MAX8688ALETG+ Datasheet MAX8688AHETG+, MAX8688ALETG+, MAX8688BHETG+T | P10-P12

MAX8688

Digital Power-Supply Controller/Monitor

with PMBus Interface

10 ______________________________________________________________________________________

MAX8688 Operating Modes

Reference Input (REFIN) Mode

Figure 3 shows the typical manner in which the

MAX8688 is used in an application where the POL has a

reference input, REFIN, to which it regulates its output

voltage between the VO+ and VO- terminals. In the

REFIN application, the DACOUT of the MAX8688 is con-

nected to the REFIN input of the POL. The output voltage

of the POL is sensed using RS+ and RS-. The sensed

voltage is suitably filtered by an internal 200Ω resistor

and external capacitor connected to RS_C and is multi-

plexed to a 12-bit ADC that uses an accurate internal ref-

erence voltage. On receipt of either an OPERATION ON

command or a turn-on signal from A3/ONOFF, the

MAX8688 commences the startup operation that has

been programmed for the POL being controlled.

After the programmed t

ON_DELAY

time, the MAX8688

open-drain ENOUT output goes active and the POL out-

put voltage is ramped up to its target VOUT_COMMAND

value precisely in the programmed t

ON_RISE

time. This

facilitates easy implementation of tracking of multiple out-

put rails. On reaching the target output voltage, the

MAX8688 continuously monitors the POL output voltage

obtained at the RS+ and RS- inputs, and regulates it to

within ±0.2% for line, load, and temperature variations by

incrementing or decrementing the DACOUT output 1 LSB

(0.5mV) at a time. The MAX8688 output-voltage correc-

tion rate is programmable up to 10kHz by the

MFR_VOUT_CORRECTION_RATE parameter. Once the

requested target POL voltage is reached, it is possible to

easily margin up or down the POL voltage at a prepro-

grammed slew rate set by the parameter VOUT_TRANSI-

TION_RATE. To achieve this, the MAX8688 increments or

decrements the DACOUT output in a suitable number of

steps that depend on the programmed transition rate. In

addition, the user needs to program the

VOUT_SCALE_LOOP parameter equal to any voltage-

divider ratio implemented on the POL from its output volt-

age node to the inverting input of its error amplifier. This

allows the MAX8688 to correctly calculate the number of

DACOUT steps and voltage increments/decrements per

step and thus achieves the programmed rise time and

transition time.

SCL

SDA

SYSTEM

CONTROLLER

IRQ

RST

SCL

SDA

FLT

RST

A3/ONOFF A2/SDAE A1/SCLE

MAX8688

VO+

VO-

IO+

POL 1

IO-

RS+

RS-

ISN+

ISN-

DACOUT

ENOUT

SCL

SDA

FLT

RST

MAX8688

VO+

VO-

IO+

POL 2

IO-

RS+

RS-

ISN+

ISN-

DACOUT

ENOUT

SCL

SDA

FLT

RST

MAX8688

VO+

VO-

IO+

POL 3

IO-

RS+

RS-

ISN+

ISN-

DACOUT

ENOUT

SCL

SDA

FLT

RST

MAX8688

VO+

VO-

IO+

POL N

IO-

RS+

RS-

ISN+

ISN-

DACOUT

ENOUT

Figure 2. System Application Showing Multiple MAX8688s Controlling POL Power Supplies

MAX8688

Digital Power-Supply Controller/Monitor

with PMBus Interface

______________________________________________________________________________________ 11

Since the reference voltage input is provided by the

MAX8688, the REFIN mode provides complete control

of the POL in terms of soft-start, soft-stop, and margin-

ing transitions. It may be noted that the slew rates dur-

ing soft-start (t

ON_RISE

) and during margining

(VOUT_TRANSITION_RATE) should be programmed

with POL current-limit consideration. An excessively

fast slew rate causes the POL to trip due to overcurrent.

A general guideline for setting the output-voltage slew

rate is as follows:

where C

OUT

is the output capacitance on the POL out-

put, I

LOAD

is the load current being delivered by the

POL, and I

LIMIT

is the current-limit setting of the POL.

