MAX8677CETG/V+T Datasheet MAX8677CETG+, MAX8677CETG+T, MAX8677CETG/V+T | P13-P15

MAX8677C

1.5A Dual-Input USB/AC Adapter Charger

and Smart Power Selector

______________________________________________________________________________________ 13

Table 1. External Components List for Figures 2 and 3

COMPONENT

(FIGURES 2, 3)

FUNCTION PART

Input filter capacitor 4.7µF ceramic capacitor

CL VL filter capacitor 0.1µF ceramic capacitor

SYS

SYS output bypass capacitors 10µF ceramic capacitor

BAT

Battery bypass capacitor 4.7µF ceramic capacitor

CT Charger timing capacitor 0.068µF low TC ceramic capacitor

(x 4) Logic output pullup resistors 560kΩ

THM Negative TC thermistor

Phillips NTC thermistor, P/N 2322-640-63103, 10kΩ ±5% at +25°C

RT THM pullup resistor 10kΩ ±1%

PSET

Input current-limit programming resistor 1.5kΩ ±1% for 2A limit

ISET

Fast-charge current programming resistor

3kΩ ±1% for 1A charging

The MAX8677C features OVP. Part of this protection is

a 4.35V voltage limiter at SYS. If the DC or USB input

exceeds 4.35V, SYS still limits at 4.35V.

The MAX8677C has numerous other charging and

power-management features, which are detailed in the

following sections.

Smart Power Selector

The MAX8677C Smart Power Selector seamlessly dis-

tributes power between the external inputs, the battery,

and the system load (Figure 4). The basic functions

performed are:

• With both an external power supply (USB or

adapter) and battery connected:

When the system load requirements are less

than the input current limit, the battery is

charged with residual power from the input.

When the system load requirements exceed the

input current limit, the battery supplies supple-

mental current to the load.

• When the battery is connected and there is no

external power input, the system is powered from

the battery.

• When an external power input is connected and

there is no battery, the system is powered from the

external power input.

A thermal-limiting circuit reduces the battery charge

rate and external power-source current to prevent the

MAX8677C from overheating.

System Load Switch

An internal 40mΩ MOSFET connects SYS to BAT (Q3,

Figure 4) when no voltage source is available at DC or

USB. When an external source is detected at DC or

USB, this switch is opened and SYS is powered from

the valid input source through the input limiter.

The SYS-BAT switch also holds up SYS when the sys-

tem load exceeds the input current limit. If that should

happen, the SYS-BAT switch turns on so that the bat-

tery supplies additional SYS load current. If the system

load continuously exceeds the input current limit, the

battery does not charge, even though external power is

connected. This is not expected to occur in most

cases, since high loads usually occur only in short

peaks. During these peaks, battery energy is used, but

at all other times the battery charges.

ADAPTER

CHARGE AND

SYS LOAD

SWITCH

CHARGE

CURRENT

LOAD

CURRENT

USB

BAT

SYS

GND

BATTE

SYSTEM

LOAD

MAX8677C

Figure 4. Smart Power Selector Block Diagram

MAX8677C

1.5A Dual-Input USB/AC Adapter Charger

and Smart Power Selector

14 ______________________________________________________________________________________

Input Limiter

The input voltage limiter is essentially an LDO regulator.

While in dropout, the regulator dissipates a small I

R loss

through the 0.2Ω MOSFET (Q1, Figure 4) between DC

and SYS. With an AC adapter or USB source connect-

ed, the input limiter distributes power from the external

power source to the system load and battery charger.

In addition to the input limiter’s primary function of

passing power to the system and charger loads at SYS,

it performs several additional functions to optimize use

of available power:

• Input Voltage Limiting. If an input voltage is above

the overvoltage threshold (6.9V typ), the MAX8677C

enters overvoltage lockout (OVLO). OVLO protects

the MAX8677C and downstream circuitry from high-

voltage stress up to 14V at DC and 8V at USB. In

OVLO, VL remains on, the input switch that sees

overvoltage (Q1, Q3, Figure 4) opens, and the

appropriate power-monitor output (DOK, UOK) is

high impedance, and CHG is high impedance.

