Rainbow Electronics MAX1804 User Manual

General Description

The MAX1804 feedback integrator is designed to
improve voltage regulation in power-supply systems.
The device corrects line- and load-regulation problems,
and can be used to compensate for voltage drops in
power-management distribution lines.

The regulation set point is determined by an external
reference voltage applied at the ADJ pin. The differ­ence between that reference and MAX1804 feedback is
integrated, and a correction current is applied to the
voltage regulator’s feedback.

The MAX1804’s four inputs allow voltage monitoring in
several local and remote locations to correct for distribu­tion losses caused by long traces, connectors, and
switches. This eliminates the wasteful practice of perma­nently adjusting the regulator’s output voltage high
enough to correct for worst-case voltage drops. This is
especially helpful in systems where the load current
varies widely and portions of the load are often shut down
or disconnected (for example, in notebook computers).

If a load switch is opened and the output voltage
sensed at one input falls, the MAX1804 automatically
disables that input and adjusts the regulator’s output
voltage to the lowest of remaining active inputs. If all
inputs are disabled, the OUT pin is in a high-imped­ance state and makes no output voltage correction.

When SHDN is low, the MAX1804 draws less than 1µA
and its inputs and output are high impedance. The
MAX1804 is available in a 10-pin µMAX package, about
half the size of an 8-pin SOIC.

Features

♦ High Precision (±1% max Error)
♦ Low Power (25µA Supply Current)
♦ Automatic Input Disable
♦ Shutdown (1µA max Supply Current)
♦ Inputs and Outputs High Impedance in Shutdown

or When V+ = 0

♦ +2.7V to +5.5V Supply Range
♦ 2.8V to 5.5V Regulation Adjust Range
♦ Parallelable for More than Four Inputs
♦ 10-Pin µMAX Package

MAX1804

External Four-Input Feedback Integrator for

Power Supplies

________________________________________________________________ Maxim Integrated Products 1

19-1737; Rev 0; 7/00

For free samples and the latest literature, visit www.maxim-ic.com or phone [phone].
For small orders, phone [phone].

Ordering Information

Applications

Pin Configuration appears at end of data sheet.

Remote Feedback Regulators

High-Accuracy Regulators

Desktop/Notebook Computers

Servers and Workstations

Subnotebooks and PDAs

Typical Operating Circuit

PART TEMP. RANGE PIN-PACKAGE

MAX1804EUB -40°C to +85°C 10 µMAX

V

IN

DH

SYSTEM
SUPPLY

DL

R

TOP

FBREF

SHDN

IN1

IN2
IN3
IN4

R

BOTTOM

V+

R

OUT

OUT

ADJ

MAX1804

COMP

GND

V

A

OUT

V

B

OUT

V

C

OUT

D

V

OUT

MAX1804

External Four-Input Feedback Integrator for
Power Supplies

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(TA= 0°C to +85°C, VV+ = 3.3V, V

ADJ

= 1.2V, V

OUT

= 2V, CV+ = 0.1µF, C

COMP

= 470pF. Typical values are at TA= +25°C, unless

otherwise noted.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

V+, IN1, IN2, IN3, IN4, OUT, ADJ, SHDN to GND...…-0.3V, +6V

COMP to GND ....................................................-0.3V, V+ +0.3V

Continuous Power Dissipation (T

A

= +70°C)