On receipt of either an OPERATION OFF command or a

turn-off signal from A3/ONOFF, the MAX8688 com-

mences the shutdown operation that has been pro-

grammed for the POL being controlled. After the

programmed t

OFF_DELAY

time, the MAX8688 ramps

down the output voltage to zero precisely in the pro-

grammed t

OFF_FALL

time, and deasserts its open-drain

ENOUT output.

SLEW RATE

LIMIT LOAD

OUT

()

≤

−

DVDD

DGND

AVDD

AGND

0.1µF

1µF

200Ω

RS-

RS+

RS_C

200Ω

ISN-

ISN+

ISN_C

REFO

DACOUT

REFIN

VO+ VO-

POWER MONITOR

SMBus

CONTROLLER

INTERNAL REFERENCE

TEMP SENSOR

ADC

IN+

SENSE

LOAD

IN-

MAX8688

A1/SCLE

A2/SDAE

A3/ONOFF

SCL

SDA

RST

0.1µF

3.3V

POL

PMBus COMMAND AND

STATUS REGISTERS

DAC

MAXQ µC

ENOUT

FLT

CLKIO

CLKOUT

IRQ

SYSTEM

CONTROLLER

Figure 3. Typical System Application—REFIN Mode

MAX8688

Digital Power-Supply Controller/Monitor

with PMBus Interface

12 ______________________________________________________________________________________

Feedback (FB) Mode

In applications where a REFIN input is not available, the

MAX8688 interfaces to the inverting input of the POL

error amplifier, the feedback node (FB), through a

resistor R

as shown in Figure 5. In steady-state oper-

ation, the MAX8688 controls the POL voltage as mea-

sured between RS+ and RS- to 0.2% accuracy over

line, load, and temperature variations, by adjusting

DACOUT 1 LSB (0.5mV) at a time, up and down as

required. This mode of operation is termed FB mode.

Since the MAX8688 does not have control over the POL

error-amplifier reference voltage, this mode relies on

the POL soft-start setting to implement the required

soft-start time. On receipt of either an OPERATION ON

command or a turn-on signal from A3/ONOFF, the

MAX8688 commences startup operations that have

been programmed for the POL being controlled. After

the programmed t

ON_DELAY

time, the MAX8688 open-

drain ENOUT output goes active, causing the POL to

ramp up its output voltage to its target value. The soft-

start time taken by the POL to ramp from zero to its

commanded output voltage should be entered into the

MAX8688 with the t

ON_RISE

parameter.

During t

ON_RISE

, the MAX8688 maintains DACOUT in a

high-impedance state by keeping the S1 switch open.

This allows the voltage at DACOUT to equal that of the

FB node of the POL. At the end of the t

ON_RISE

delay

time, the internal DAC output is initialized to the external

voltage measured on DACOUT and switch S1 is closed.

If the POL has completed its soft-start and settled down

at its output voltage, the DAC output is initialized to the

steady-state value of the POL FB voltage. Therefore,

when switch S1 is closed, the voltages on either side of

the resistor R

are equal. Under these conditions, zero

current flows into the FB node from DACOUT and no

perturbations are introduced to the output voltage. From

this point on, the MAX8688 adjusts the voltage at

DACOUT to provide accurate output voltage control. In

FB mode, the user is required to supply t

ON_DELAY

and

ON_RISE

parameters. If those parameters are not set

(the default values are zero), S1 closes prematurely and

cause an initial error in the voltage monitor.

On receipt of either an OPERATION OFF command or a

turn-off signal from A3/ONOFF, the MAX8688 com-

mences the shutdown operation that has been pro-

grammed for the POL being controlled. After the

programmed t

OFF_DELAY

time, the MAX8688 deasserts

its open-drain ENOUT output, and turns off the POL.

For the FB mode, the value of R

is selected based on

following formula:

DAC

=×

∆

PMB_RSP

ON_DELAY

CLOSE S1

OFF_DELAY

OFF_FALL

ON_RISE

TURN-ON

TURN-OFF

DAC

OUTPUT

ENOUT

POL

OUT

POL

OPERATION

Figure 4. REFIN Mode Timing

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MAX8688ALETG+

Mfr. #:

Buy MAX8688ALETG+

Manufacturer:

Maxim Integrated

Description:

Current & Power Monitors & Regulators Digital Power-Supply Controller/Monitor

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MAX8688ALETG+

MAX8688ALETG+

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