If both DC and USB see overvoltage, both input

switches (Q1 and Q2, Figure 4) open and the

charger turns off. The BAT-SYS switch (Q3, Figure

4) closes, allowing the battery to power SYS.

An input is also invalid if it is less than BAT, or less

than the DC undervoltage threshold of 3.5V (falling).

With an invalid input voltage, SYS connects to BAT

through a 40mΩ switch (Q3, Figure 4).

• Input Overcurrent Protection. The current at DC

and USB is limited to prevent input overload. This cur-

rent limit can be selected to match the capabilities of

the source, whether it is a 100mA or 500mA USB

source, or an AC adapter. When the load exceeds

the input current limit, SYS drops to 68mV below BAT

and the battery supplies supplemental load current.

• Thermal Limiting. The MAX8677C reduces input lim-

iter current by 5%/°C when its die temperature

exceeds +100°C. The system load (SYS) has priority

over the charger current, so input current is first

reduced by lowering charge current. If the junction

temperature still reaches +120°C in spite of charge-

current reduction, no input (DC or USB) current is

drawn, the battery supplies the entire system load, and

SYS is regulated at 68mV below BAT. Note that this

on-chip thermal-limiting circuitry is not related to and

operates independently from the thermistor input.

• Adaptive Battery Charging. While the system is

powered from DC, the charger draws power from

SYS to charge the battery. If the charger load plus

system load exceeds the input current limit, an

adaptive charger control loop reduces charge cur-

rent to prevent the SYS voltage from collapsing.

Maintaining a higher SYS voltage improves efficien-

cy and reduces power dissipation in the input limiter.

The total current through the switch (Q1 or Q2 in

Figure 4) is the sum of the load current at SYS and

the battery charging current. The limiter clamps at

4.35V, so input voltages greater than 4.35V can

increase power dissipation in the limiter. The limiter

power loss is (V

- 4.35) x I, but not less than I

0.2Ω. Also note that the MAX8677C turns off any

input that exceeds 6.9V (nominal).

DC and USB Connections and

Current-Limit Options

Input Current Limit

The input and charger current limits are set as shown in

Table 2. It is often preferable to change the input cur-

rent limit as the input power source is changed. The

MAX8677C facilitates this by allowing different input

current limits for DC and USB as shown in Table 2.

POWER SOURCE DOK UOK PEN1

PEN2 USUS

DC INPUT

CURRENT

LIMIT

USB INPUT

CURRENT

LIMIT

MAXIMUM

CHARGE

CURRENT*

AC adapter at DC input L X H X X

3000/R

PSET

3000/R

ISET

L X L L L 100mA 100mA

L X L H L 500mA 500mAUSB power at DC input

LX L X H

USB suspend

USB input off;

DC input has

priority

H L X L L 100mA

H L X H L 500mA

3000/R

ISET

USB power at USB input;

DC unconnected

HL X X H

USB suspend

DC and USB unconnected H H X X X

No DC input

No USB input

Table 2. Input Limiter Control Logic

Charge current cannot exceed the input current limit. Charge may be less than the maximum charge current if the total SYS load

exceeds the input current limit.

MAX8677C

1.5A Dual-Input USB/AC Adapter Charger

and Smart Power Selector

______________________________________________________________________________________ 15

When the input current limit is reached, the first action

taken by the MAX8677C is to reduce the battery charge

current. This allows the regulator to stay in dropout, or

at 4.35V, during heavy loads, thus reducing power dis-

sipation. If, after the charge current is reduced to 0mA,

the load at SYS still exceeds the input current limit, SYS

begins to fall. When the SYS voltage drops to BAT, the

SYS-BAT switch turns on, using battery power to sup-

port the system load during the load peak.

