AN1488.0 INTERSIL [Intersil Corporation], AN1488.0 Datasheet
![no-image](/images/no-image-200.jpg)
AN1488.0
Manufacturer Part Number
AN1488.0
Description
Fixed DC/DC Regulator Output Margining Using Digitally Controlled Potentiometer
Manufacturer
INTERSIL [Intersil Corporation]
Datasheet
1.AN1488.0.pdf
(2 pages)
VIN
Many applications using a DC/DC regulator are implemented
with a resistor divider in the feedback path to set up Vout.
However, for some high end applications, the voltage needs
to be adjusted in order to optimize the application’s
performance. A Digitally Controlled Potentiometer (DCP)
can be used in both high end and low end applications to
provide this flexibility.
The typical Buck DC/DC converter scheme includes a PWM
driver, switches, inductor, filter, and feedback divider (Figure 1).
The feedback voltage, FB, from the resistor divider R1 and
R2, is a negative input for the error amplifier of the voltage
control loop. In addition to the point of load regulation, the
feedback divider also sets up the desired output voltage,
Vout. In other words, Vout is programmable through an
external resistor divider based on Equation 1.
where Vref is a reference voltage and a positive input for the
error amplifier.
The minimum output voltage can be set as low as Vref,
according to Equation 1. The maximum output is also
determined by the resistor divider ratio, as well as input
voltage and the maximum duty cycle. The reference voltage,
Vref, can be obtained from a PWM controller datasheet.
Thus, making the feedback divider adjustable makes the
DC/DC regulator also adjustable, as shown in Figure 2.
The adjustable output voltage of the DC/DC regulator can be
expressed by Equation 2.
where R1 and R2 are fixed resistors, Rw is a wiper resistance,
Rtotal is the total resistance of the DCP, code is a decimal
code of wiper position and n is a total number of wiper taps.
Vout
Vout
=
=
FIGURE 1. TYPICAL BUCK DC/DC CONVERTER
Vref
Vref
ERROR
CONTROLLER
•
•
⎛
⎝
PWM
⎛
⎝
---------------------------------------------------------------------------------------- -
R2
R1
------- -
R2
+
+
VREF
Rw
1 )
+
Rtotal code
FB
®
R1
1
•
D1
Application Note
Fixed DC/DC Regulator Output Margining Using
L1
⁄
(
n 1
–
)
+
C1
1
⎞
⎠
VOUT
(EQ. 1)
(EQ. 2)
1-888-INTERSIL or 1-888-468-3774
R1
R2
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Digitally Controlled Potentiometer
VIN
FIGURE 2. ADJUSTABLE POINT OF LOAD DC/DC REGULATOR
The advantages of having the DCP in serial with the R2 resistor
(rather than in parallel) are easier calculation and bandwidth
limitation of the DCP. The DCP terminals have a parasitic
capacitance of 10pF to 25pF, which is shown in Figure 2 as
Crw. Connecting any of the DCP terminals to the FB node will
cause phase shift and may decrease DC/DC regulator
performance, such as transient response and stability.
The right DCP should be chosen to meet the application
requirements based on knowing the Vout adjusting range
and desired resolution. For example, we would like to adjust
the Vout output voltage of a DC/DC regulator, built on the
ISL85001 PWM controller, in a range from 0.8V to 3.3V.
The DCP total resistance, Rtotal, R1 and R2 resistor values
can be derived from Equation 2. For simplicity, it can be
overwritten as Equation 3, where the wiper resistance is
counted together with the R2 serial resistor. The suggested
value of the R2 resistor should be less than or equal to
0.1*Rtotal in order to have a bigger adjusting range.
The maximum Vout will be at code = 0
and the minimum Vout will be at max code = n -1
According to the ISL85001 datasheet, the internal Vref = 0.6V
and the recommended value for the R1 resistor is from 1kΩ to
10kΩ. Since the number of Rtotal values of the DCPs are
limited, let’s select the Rtotal = 10kΩ first and then calculate
R1 and R2.
R1
R2
Vout
Vout max
Vout min
≤
=
1kΩ
(
(
August 17, 2009
0.1Rtotal
=
Vref
All other trademarks mentioned are the property of their respective owners.
ERROR
CONTROLLER
)
)
|
=
=
Intersil (and design) is a registered trademark of Intersil Americas Inc.
PWM
•
Vref
Vref
R1
---------------------------------------------------------------------------------------------------------- -
•
0.1Rtotal
Vout max
------------------------------------------------ -
+
VREF
•
•
0.1Rtotal
R1
---------------------------------------- -
Copyright Intersil Americas Inc. 2009. All Rights Reserved
R1
---------------------------------------- -
(
1.1Rtotal
0.1Rtotal
+
Vref
+
FB
1.1Rtotal
0.1Rtotal
+
) Vref
–
Rtotal code
+
D1
Rtotal code
L1
=
•
1kΩ
•
•
⁄
3.3V 0.6V
------------------------------ -
(
n 1
C1
⁄
(
–
0.6V
n 1
–
AN1488.0
–
VOUT
)
)
RW
R1
R2
CRW
=
(EQ. 3)
(EQ. 4)
(EQ. 5)
(EQ. 6)
4.5kΩ
DCP
Related parts for AN1488.0
AN1488.0 Summary of contents
Page 1
... Rtotal = 10kΩ first and then calculate (EQ ⎠ ⁄ – R1 and R2 0.1Rtotal ≤ R2 1kΩ CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 AN1488.0 L1 VOUT D1 C1 PWM CONTROLLER FB ERROR VREF • ⁄ 0.1Rtotal + Rtotal code n 1 – ...
Page 2
... Vout resolution per step changes with the tap position, from 50mV per tap at the low end to 2mV per tap at the high end of the DCP settings in our example. 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 FIGURE 3. THE ISL85001 VOUT vs THE ISL22317W TAP POSITION 100 120 TAP POSITION (DECIMAL) AN1488.0 August 17, 2009 ...