10-Pin µMAX (derate 8.3mW/°C above +70°C) ..........667mW

Operating Temperature Extended Range ...........-40°C to +85°C

Junction Temperature ..................…………………………+150°C

Storage Temperature Range.……...……………-65°C to +165°C

Lead Temperature (soldering, 10s) .................................+300°C

PARAMETER CONDITIONS

U N IT S

Supply Voltage Range (V+) 2.7 5.5 V
Shutdown Supply Current (V+) SHDN = GND

1 µA

Quiescent Supply Current (V+) SHDN = V+ 25 35 µA

UVLO Threshold, Rising 2.5 2.6 V

UVLO Threshold, Falling 2.3 2.4 V

INTEGRATOR INPUTS IN1–IN4

IN_ Voltage Range 0 5.5 V

IN_ Offset VV+ = 2.7V to 5.5V -15 15

mV

V

ADJ

= 0.7V to 1.4V

V/V

V

IN_

= 4.75V to 4.85V, V

COMP

= 1.3V 12 30

48

µS

V

IN_

= 4.8V 1.5 2.7 µA

SHDN = GND 0.2 µA

VV+ = 0, V

SHDN

= 0 0.2 µA

IN_ Disable Threshold

V

ADJ

= 0.7V to 1.4V, V

ADJ

x 4, falling edge 87 90 93

%

V

ADJ

= 0.7V to 1.4V, rising edge 50

mV

ADJ INPUT

0.7 1.4 V

-20 20 nA

SHDN = GND 0.2 µA

VV+ = 0, V

SHDN

= 0 0.2 µA

COMP OUTPUT

V

COMP

= 1.3V, V

IN_

= 4.6V 1.6 2.0 2.4 µA

V

COMP

= 1.3V, V

IN_

= 5.0V 1.6 2.0 2.4 µA

Sink 1µA, V

IN_

= 5.0V 0.2 V

Source 1µA, V

IN_

= 4.6V 2.4 V

OUT OUTPUT

I

OUT

= 10 µA, V

IN_

= 4.6V 0.2 V

V

SHDN

= 0 0.1

VV+ = 0 0.1

V

IN_

= 5.0V 0.1

µA

V

COMP

= 1V to 2V 4 10 22 µS

IN_ Scale Factor

IN_ Transconductance

IN_ Bias Current

IN_ Shutdown Current

IN_ Leakage Current

IN_ Disable Threshold Hysteresis

ADJ Voltage Range

ADJ Bias Current

ADJ Shutdown Current

ADJ Leakage Current

COMP Source Current

COMP Sink Current

COMP Output Voltage Low

COMP Output Voltage High

OUT Output Voltage Low

OUT Output Leakage Current

OUT Transconductance

MIN TYP MAX

0.01

3.988 4.000 4.016

MAX1804

External Four-Input Feedback Integrator for

Power Supplies

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(TA= 0°C to +85°C, VV+ = 3.3V, V

ADJ

= 1.2V, V

OUT

= 2V, CV+ = 0.1µF, C

COMP

= 470pF. Typical values are at TA= 25°C, unless oth-

erwise noted.)

PARAMETER CONDITIONS

MIN

TYP

MAX

U N IT S

SHDN INPUT

Logic Input Voltage High VV+ = 2.7V to 5.5V 2.0 V

Logic Input Voltage Low VV+ = 2.7V to 5.5V 0.6 V

Logic Input Current -1 1 µA

ELECTRICAL CHARACTERISTICS

(TA= -40°C to +85°C, VV+ = 3.3V, V

ADJ

= 1.2V, V

OUT

= 2V, CV+ = 0.1µF, C

COMP

= 470pF.) (Note 1)

PARAMETER CONDITIONS

U N IT S

Supply Voltage Range (V+) 2.7 5.5 V
Shutdown Supply Current (V+) SHDN = GND 1 µA
Quiescent Supply Current (V+) SHDN = V+ 35 µA

UVLO Threshold, Rising 2.6 V

UVLO Threshold, Falling 2.3 V

INTEGRATOR INPUTS IN1–IN4

IN_ Voltage Range 0 5.5 V

IN_ Offset VV+ = 2.7V to 5.5V -20 20

mV

V

ADJ

= 0.7V to 1.4V

V/V

V

IN_

= 4.75V to 4.85V, V

COMP

= 1.3V 12

60

µS

V

IN_

= 4.8V 2.7 µA

SHDN = GND 0.2 µA

VV+ = 0, V

SHDN

= 0 0.2 µA

IN_ Disable Threshold

V

ADJ

= 0.7V to 1.4V, V

ADJ

x 4, falling edge 87 93

%

ADJ INPUT

0.7 1.4 V

-30 30 nA

SHDN = GND 0.2 µA

V V+ = 0, V

SHDN

= 0 0.2 µA

COMP OUTPUT

V

COMP

= 1.3V, V

IN_

= 4.6V 1.6 2.4 µA

V

COMP

= 1.3V, V

IN_

= 5.0V 1.6 2.4 µA

Sink 1µA, V

IN_

= 5.0V 0.2 V

Source 1µA, V

IN_

= 0.6V 2.4 V

I

OUT

= 10µA, V

IN_

= 4.6V 0.2 V

V

SHDN

= 0 0.1

VV+ = 0 0.1

VIN = 5.0V 0.1

µA

OUT Transconductance V

COMP

= 1V to 2V 3 23 µS

IN_ Scale Factor

IN_ Transconductance

IN_ Bias Current

IN_ Shutdown Current

IN_ Leakage Current

ADJ Voltage Range

ADJ Bias Current

ADJ Shutdown Current

ADJ Leakage Current

COMP Source Current

COMP Sink Current

COMP Output Voltage Low

COMP Output Voltage High

OUT Output Voltage Low

OUT Output Leakage Current

MIN TYP MAX

3.98 4.02

ELECTRICAL CHARACTERISTICS (continued)

