11
4009fd
LTC4009
LTC4009-1/LTC4009-2
operaTion
Overview
The LTC4009 is a synchronous step-down (buck) current
mode PWM battery charger controller. The maximum
charge current is programmed by the combination of a
charge current sense resistor (R
SENSE
), matched input
resistors (R
IN
, Figure 1), and a programming resistor
(R
PROG
) between the PROG and GND pins. Battery volt-
age is programmed either with an external resistor divider
between FBDIV and GND (LTC4009) or two digital battery
voltage select pins (LTC4009-1/LTC4009-2). In addition,
the PROG pin provides a linearized voltage output of the
actual charge current.
The LTC4009 family does not have any built-in charge
termination and is flexible enough for charging any type
of battery chemistry. These are building block ICs intended
for use with an external circuit, such as a microcontroller,
capable of managing the entire algorithm required for
the specific battery being charged. Each member of the
LTC4009 family features a shutdown input and various state
indicator outputs, allowing easy and direct management by
a wide range of external (digital) charge controllers. Due
to the popularity of rechargeable lithium-ion chemistries,
the LTC4009-1 and LTC4009-2 also offer internal precision
resistors that can be digitally selected to produce one of
four preset output voltages for simplified design of those
charger types.
Shutdown
The LTC4009 remains in shutdown until DCDIV exceeds
1.2V, and SHDN is driven above 1.4V. In shutdown, current
drain from the battery is reduced to the lowest possible
level, thereby increasing standby time. When in shutdown,
the ITH pin is pulled to GND and the CHRG, ICL, FET gate
drivers and INTV
DD
output are all disabled. The ACP status
output indicates sensed adapter input voltage during all
LTC4009 states. Charging can be stopped at any time by
forcing SHDN below 300mV.
Soft-Start
Exiting the shutdown state enables the charger and releases
the ITH pin. When enabled, switching will not begin until
CLP exceeds BAT by 100mV and ITH exceeds a threshold
that assures initial current will be positive (about 5% to
25% of the maximum programmed current). To limit inrush
current, soft-start delay is created with the compensation
values used on the ITH pin. Longer soft-start times can be
realized by increasing the filter capacitor on ITH, if reduced
loop bandwidth is acceptable. The actual charge current at
the end of soft-start will depend on which loop (current,
voltage or adapter limit) is in control of the PWM. If this
current is below that required by the ITH start-up threshold,
the resulting charge current transient duration depends on
loop compensation but is typically less than 100µs.
Bulk Charge
When soft-start is complete, the LTC4009 begins sourc-
ing the current programmed by the external components
connected to CSP, CSN and PROG. Some batteries may
require a small conditioning trickle current if they are heavily
discharged. As shown in the Applications Information sec-
tion, the LTC4009 can address this need through a variety
of low current circuit techniques on the PROG pin. Once
a suitable cell voltage has been reached, charge current
can be switched to a higher, bulk charge value.
End-of-Charge and CHRG Output
As the battery approaches the programmed output volt-
age, charge current will begin to decrease. The open-
drain CHRG output can indicate when the current drops
to 10% of its programmed full-scale value by turning off
the strong pull-down (open-drain FET) and turning on a
weak 25µA pull-down current. This weak pull-down state
is latched until the part enters shutdown or the sensed
current rises to roughly C/6. C/10 indication will not be
set if charge current has been reduced due to adapter
input current limiting or DCIN/battery overvoltage. As
the charge current approaches 0A, the PWM continues to
operate in full continuous mode. This avoids generation
of audible noise, allowing bulk ceramic capacitors to be
used in the application.
