MAX1874 Maxim, MAX1874 Datasheet - Page 12

• Manufacturer Part Number: MAX1874
• Description: Dual-Input / USB/AC Adapter / 1-Cell Li+ Charger with OVP and Thermal Regulation
• Manufacturer: Maxim

The MAX1874’s thin QFN package includes a bottom
metal plate that reduces thermal resistance between the
die and the PC board. The external pad should be sol-
dered to a large ground plane. This helps dissipate
power and keeps the die temperature below the thermal
limit. The MAX1874 thermal resistance from the die to the
package thermal pad is typically 5°C/W. The thermal
resistance of 1in
material in free air is +42°C/W (typ). Consequently, the
PC board pad area dominates the MAX1874’s ability to
dissipate heat. The MAX1874’s thermal regulator is set
for a +105°C die temperature. With the example thermal
resistance of +47°C/W, the MAX1874 charge-current
thermal limiting can be expected to occur when dissipat-
ing approximately 1.7W at +25°C ambient, and when
dissipating approximately 0.75W at +70°C ambient.
The power dissipated in the charger is P
(either V
sipation drops as the battery voltage rises, so thermal-
charge current limiting, if it occurs, typically releases
soon after charging begins and has little impact on
charge time.
The MAX1874 features an internal window comparator to
monitor battery pack temperature or ambient tempera-
ture with an external negative temperature coefficient
thermistor. In typical systems, temperature is monitored
to prevent charging at ambient temperature extremes
(below 0°C or above +50°C). When the temperature
moves outside these limits, charging is stopped. If the
V
resumes. Connect THRM to GND when not using this
feature. The THRM block diagram is detailed in Figure 1.
Note that the temperature monitor at THRM is entirely
separate from the on-chip temperature limiting dis-
cussed in the Package Thermal Limiting section.
The input thresholds for the THRM input are 0.74
V
HOT trip point.
The DCLV input from an AC adapter or other source
can be protected against overvoltage of up to 18V by
connecting an external P-channel MOSFET (Q2 in
Figures 3, 4, and 5) between DC and DCLV. When V
exceeds 6.2V, the DCOK output turns the P-channel
MOSFET off. On power-up, DCOK remains high until it
has been verified that V
above 6.5V is not needed, then the MOSFET from the
DC to DCLV can be omitted (Figure 2).
Dual-Input, USB/AC Adapter, 1-Cell
Li+ Charger with OVP and Thermal Regulation
12
THERM
REF
______________________________________________________________________________________
for the COLD trip point and 0.29
USB
Input Overvoltage Protection Switch
External Thermistor Monitor (THRM)
returns to within its normal window, charging
or V
2
Applications Information
DCLV
of 1oz copper on typical FR4 PC board
) - V
DC
BATT
is in range. If protection
]
✕
I
CHARGE
✕
V
DISS
. Power dis-
REF
= [V
for the
DC
IN
✕
When input power is connected to the charger, some
systems prefer that the battery is disconnected from the
load and that system load current is taken directly from
the DC input or USB source. This is an alternative to the
basic case where the system load is permanently con-
nected to the battery. The later setup is lower cost but
has the disadvantage that if the battery is completely
discharged, the system might not be ready to operate
immediately, or might have limited functionality immedi-
ately upon plugging in the charger. If the battery has a
load-disconnect switch, the system is more complex,
but operation does not depend on the state of the bat-
tery. When system power is taken from the DC or USB
input source, use D1, D2, Q1, and Q2 (Figure 4).
A partial approach to battery-load switching can con-
nect the AC power adapter (DC) directly to the load, but
not USB power (Figure 5). This can be useful when USB
power is insufficient to fully power the system and
charge the battery. When DC is powered, D2 provides a
direct connection to the system and Q3 disconnects the
battery. The battery does not power the load while it is
charging. When only USB is connected, there is no
bypass path from USB to the system. The battery is
charged from the BATT output, and any system power is
drawn from the battery through D5. If the system load
exceeds the current supplied by the charger from USB
(500mA or 100mA), then the battery can still discharge.
In addition, if the system load does not allow the BATT
current to fall below the USB battery full current thresh-
old listed in Table 2, then CHG does not go high to indi-
cate a full battery.
Figure 1. Thermistor Sensing Block Diagram
10KΩ AT +25°C
0.1µF
THERMISTOR
10kΩ
THRM
REF
100mV
Battery-Load Switch
T
T
HOT
COLD
TO REGULATOR