MAX8731AETI+ Datasheet MAX8731AETI+, MAX8731AETI+T | P22-P24

MAX8731A

SMBus Level 2 Battery Charger

with Remote Sense

22 ______________________________________________________________________________________

Battery-Charger Commands

The MAX8731A supports four battery-charger com-

mands that use either Write-Word or Read-Word proto-

cols, as summarized in Table 4. ManufacturerID() and

DeviceID() can be used to identify the MAX8731A. On

the MAX8731A, the ManufacturerID() command always

returns 0x004D and the DeviceID() command always

returns 0x0008.

DC-DC Converter

The MAX8731A employs a synchronous step-down DC-

DC converter with an n-channel high-side MOSFET

switch and an n-channel low-side synchronous rectifier.

The MAX8731A features a pseudo-fixed-frequency, cur-

rent-mode control scheme with cycle-by-cycle current

limit. The controller’s constant off-time (t

OFF

) is calculat-

ed based on V

CSSP

, V

CSIN

, and a time constant with a

minimum value of 300ns. The MAX8731A can also oper-

ate in discontinuous-conduction mode for improved

light-load efficiency. The operation of the DC-DC con-

troller is determined by the following four comparators

as shown in the functional diagrams in Figures 2 and 6:

The IMIN comparator triggers a pulse in discontinuous

mode when the accumulated error is too high. IMIN

compares the control signal (LVC) against 100mV (typ).

When LVC is less than 100mV, DHI and DLO are both

forced low. Indirectly, IMIN sets the peak inductor cur-

rent in discontinuous mode.

The CCMP comparator is used for current-mode regu-

lation in continuous-conduction mode. CCMP com-

pares LVC against the inductor current. The high-side

MOSFET on-time is terminated when the CSI voltage is

higher than LVC.

The IMAX comparator provides a secondary cycle-by-

cycle current limit. IMAX compares CSI to 2V (corre-

sponding to 10A when RS2 = 10mΩ). The high-side

MOSFET on-time is terminated when the current-sense

signal exceeds 10A. A new cycle cannot start until the

IMAX comparator’s output goes low.

The ZCMP comparator provides zero-crossing detec-

tion during discontinuous conduction. ZCMP compares

the current-sense feedback signal to 750mA (RS2 =

10mΩ). When the inductor current is lower than the

750mA threshold, the comparator output is high and

DLO is turned off.

The OVP comparator is used to prevent overvoltage at

the output due to battery removal. OVP compares FBS_

against the set voltage (ChargeVoltage()). When FBS_

is 200mV above the set value, the OVP comparator out-

put goes high and the high-side MOSFET on-time is ter-

minated. DHI and DLO remain off until the OVP

condition is removed.

CCV, CCI, CCS, and LVC Control Blocks

The MAX8731A controls input current (CCS control

loop), charge current (CCI control loop), or charge volt-

age (CCV control loop), depending on the operating

condition. The three control loops—CCV, CCI, and

CCS—are brought together internally at the lowest volt-

age-clamp (LVC) amplifier. The output of the LVC

amplifier is the feedback control signal for the DC-DC

controller. The minimum voltage at the CCV, CCI, or

CCS appears at the output of the LVC amplifier and

clamps the other control loops to within 0.3V above the

COMMAND

COMMAND NAME READ/WRITE DESCRIPTION POR STATE

0x14 ChargeCurrent() Write Only 6-Bit Charge-Current Setting 0x0000

0x15 ChargeVoltage() Write Only 11-Bit Charge-Voltage Setting 0x0000

0x3F InputCurrent() Write Only 6-Bit Charge-Current Setting 0x0080

0xFE ManufacturerID() Read Only Manufacturer ID 0x004D

0xFF DeviceID() Read Only Device ID 0x0008

Table 4. Battery-Charger Command Summary

IMAX

CCMP

IMIN

ZCMP

OVP

CSI

100mV

150mV

ChargeVoltage ( )

+200mV

FBS_

DCIN

CSIN

LVC

OFF-TIME

ONE-SHOT

OFF-TIME

COMPUTE

DRIVER

Figure 6. DC-DC Converter Functional Diagram

MAX8731A

SMBus Level 2 Battery Charger

with Remote Sense

______________________________________________________________________________________ 23

control point. Clamping the other two control loops

close to the lowest control loop ensures fast transition

with minimal overshoot when switching between differ-

ent control loops (see the Compensation section).

Continuous-Conduction Mode

With sufficient charge current, the MAX8731A’s induc-

tor current never crosses zero, which is defined as con-

tinuous-conduction mode. The regulator switches at

400kHz (nominal) if V

CSIN

< 0.88 x V

CSSP

. The con-

troller starts a new cycle by turning on the high-side

MOSFET and turning off the low-side MOSFET. When

the charge-current feedback signal (CSI) is greater

than the control point (LVC), the CCMP comparator out-

put goes high and the controller initiates the off-time by

turning off the high-side MOSFET and turning on the

low-side MOSFET. The operating frequency is gov-

erned by the off-time and is dependent upon V

CSIN

and

CSSP

. The off-time is set by the following equation:

The on-time can be determined using the following

equation:

where:

The switching frequency can then be calculated:

These equations describe the controller’s pseudo-

fixed-frequency performance over the most common

operating conditions.