(TA= -40°C to +85°C, VV+ = 3.3V, V

ADJ

= 1.2V, V

OUT

= 2V, CV+ = 0.1µF, C

COMP

= 470pF.) (Note 1)

MAX1804

External Four-Input Feedback Integrator for
Power Supplies

4 _______________________________________________________________________________________

Typical Operating Characteristics

(VV+ = V

SHDN

= 3.3V, V

ADJ

= 1.2V, V

OUT

= 2V, C

COMP

= 470pF, TA= +25°C, unless otherwise noted.)

Note 1: Specifications to -40°C are guaranteed by design and not production tested.

PARAMETER CONDITIONS MIN TYP MAX U N IT S

SHDN INPUT

Logic Input Voltage High VV+ = 2.7V to 5.5V 2.0 V

Logic Input Voltage Low VV+ = 2.7V to 5.5V 0.6 V

Logic Input Current -1 1 µA

29.0

28.5

28.0

27.5

27.0

26.5

26.0

25.5

QUIESCENT CURRENT (µA)

25.0

24.5

24.0

-40 0-20 20 40 60 80

QUIESCENT CURRENT vs.

TEMPERATURE

VV+ = 5V

VV+ = 3.3V

TEMPERATURE (°C)

IN_REGULATION SET-POINT ERROR

vs. TEMPERATURE

0.050

V

0.045

MAX1804 toc01

0.040

0.035

0.030

0.025

0.020

0.015

0.010

IN_REGULATION SET-POINT ERROR (%)

0.005

0

-40

NOTE: Circuit of Figure 1.

= 0.7V

ADJ

V

= 1.4V

ADJ

V

= 1.2V

ADJ

0-20 20 40 60 80
TEMPERATURE (°C)

MAX1804 toc02

IN_ (TO COMP) TRANSCONDUCTANCE

34

32

30

28

26

24

IN_TRANSCONDUCTANCE (µS)

22

20

-40 0-20 20 40 60 80

vs. TEMPERATURE

V

∆

= 4.85V TO 4.75V

IN_

= 1.3V

V

COMP

TEMPERATURE (°C)

MAX1804 toc03

OUT CURRENT vs. COMP VOLTAGE

30

25

20

15

OUT CURRENT (µA)

10

5

0

0 1.5 2.00.5 1.0 2.5 3.0 3.5

COMP VOLTAGE (V)

MAX1804toc04

IN_SCALE FACTOR vs. ADJ VOLTAGE

4.05

4.00

3.95

3.90

IN_SCALE FACTOR

3.85

3.80

3.75

0.4

NOTE: Circuit of Figure 1.

ADJ VOLTAGE (V)

1.0 1.20.6 0.8 1.4

0

MAX1804toc05

-50

-100

-150

-200

CHANGE IN OUTPUT VOLTAGE (mV)

-250

-300

0.001

vs. OUTPUT CURRENT

MAX1804 OFF

V

= 5V (NOMINAL)

OUT

= 470pF

C

COMP

OUTPUT CURRENT (A)

NOTE: Application Circuit of Figure 2.

CHANGE IN OUTPUT VOLTAGE

MAX1804toc06

MAX1804 ON

1100.01 0.1

MAX1804

External Four-Input Feedback Integrator for

Power Supplies

_______________________________________________________________________________________ 5

Typical Operating Characteristics (continued)

(VV+ = V

SHDN

= 3.3V, V

ADJ

= 1.2V, V

OUT

= 2V, C

COMP

= 470pF, TA= +25°C, unless otherwise noted.)

