MAX1873 Maxim, MAX1873 Datasheet - Page 10

• Manufacturer Part Number: MAX1873
• Description: Simple Current-Limited Switch-Mode Li Charger Controller
• Manufacturer: Maxim

The charging-current regulator limits the battery charg-
ing current. Current is sensed by the current-sense
resistor (R
and CSB. The voltage on ICHG/EN can also adjust the
charging current. Full-scale charging current (I
0.2V / RCSB) is achieved by connecting ICHG/EN to
REF. See Setting the Charging-Current Limit section for
more details.
The charging-current error amplifier is compensated at
CCI (Figure 1). A 47nF capacitor from CCI to GND pro-
vides suitable performance for most applications.
The input-current regulator limits the source current by
reducing charging current when the input current
reaches the set input-current limit. In a typical portable
design, system load current will normally fluctuate as
Simple Current-Limited Switch-Mode
Li+ Charger Controller
Figure 2. Functional Block Diagram
10
CSSN
CSSP
ICHG
VADJ
IOUT
CCS
CCV
CCI
/EN
______________________________________________________________________________________
CSB
A = 1
in Figure 1) connected between BATT
9R
GND
Charging-Current Regulator
SHUTDOWN
FOR ALL
BLOCKS
ERROR AMP
CURRENT
Input-Current Regulator
R
CONTROL
LOGIC
R
ERROR AMP
VOLTAGE
REGULATOR
UNDERVOLTAGE
COMPARATOR
5.4
VH
REFERENCE
4.2V
BATT
DRIVER
CHG
DCIN
BATT
GND
CSB
EXT
REF
VL
VH
=
portions of the system are powered up or put to sleep.
Without the benefit of input-current regulation, the input
source would have to be able to supply the maximum
system current plus the maximum charger-input cur-
rent. The MAX1873 input-current loop ensures that the
system always gets adequate power by reducing
charging current as needed. By using the input-current
limiter, the size and cost of the AC adapter can be
reduced. See Setting the Input-Current Limit section for
design details.
Input current is measured through an external sense
resistor, R
current limit feature may be bypassed by connecting
CSSP to CSSN.
The input-current error amplifier is compensated at
CCS. A 47nF capacitor from CCS to GND provides suit-
able performance for most applications.
The pulse-width modulation (PWM) controller drives the
external MOSFET at a constant 300kHz to regulate the
charging current and voltage while maintaining low
noise. The controller accepts inputs from the CCI, CCV,
and CCS error amplifiers. The lowest signal of these
three drives the PWM controller. An internal clamp limits
the noncontrolling signals to within 200mV of the con-
trolling signal to prevent delay when switching between
the battery-voltage control, charging-current control,
and input-current regulation loops.
The MAX1873 stops charging when ICHG/EN is pulled
low (below 0.5V) and shuts down when the voltage at
DCIN falls below the voltage at BATT. In shutdown, the
internal resistive voltage-divider is disconnected from
BATT to reduce the battery drain. When AC-adapter
power is removed, or when the part is shut down, the
MAX1873 typically draws 1.5µA from the battery.
The DCIN voltage is compared to the voltage at BATT.
When the voltage at DCIN drops below BATT + 50mV,
the charger turns off, preventing drain on the battery
when the input source is not present or is below the
battery voltage.
A diode is typically connected between the input
source and the charger input. This diode prevents the
battery from discharging through the body diode of the
high-side MOSFET should the input be shorted to GND.
It also protects the charger, battery, and systems from
reversed polarity adapters and negative input voltages.
Source Undervoltage Shutdown (Dropout)
CSS
, between CSSP and CSSN. The input-
PWM Controller
Shutdown