Rainbow Electronics MAX1804 - Manual (Page 10)

MAX1804
External Four-Input Feedback Integrator for
Power Supplies
10 ______________________________________________________________________________________
Applications Information
Layout Information
Proper layout is essential due to high current levels and
switching waveforms that generate noise. Place the
regulator’s feedback resistors and R
OUT (from the reg-
ulator feedback point to OUT of MAX1804) close to the
regulator’s feedback point to prevent noise pickup
(Figure 5). Minimize leakage of the COMP capacitor and
traces around COMP because they cause IN_ set-point
errors according to the IN_ transconductance. A leakage
of I
COMP at COMP will result in an error given by:
IN_ error voltage = I COMP / (IN_ transconductance)
For a typical value of IN_ transconductance of 30µS, a
leakage of 0.1µA will result in an IN_ error voltage of
3mV in the set point.
Design Example (Step-Down
Converter Application)
Figure 2 is an example of an application circuit using
the MAX1804 for point-of-load regulation when using a
step-down controller (MAX1653). Use the following
design procedure to optimize the basic schematic.
Begin the design by establishing the following:
• Input voltage to the MAX1653 controller (7V to 20V)
• Typical output voltage and accuracy of MAX1653
(5V ±5%)
This gives a target of:
V OUT, min = 5V - 5% = 4.75V
To minimize power dissipation, set regulator output
V
OUT, R = 4.8V with MAX1653 feedback resistors.
Choose feedback resistors (R TOP and R BOTTOM ) for
about 25µA current in the resistive divider for noise
immunity.
Therefore:
(R TOP + R BOTTOM ) = 4.8V / 25µA = 192k Ω
For the MAX1653, the voltage at its feedback point
V
FB = 2.5V.
Choose R TOP = 91k
Ω and therefore, R
BOTTOM =
100k
Ω.
If the desired adjustment range is +400mV, max OUT
current (MAX1804) is given by:
I OUT (MAX) = 0.4V / 91k
Ω = 4.4µA
R OUT = V FB / I OUT (MAX) = 2.5V / 4.4µA = 562k
Ω
To guarantee that the output will not go below the tar-
get of 4.75V, including all error sources, set the
MAX1804 nominal regulation point to 4.85V.
Since:
IN_scale factor = 4 x V ADJ , V ADJ = 1.2125V
Divide the reference voltage down and bypass as
required if using the MAX1653 reference voltage as
shown in Figure 2. Assuming 0.1% resistor accuracy
and using the MAX1653 reference, the error budget
can be calculated as:
Reference Accuracy -1.6%
ADJ Divider Accuracy -0.2%
Scale Factor Error -0.4%
IN_ Offset Voltage (-15mV) -0.3%
Total -2.5%
This gives a worst-case output voltage of 4.73V, which is
below the target of 4.75V. The most-significant error
source is the MAX1653 reference. Therefore, use a better
reference, if available, in the system or increase the VADJ
to increase the worst-case output voltage to 4.75V.
The MAX1653 works well with the MAX1804’s GBW
product set at 40kHz, so use a 470pF capacitor on
COMP.
Chip Information
TRANSISTOR COUNT: 636
Figure 5. Proper Layout of Feedback Resistors for Regulator
R
BOTTOM
R OUT
R TOP
TO
MAX1804
FB
G
ND
MAX1653
SHORT COMPACT PLACEMENT
OF FEEDBACK RESISTORS
REDUCES NOISE PICKUP