CH1

CH5

CH4

CH3

CH2

LOAD TRANSIENT RESPONSE

(APPLICATION CIRCUIT OF FIGURE2)

100µs/div

CH1 = I

; 1.00A/div

LOAD

; 500mV/div; AC-COUPLED (C

CH5 = V

IN1

; 500mV/div; AC-COUPLED (C

CH4 = V

IN1

; 500mV/div; AC-COUPLED (C

CH3 = V

IN1

; 500mV/div; AC-COUPLED (MAX1804 DISABLED)

CH2 = V

IN1

(MAX1653) = 8V

V

IN

STARTUP/SHUTDOWN WAVEFORM

COMP
COMP
COMP

= 1000pF)
= 470pF)
= 220pF)

MAX1804toc07

CH1

CH2

CHR4

CHR3

CHR2

CHR1

CH1 = I
CH2 = V
CHR4 = V
CHR3 = V
CHR2 = V
CHR1 = V

(MAX603) = 8V

V

IN

NOTE: Circuit of Figure 3.

AUTOMATIC INPUT DISABLE WAVEFORM

LOAD TRANSIENT RESPONSE

400µs/div

; 500mA/div

LOAD

; 500mV/div; AC-COUPLED (C

IN1

; 500mV/div; AC-COUPLED (C

IN1

; 500mV/div; AC-COUPLED (C

IN1

; 500mV/div; AC-COUPLED (C

IN1

; 500mV/div; AC-COUPLED (MAX1804 DISABLED)

IN1

MAX1804toc08

COMP

COMP
COMP
COMP

= 0.047µF)

= 1000pF)
= 2200pF)
= 4700pF)

0.5

0.01

CH1

CH2

CH1 = SHDN, 2V/div

; 200mV/div; DC OFFSET = 4.7V

CH2 = V

IN2

0.5A

I

LOAD =

(MAX1653) = 8V

V

IN

NOTE: Circuit of Figure 2.

200µs/div

MAX1804toc09

CHR1

CH1

CH2

CHR1 = V
CH1 = V
CH2 = V
V
NOTE: Circuit of Figure 2.

IN2 ENABLE

IN2 DISABLE IN2 DISABLE

200µs/div

; 5V/div

G(PFET)

; 200mV/div; DC OFFSET = 4.8V

IN1

; 2V/div

IN2

(MAX1653) = 8V

IN

MAX1804toc10

MAX1804

External Four-Input Feedback Integrator for
Power Supplies

6 _______________________________________________________________________________________

Detailed Description

The MAX1804 is a precision external feedback integra­tor used for tight point-of-load regulation in power-man­agement systems. The dynamic regulation of the output
is still controlled by the power-supply regulator. The
MAX1804 is used only to fine tune the output voltage,
removing load- and line-regulation errors and voltage
drops across switches and connectors that occur in the
power-supply distribution line.

The MAX1804 includes four remote-sense inputs and
an output that connects to a voltage regulator’s feed­back divider. It can be used with any regulator or con­troller with a high-impedance feedback input. Its OUT
pin sinks current to increase the regulator’s set point so
that the four IN_ inputs are at or above MAX1804’s reg­ulation set point, which is four times the voltage applied
to the ADJ pin. The MAX1804 monitors the input signals
and compares the lowest to the voltage set point. The
error between the input signal and the set point is inte­grated. The MAX1804 output stage generates a current
based on the integrator output to adjust the regulator’s
output voltage to meet the set point.

Pin Description

NAME PIN FUNCTION

IN1 1

Sense Input 1. Connect to one of up to four inputs to be monitored. If IN_ is the lowest active input,
IN_ will be regulated at 4 x V

ADJ

. If IN_ is less than 90% of the regulation set point (4 x V

ADJ

), it is

automatically disabled.

IN2 2 Sense Input 2

IN3 3 Sense Input 3

IN4 4 Sense Input 4

ADJ 5

Regulation Adjustment Input. The voltage at this input adjusts the regulation set point of
IN1–IN4.There is a 4x scale factor between V

ADJ

and the regulation set point.

GND 6 Ground

COMP 7

Integrator Capacitor Connection. Connect a capacitor (>100pF) to GND to set the integrator time
constant. See Integrator Gain-Bandwidth Product and Regulator Stability section.

OUT 8

O p en- D r ai n O utp ut. C an si nk at l east 10µ A i f any of the i np uts r em ai n b el ow the r eg ul ati on set p oi nt.
C onnect OU T to the feed b ack p oi nt of the r eg ul ator - d i vi d er r esi stor s. If OU T i s connected thr oug h a
r esi stor , i ts val ue r el ati ve to the r eg ul ator ’ s feed b ack- d i vi d er va l ues sets the m axi m um p osi ti ve
ad j ustm ent r ang e. S ee S etti ng the M axi m um Reg ul ator O utp ut Ad j ustm ent Incr ease secti on.

