Detailed Description

The MAX1874 charges a single-cell Li+ battery from

either USB power sources or AC adapter sources. It

contains a complete two-input linear charger that controls

both battery charge current and voltage. In addition to

all charging functions, the MAX1874 includes voltage-

sensing and switchover circuitry that selects the active

input source. When both inputs are active, priority is

given to the AC adapter (DC). Charging current is regu-

lated with on-chip power MOSFETs, so no external

MOSFETs are required for a basic two-input charger.

Additional features such as input-voltage protection and

battery-load switching can be added with external

MOSFETs that are driven directly from MAX1874 outputs.

The MAX1874 also features a thermal regulation loop

that adjusts charging current so the die temperature

remains below +105°C. See the Package Thermal

Limiting section. This on-chip thermal control simplifies

PC board layout and allows the optimum charging rate

to be set without the thermal limits imposed by worst-

case battery and input voltage. When the MAX1874

thermal limit is reached, the charger does not shut

down but reduces charging current.

In addition to, and separate from, its internal die tem-

perature control, the MAX1874 can also monitor ambi-

ent or cell temperature with an external thermistor

connected to THRM. When the thermistor temperature

is out of range (greater than +50°C or less than 0°C),

charging stops until the temperature returns to normal.

See the External Thermistor Monitor (THRM) section.

Other features include a CHG output to indicate battery

full (when charge current tapers to a percentage of

fast-charge current). DCOK, UOK, and power-on (PON)

outputs indicate when valid power is present. These

outputs can drive overvoltage protection and power

selection MOSFETs (Figures 3, 4, and 5).

When charging is stopped or input power is removed,

battery leakage is typically 5µA. No input blocking

diodes are required to prevent battery drain.

With USB power connected, but without power at the

DC input, charge current can be set to either 500mA or

100mA through the USEL input. When power is taken

from the DC input, charge current is linearly set by the

voltage at DCI. The MAX1874 charge current can also

be DAC controlled with the output of a DAC connected

to DCI. See the Charging Current section.

Enable (EN)

The enable input, EN, switches the MAX1874 on or off.

With EN high, the MAX1874 is on and can begin charg-

ing. When EN is low, UOK, DCOK, PON, and REF remain

active. Charging stops when EN is low, but the chip

remains biased and continues to draw current from the

input supplies so power-monitoring outputs can remain

valid.

USB-to-Adapter Power Handoff

The MAX1874 can charge from either the USB input or

the DC input. It cannot charge from both sources at the

same time. The IC automatically selects the active input

and charges from that. If both power sources are

active, the adapter input (DC) takes precedence. Table

1 describes the switchover between DC and USB.

DC serves as the sense input for the adapter power

source. This input senses when DC is above 6.2V (maxi-

mum range is 18V) or below 4V. When it senses the DC

source is above 6.2V, DCOK goes high, indicating an

invalid DC input. See the DC Power-OK (DCOK) section.

When power is connected to DC, the MAX1874 requires

20ms to validate the input. Consequently, charging is

interrupted for 20ms until it is determined that input

power is good. Also, when DC power is removed while

valid USB power is present, charging is interrupted for

20ms before transferring to the USB source.

DC Power-OK (

DCOK)

DCOK is an active-low, open-drain output that goes low

used as a logic output, but is also designed to drive an

external MOSFET (Q2 in Figures 3, 4, and 5). This allows

the charger to protect the input from overvoltage up to

18V. Charging is disabled for inputs over 6.2V. An exter-

nal 1k

Ω pullup resistor keeps DCOK high (external

MOSFET off) until it is certain the voltage is within the

Dual-Input, USB/AC Adapter, 1-Cell

Li+ Charger with OVP and Thermal Regulation

10

______________________________________________________________________________________

Exceeds operating input

range. Not allowed. See the

Absolute Maximum Ratings

section.

DCLV powers device

and supplies charging

current.

1)

USB powers device and supplies charging

current.

2)

DCLV disconnected from DC source through

external MOSFET (Q2 Figures 3, 4, and 5).

No charging

Table 1. USB and DC Input Selection