At the end of the fixed off-time, the controller initiates a

new cycle if the control point (LVC) is greater than

100mV and the peak charge current is less than the

cycle-by-cycle current limit. Restated another way,

IMIN must be high, IMAX must be low, and OVP must

be low for the controller to initiate a new cycle. If the

peak inductor current exceeds the IMAX comparator

threshold or the output voltage exceeds the OVP

threshold, then the on-time is terminated. The cycle-by-

cycle current limit effectively protects against overcur-

rent and short-circuit faults.

If during the off-time the inductor current goes to zero,

the ZCMP comparator output pulls high, turning off the

low-side MOSFET. Both the high- and low-side

MOSFETs are turned off until another cycle is ready to

begin. ZCOMP causes the MAX8731A to enter into dis-

continuous-conduction mode (see the Discontinuous

Conduction section).

There is a 0.3µs minimum off-time when the (V

CSSP

CSIN

) differential becomes too small. If V

CSIN

≥ 0.88 x

CSSP

, the threshold for the 0.3µs minimum off-time is

reached. The switching frequency in this mode varies

according to the equation:

Discontinuous Conduction

The MAX8731A can also operate in discontinuous-con-

duction mode to ensure that the inductor current is

always positive. The MAX8731A enters discontinuous-

conduction mode when the output of the LVC control

point falls below 100mV. This corresponds to peak

inductor current = 500mA:

charge current for RS2 = 10mΩ.

In discontinuous mode, a new cycle is not started until

the LVC voltage rises above 100mV. Discontinuous-

mode operation can occur during conditioning charge

of overdischarged battery packs, when the charge cur-

rent has been reduced sufficiently by the CCS control

loop, or when the charger is in constant-voltage mode

with a nearly full battery pack.

CHG

=×

100

20 2

250

RIPPLE

CSSN BATT

−

03. μ

ON OFF

RIPPLE

BATT OFF

RIPPLE

CSSN BATT

−

OFF

CSSP CSIN

CSSP

=×

−

25. μ

MAX8731A

SMBus Level 2 Battery Charger

with Remote Sense

24 ______________________________________________________________________________________

Compensation

The charge-voltage and charge-current regulation

loops are independent and compensated separately at

the CCV, CCI, and CCS.

CCV Loop Compensation

The simplified schematic in Figure 7 is sufficient to

describe the operation of the MAX8731A when the volt-

age loop (CCV) is in control. The required compensa-

tion network is a pole-zero pair formed with C

and

. The zero is necessary to compensate the pole

formed by the output capacitor and the load. R

ESR

the equivalent series resistance (ESR) of the charger

output capacitor (C

OUT

). R

is the equivalent charger

output load, where R

= ΔV

BATT

/ ΔI

CHG

. The equiva-

lent output impedance of the GMV amplifier, R

OGMV

, is

greater than 10MΩ. The voltage amplifier transconduc-

tance, GMV = 0.125µA/mV. The DC-DC converter

transconductance is dependent upon the charge-cur-

rent sense resistor RS2:

OUT

where A

CSI

= 20V/V, and RS2 = 10mΩ in the typical

application circuits, so GM

OUT

= 5A/V. The loop-trans-

fer function is given by:

The poles and zeros of the voltage loop-transfer function

are listed from lowest frequency to highest frequency in

Table 5.

Near crossover C

is much lower impedance than

OGMV

. Since C

is in parallel with R

OGMV

, C

dom-

inates the parallel impedance near crossover.

Additionally, R

is much higher impedance than C

and dominates the series combination of R

and C

so near crossover:

RsCR

sC R

OGMV CV CV

CV OGMV

+×

≅

()

LTF GM R GMV R

sC R sC R

OUT L OGMV

OUT ESR CV CV

CV OGMV OUT L

=×××

+×+×

+× + ×

()()

2ARS

CSI

OUT

ESR

OGMV

CCV

FBS_

GMV

ChargeVoltage( )

OUT

Figure 7. CCV Loop Diagram

NAME EQUATION DESCRIPTION

CCV Pole

Lowest frequency pole created by C

and GMV’s finite output resistance.

CCV Zero

Voltage-loop compensation zero. If this zero is at the same frequency or

lower than the output pole f

P_OUT

, then the loop-transfer function

approximates a single-pole response near the crossover frequency. Choose

to place this zero at least 1 decade below crossover to ensure

adequate phase margin.

Output

Pole

Output pole formed with the effective load resistance R

and the output

capacitance C

OUT

. R

influences the DC gain but does not affect the

stability of the system or the crossover frequency.

Output

Zero

Output ESR Zero. This zero can keep the loop from crossing unity gain if

Z_OUT

is less than the desired crossover frequency; therefore, choose a

capacitor with an ESR zero greater than the crossover frequency.

Table 5. CCV Loop Poles and Zeros

PCV

OGMV CV

2π

ZCV

CV CV

2π

P OUT

L OUT

2π

P OUT

L OUT

2π

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MAX8731AETI+

Mfr. #:

Buy MAX8731AETI+

Manufacturer:

Maxim Integrated

Description:

Battery Management SMBus Level 2 Battery Charger

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MAX8731AETI+

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