set the zero
formed by the resistor and compensation capacitor, CCOMP, to
one-fourth of the crossover frequency, or
CCOMP
=
π×
fC
2
× RCOMP
(31)
Capacitor C2 is chosen to cancel the zero introduced by the output
capacitance ESR. Thus, C2 should be set to (see Figure 31)
C2 = ESR × COUT
(32)
RCOMP
where ESR represents the ESR of COUT.
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.
REF
2
COMP
gm
3
RCOMP
C2
CCOMP
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
2
×
RS
×
1
ISC,PK ×
− tOFF,MIN
fSW
× fSW
> RCS × VOUT
+ VD − VIN
L
(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, and L is the power-stage inductor. In the
case of lossless current sensing, RCS is equal to the on resistance,
Rev. B | Page 17 of 32
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