SHDN 9

Shutdown Control Input. When SHDN is low, the device is off and the supply current is reduced to
about 10nA. All inputs are in high-impedance state. When SHDN is high, the device is on.

V+ 10 Power-Supply Input. Bypass with 0.1µF capacitor to GND.

Figure 1. Test Circuit for MAX1804 IN_ Regulation Set Point

0.1µF

1M
1%

3.3V

3.3V

1k

1%

+10V

-10V

1k

1%

75k
1%

25k
1%

470pF

ADJ

COMP

EXTERNAL

REF

IN2

IN3

IN4

IN1

V+

OUT

MAX1804

MAX400

SHDN

MAX1804

External Four-Input Feedback Integrator for

Power Supplies

_______________________________________________________________________________________ 7

The MAX1804 only sinks current at OUT (Figure 4). The
maximum sink current and maximum regulator voltage
increase are limited by the minimum guaranteed OUT
current of 10µA. The maximum regulator output voltage
increase can be further limited by including a resistor
between OUT and the feedback point of the regulator.

If any of the IN_ inputs fall to 90% of the regulation set
point, they are automatically disabled and OUT contin­ues to regulate according to the lowest remaining
active input. See Automatic Input Disable.

Adjusting the Regulation Set Point

The MAX1804 monitors the voltage at the IN1–IN4
sense inputs and regulates the lowest active input to
four times the reference voltage at ADJ. The IN1–IN4
regulation set point is calculated as:

V

IN_

= 4 ✕V

ADJ

The total error in the MAX1804 regulation set point is:

total percentage error = [(IN_scale factor x V

ADJ

+

IN_offset) / (4 x V

ADJ

)] x 100%

The maximum IN_ voltage is 5.5V. Use the MAX1804 for
higher output voltages by using an external divider to
divide down the desired output voltage at the IN1–IN4
pins. Since the IN_ input bias current is fairly high (1.5µA
typ), choose the resistive divider accordingly.

Automatic Input Disable

The MAX1804 has an input buffer stage that monitors
the sense inputs (IN1–IN4) and connects the lowest to
its internal integrator (Figure 4). Each input is com­pared to 90% of the set point. If the input falls below
this value, the input buffer is disabled and the output
voltage is adjusted according to the lowest of the
remaining active inputs (see Automatic Input Disable in
Typical Operating Characteristics). If all four sense

4.5V TO 28V

VIN

VOUT

SHDN

SYNC

REF

3

10 16

1

2

3

14

11

15

13

12

8

9

7

DH

BST

VL

LX

DL

PGND

CSH

CSL

FB

6

6

4

3

1

5

827

2

5

3

4

1

V+

SHDN

N1B
IRF7303

D1

CMPSH-3

C4

0.1µF

N1A
IRF7303

D2
MBR0530

SKIP

SYNC

REF

GND

1M

SS

JU3

2

JU4

EXT
REF

COMP

ADJ

IN2

IN3

IN4

IN1

OUTV+

VL

C6

0.01µF

0.01µF

C5

0.33µF

100k

100k

JU2

JU1

3

VL

VL

2

3

1

C2

0.1µF

C1
22µF
35V

C7
220µF
10V

R

TOP

91k
1%

5V± 5%

PFET

I

LOAD

R

BOTTOM

100k

1%

562k

C3

4.7µF
16V

L1

15µH

R1

0.033Ω
1%

2

U1

MAX1653

MAX1804

SHDN

Figure 2. Application Circuit with Step-Down Regulator

MAX1804

External Four-Input Feedback Integrator for
Power Supplies

8 _______________________________________________________________________________________

inputs are disabled, OUT is high impedance. Connect
any unused input to GND.

Setting the Maximum Regulator Output

Voltage Increase

The ratio between the maximum guaranteed OUT cur­rent of 10µA and the current in the regulator’s feedback
resistive divider determines the maximum increase. The
maximum increase in the regulator output voltage
depends on the regulator’s upper feedback resistor
(R

TOP

) and the sink current into OUT:

∆V

OUT

(MAX) = I

OUT

(MAX) x R

TOP

The maximum adjust current I

OUT

(MAX) can be further

limited with a resistor (R

OUT

) between OUT and the
feedback point of the regulator (see Typical Operating
Circuit):

I

OUT

(MAX) = VFB/ R

OUT

where VFBis the voltage at the regulator’s feedback
point.

