TC3827 Datasheet, PDF (5/10 Pages) Microchip Technology – Lithium-Ion Battery Charger

Lithium-Ion Battery Charger
TC3827
control loop changes from current limiting to voltage regula-
tion. If an external micro-controller determines battery con-
ditions are unsafe for charge it can toggle the shutdown pin
low and interrupt the charge cycle. Otherwise, once the pre-
determined cell voltage is reached the TC3827 shifts into a
constant-voltage mode (linear regulation) and a variable
charge current is applied as required to maintain the battery
cell voltage to within 1% accuracy of the cell voltage set-
point.
I_MON – Charge Current Status
The I_MON pin provides an output voltage that is propor-
tional to the battery charging current . It is an amplified
version of the sense resistor voltage drop that the current
loop uses to control the PMOS device. This voltage signal
can be applied to the input of an A/D Converter and used by
a controller to display information about the state of the
battery or charge current profile.
Higher Current Option
The current sense resistor for the circuit shown in
Figure 1 is calculated by: R_SENSE = V_CS /I_MAX .
Where V_CS is the current limit threshold voltage of
40mV to 75mV, 50mV typical. If I_MAX = 1A is desired,
R_SENSE = 50mΩ.
Pre-regulated Input Voltage (5V ± 0%)
For this application, the required θ_JA thermal imped-
ance is calculated as follows:
if:
then:
the PMOS data sheet allows a max
junction temperature of T_JMAX = 150°C,
at 50°C ambient with convection
cooling, the maximum allowed
junction temperature rise is:
MODE – Charge Mode Status LED
The MODE pin indicates the battery charging mode. An
LED can be connected to the MODE for a visible indicator.
Alternatively, a pull-up resistor (typically 100kΩ) from the
interfacing logic supply to MODE provides a logic-level
output. The MODE pin will toggle LOW and the LED will
illuminate when the charger is in the current limited mode.
The MODE pin toggles to a high impedance state and the
LED will be off during constant-voltage mode charging or if
the battery is not connected. The MODE pin toggles at a
V_OUT of V_REG, typically.
APPLICATION CIRCUIT DESIGN
Due to the low efficiency of Linear Regulator Charging,
the most important factors are thermal design and cost,
which is a direct function of the input voltage, output current
and thermal impedance between the PMOS and the ambi-
ent cooling air. The worst-case situation is when the battery
is shorted since the PMOS has to dissipate the maximum
power. A tradeoff must be made between the charge cur-
rent, cost and thermal requirements of the charger. Higher
current requires a larger PMOS with more effective heat
dissipation leading to a more expensive design. Lowering
the charge current reduces cost by lowering the size of the
PMOS, possibly allowing a smaller package such as 6-Pin
SOT. The following designs consider both options.
T_JMAX – T_AMAX = 150°C – 50°C = 100°C.
θ_JA = ∆T/(I_Ox k x V_IN) = 100/(1 x 0.46 x 5.5)
= 39.5 °C/W
This k factor is: k = I_SC/I_MAX ≈ 0.46.
This thermal impedance can be realized using the
transistor shown in Figure 1 when mouted to a heat sink.
The θ_SA or thermal impedance of a suitable heatsink is
calculated below:
θ_SA ≤ (θ_JA – θ_JC – θ_CS) = 39.5 – 2.5 – 0.3 = 36.7°C/W
Where the θ_JC, or junction-to-case thermal impedance
is for the PMOS from the PMOS data sheet. A low cost
heatsink is Thermalloy type PF430, with a θ_SA = +25.3°C/W.
© 2001 Microchip Technology Inc. DS21558A
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TC3827-2 12/12/00