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MAX1804EUB

External Four-Input Feedback Integrator for Power Supplies

Maxim Integrated 

External Four-Input Feedback Integrator for

Power Supplies

MAX1653 FB

GND

RTOP

ROUT

RBOTTOM

TO

MAX1804

SHORT COMPACT PLACEMENT

OF FEEDBACK RESISTORS

REDUCES NOISE PICKUP

Figure 5. Proper Layout of Feedback Resistors for Regulator

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 ROUT (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 ICOMP at COMP will result in an error given by:

IN_ error voltage = ICOMP / (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:

VOUT, min = 5V - 5% = 4.75V

To minimize power dissipation, set regulator output

VOUT, R = 4.8V with MAX1653 feedback resistors.

Choose feedback resistors (RTOP and RBOTTOM) for

about 25µA current in the resistive divider for noise

immunity.

Therefore:

(RTOP + RBOTTOM) = 4.8V / 25µA = 192kΩ

For the MAX1653, the voltage at its feedback point

VFB = 2.5V.

Choose RTOP = 91kΩ and therefore, RBOTTOM =

100kΩ.

If the desired adjustment range is +400mV, max OUT

current (MAX1804) is given by:

IOUT(MAX) = 0.4V / 91kΩ = 4.4µA

ROUT = VFB / IOUT(MAX) = 2.5V / 4.4µA = 562kΩ

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 VADJ, VADJ = 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

10 ______________________________________________________________________________________

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