The MAX8677C features flexible input connections (at

the DC and USB input pins) and current-limit settings

(set by PEN1, PEN2, PSET, and ISET) to accommodate

nearly any input power configuration. However, it is

expected that most systems use one of two external

power schemes: separate connections for USB and an

AC adapter, or a single connector that accepts either

USB or AC adapter output. Input and charger current

limit are controlled by PEN1, PEN2, R

PSET

, and R

ISET

as shown in Table 2.

Separate Adapter and USB Connectors

When the AC adapter and USB have separate connec-

tors, the adapter output connects to DC and the USB

source connects to USB. PEN1 is permanently tied high

(to DC or VL). The DC current limit is set by R

PSET

while the USB current limit is set by PEN2 and USUS.

Single Common Connector for USB or Adapter

When a single connector is used for both AC adapter

and USB sources, the DC input is used for both input

sources. When an AC adapter is connected at DC,

PEN1 should be pulled high to select the current limit

set by R

PSET

. When a USB source is connected, PEN1

should be low to select 500mA, 100mA, or USB sus-

pend (further selected by PEN2 and USUS). PEN1 can

be pulled up by the AC adapter power to implement

hardware adapter/USB selection.

USB Suspend

Driving USUS high when PEN1 is low turns off charging,

as well as the SYS output and reduces input current to

190µA to accommodate USB suspend mode.

Power Monitor Outputs (

UOK

DOK

)

DOK is an open-drain output that pulls low when the

DC input has valid power. UOK is an open-drain output

that pulls low when the USB input sees valid power. A

valid input for DC or USB is between 4.1V and 6.6V. If a

single power-OK output is preferred, DOK and UOK

can be wire-ORed together. The combined output then

pulls low if either USB or DC sees a valid input.

Soft-Start

To prevent input transients that can cause instability in

the USB or AC adapter power source, the rate of change

of input current and charge current is limited. When a

valid DC or USB input is connected, the input current limit

is ramped from zero to the set current-limit value (as

shown in Table 2). If DC is connected with no USB power

present, input current ramps in 1.5ms. If DC is connected

with USB already present, input current ramps in 50µs.

When USB is connected with no DC present, input cur-

rent also ramps in 50µs. If USB is connected with DC

already present, the USB input is ignored.

If an adapter is plugged into DC while USB is already

powered, the input current limit reramps from zero back

up to the DC current limit so that the AC adapter does

not see a load step. During this transition, if the input

current limit is below the SYS load current, the battery

supplies the additional current needed to support the

load. Additionally, capacitance can be added to SYS to

support the load during input power transitions.

When the charger is turned on, charge current ramps

from zero to the ISET current value in typically 1.5ms.

Charge current also ramps when transitioning to fast-

charge from prequal and when changing the USB

charge current from 100mA to 500mA with PEN2. There

is no dI/dt limiting, however, if ISET is changed suddenly

using a switch at R

ISET

Battery Charger

The battery charger state diagram is illustrated in

Figure 5. With a valid DC or USB input, the battery

charger initiates a charge cycle when the charger is

enabled. It first detects the battery voltage. If the bat-

tery voltage is less than the BAT prequal threshold

(3.0V), the charger enters prequal mode in which the

battery charges at 10% of the maximum fast-charge

current. This reduced charge rate ensures that the bat-

tery is not damaged by the fast-charge current while

deeply discharged. Once the battery voltage rises to

3.0V, the charger transitions to fast-charge mode and

applies the maximum charge current. As charging con-

tinues, the battery voltage rises until it approaches the

battery regulation voltage (4.2V) where charge current

starts tapering down. When charge current decreases

to 5%, 10%, or 15% (as set by TSET) of the fast-charge

current, the charger enters a brief 15s top-off, and then

charging stops. If the battery voltage subsequently

drops below the 4.1V recharge threshold, charging

restarts and the timers reset.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21

MAX8677CETG/V+T

Mfr. #:

Buy MAX8677CETG/V+T

Manufacturer:

Maxim Integrated

Description:

Battery Management 1.5A Dual USB/AC Adapter Charge

Lifecycle:

New from this manufacturer.

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MAX8677CETG/V+T

MAX8677CETG/V+T

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