ADP1876-EVALZ AD [Analog Devices], ADP1876-EVALZ Datasheet - Page 18

• Manufacturer Part Number: ADP1876-EVALZ
• Description: 600 kHz Dual Output Synchronous Buck
• Manufacturer: AD [Analog Devices]

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 RAMPx pin. To implement the
slope compensation, connect a resistor between RAMPx and
the input voltage. The resistor, R
where:
3.6 × 10
L is the inductance of the inductor.
R
A
current sense amplifier (see the Setting the Current Sense Gain
section for more details).
Keep in mind that R
as much as 0.4%/
temperature. The voltage at RAMPx is fixed at 0.2 V, and the
current going into RAMPx should be between 6 µA and 200 µA.
Ensure that the following condition is satisfied:
For instance, with an input voltage of 12 V, R
exceed 1.9 MΩ. If the calculated R
select an R
Figure 29 illustrates the connection of the slope compensation
resistor, R
ADP1876
DSON_MAX
CS
is the gain, either 3 V/V, 6 V/V, 12 V/V, or 24 V/V, of the
6
R
μA
RAMP
10
Figure 29. Slope Compensation and CS Gain Connection
is the the low-side MOSFET maximum on resistance.
≤
is an internal parameter.
RAMP
RAMP
V
=
IN
R
, and the current sense gain resistor, R
A
Figure 28. Accurate Current-Limit Sensing
value that produces between 6 µA and 20 µA.
−
RAMP
CS
o
0
3
C. Choose R
2 .
×
6 .
DSON
ADP1876
R
V
ADP1876
×
DSON
10
≤
RAMP
PGNDx
200
is temperature dependent and can vary
10
PGNDx
ILIMx
SWx
DHx
_
DLx
L
ILIMx
SWx
MAX
R
DHx
μA
DLx
RAMP
DSON
RAMP
R
RAMP
ILIM
R
R
ILIM
at the maximum operating
SENSE
, is calculated by
produces less than 6 µA,
R
CSG
V
IN
V
IN
RAMP
should not
CSG
.
Rev. A | Page 18 of 24
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
amplified by a gain factor of either 3 V/V, 6 V/V, 12 V/V, or
24 V/V, which is programmable by an external resistor, R
connected to the DLx pin. This gain is sensed during power-up
only 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
and 2.2 V. Select the current sense gain such that the internal
minimum amplified voltage (V
maximum amplified voltage (V
or V
to 2.3 V. Make sure that the maximum V
not exceed 2.2 V to account for temperature and device to
device variations. The following are equations for V
V
where:
V
sense amplifier at zero output current.
V
sense amplifier at maximum output current.
R
I
I
V
A
24 V/V set by the gain resistor between DLx and PGNDx.
25 pF is an internal parameter.
t
INPUT CAPACITOR SELECTION
The input current to a buck converter is a pulse waveform. It is
zero when the high-side switch is off and approximately equal
to the load current when it is on. The input capacitor carries the
input ripple current, allowing the input power source to supply
only the direct current. The input capacitor needs a sufficient
ripple current rating to handle the input ripple, as well as an
equivalent series resistance (ESR) that is low enough to mitigate
input voltage ripple. For the usual current ranges for these con-
verters, it is good practice to use two parallel capacitors placed
close to the drains of the high-side switch MOSFETs (one bulk
capacitor of sufficiently high current rating and a 10 μF ceramic
decoupling capacitor, typically).
LPP
LOADMAX
ON
DSON_MIN
CSMAX
CSMIN
CSMAX
COMPMAX
CS
is on time for the high-side driver (DH).
is the peak-to-peak ripple current in the inductor.
is the current sense gain of either 3 V/V, 6 V/V, 12 V/V, or
CSMAX
V
V
V
COMPMAX
is the minimum amplified voltage of the internal current
, and V
CSMIN
CSMAX
is the maximum amplified voltage of the internal current
is the maximum output dc load current.
is the the low-side MOSFET minimum on resistance.
is the maximum voltage at the COMP pin.
is not the same as V
=
=
COMPMAX
. 0
. 0
=
75
75
(
V
V
25
V
IN
−
+
:
pF
−
1
2
(
I
0
×
I
LOADMAX
LPP
2 .
R
DSON
V
RAMP
×
)
t
COMP
R
ON
CSMIN
of the MOSFET. The result is
CSMAX
DSON
+
, which has a range of 0.85 V
+
1
2
) is above 0.4 V and the
V
) is 2.1 V. Note that V
_
I
CSMAX
MIN
LPP
COMP
CS
)
×
×
, is between 0.4 V
A
R
CS
DSON
(V
Data Sheet
COMPMAX
_
MAX
CSMIN
×
) does
,
A
CSG
CSMIN
CS
,