Therefore:

∆V

OUT

(MAX) = VFBx R

TOP

/ R

OUT

and:

R

OUT

= VFBx R

TOP

/ ∆V

OUT

(MAX)

Place the regulator’s feedback resistors and R

OUT

close to the regulator’s feedback pin to reduce noise
pickup at the regulator’s feedback point, which can
cause unstable switching in the regulator (Figure 5).

Shutdown Mode

SHDN is a logic input that, when held low, places the
MAX1804 in its low-power shutdown mode, reducing
the supply current to 10nA (typ). The IN1–IN4, OUT,
and ADJ are high impedance when the MAX1804 is in
shutdown or when V+ is removed. Connect SHDN to
V+ for normal operation.

Undervoltage Lockout

The MAX1804 has an undervoltage lockout (UVLO) fea­ture that deactivates the device when the supply volt­age at V+ goes below 2.4V; IN1–IN4, OUT, and ADJ go
to high impedance and do not affect the regulator oper­ation. Hysteresis holds the device in lockout until the
supply voltage at V+ rises above 2.6V.

Integrator Gain-Bandwidth Product

and Regulator Stability

The MAX1804 gain-bandwidth (GBW) product is set by
the external capacitor on COMP:

GBW = [4 x (IN_ transconductance)] / (2π✕C

COMP

)

The bandwidth is typically 40kHz with a 470pF capaci­tor on COMP.

For system stability, the integrator GBW product is typi­cally set below the regulator circuit’s crossover frequen­cy, if known. Switching regulators typically have
crossover frequencies well below their switching frequen­cies. Setting the MAX1804’s GBW product too high can
cause regulator loop instability, typically evidenced by
ringing after transients. Setting the GBW product unnec­essarily low will slow the MAX1804’s loop response to
transients, although the regulator’s loop transient
response will remain unaffected (see Load Transient
Response in Typical Operating Characteristics).

Often the regulator’s crossover frequency varies with
load and is not easily found. If the regulator’s crossover
frequency is unknown, the MAX1804’s gain-bandwidth
product can be chosen empirically. Start with a fairly
low capacitor value (470pF is a good starting point)
and increase the value until the circuit is stable with all
loads. Then increase the value further to ensure design
margin. If transient response is unimportant, choose a
large COMP capacitor value (such as 2200pF), thereby
maximizing stability.

Figure 3. MAX1804 Used with Linear Regulator

COMP

ADJ

IN2

IN3

IN4

IN1

GND

V+ SET

10µF

2200pF

R

TOP

300k

R

TOP

600k

I

LOAD

0 TO

500mA

FDV 304P

5V TO

11V

1.25V

REFERENCE

MAX1804

SHDN

10µF

R

BOTTOM

100k

GND SET

OUT

IN

MAX603

OFF

MAX1804

External Four-Input Feedback Integrator for

Power Supplies

_______________________________________________________________________________________ 9

Figure 4. MAX1804 Functional Block Diagram

V

BATT

V+

SYSTEM

DC/DC

FB3 FB5

GND

390k

UVLO

V+

EN

EN

SHDN

SHUTDOWN

LOGIC

OUT

EN

0.1µF

47k

51k

+5V SYSTEM SUPPLY

ONA

ONB

ONC

OND

MAX1804

IN1

3R

1R

SHDN

IN2

3R

1R

470pF

COMP

GND

g

m

EN

= 120µS

SHDN

EN

EN

9R1R

IN3

3R

1R

SHDN

IN4

3R

1R

SHDN

ADJ

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 reg­ulator 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 = 192kΩ

For the MAX1653, the voltage at its feedback point
V

FB

= 2.5V.

Choose R

TOP

= 91kΩ and therefore, R

BOTTOM

=

100kΩ.

If the desired adjustment range is +400mV, max OUT
current (MAX1804) is given by:

I

OUT

(MAX) = 0.4V / 91kΩ = 4.4µA

R

OUT

= VFB/ I

OUT

(MAX) = 2.5V / 4.4µA = 562kΩ

To guarantee that the output will not go below the tar­get 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

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

11 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

11 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

11 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

11 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

Package Information

Note: MAX1804 does not feature exposed pads

External Four-Input Feedback Integrator for

Power Supplies

MAX1804