6/13
Description of a DC/DC step down battery
charger
1. Voltage and Current Controller
The device has been designed to drive an external
P-Channel MOSFET in PWM mode and in Step
Down topology. Its two integrated operational am-
plifiers ensure accurate Voltage and Current Reg-
ulation loops.
The Voltage Control dedicated operational ampli-
fier acts as an error amplifier and compares a part
of the output voltage (external voltage divider) to
an integrated highly precise voltage reference
(V
ref
).
The Current Control dedicated operational amplifi-
er acts as an error amplifier and compares the
drop voltage through the sense resistor to an inte-
grated low value voltage reference (V
s
).
These two amplified errors are ORed through di-
odes, and the resulting signal (“max of”) is a refer-
ence for the PWM generator to set the switching
duty cycle of the P-Channel MOSFET transistor.
The PWM generator comprises an oscillator (saw
tooth) and a comparator which gives a variable
duty cycle from 0 to 95%. This PWM signal is the
direct command of the output totem pole stage to
drive the Gate of the P-Channel MOSFET.
Thanks to this architecture, the TSM108 is ideal to
be used from a DC power supply to control the
charging Voltage and Current of a battery in appli-
cations such as accessories for cellular phones
chargers and post-regulation in power supplies.
2. Voltage Control
The Voltage Control loop can be set by an exter-
nal voltage divider connected between the output
positive line and the Ground reference. The mid-
dle point has to be connected to the V
ctrl
pin of
TSM108, and, if R1 is the upper resistor, and R2,
the lower resistor of the voltage divider, the values
of R1 and R2 are:
❑ eq1: Vref = Vout x R2 / (R1 + R2)
In Constant Voltage Control mode, the output volt-
age is fixed by to the R1/R2 resistor.
The total value of R1 + R2 voltage divider will de-
termine the necessary bleeding current to keep
the Voltage Control loop effective, even under “no
load” conditions.
The current compensation loop is directly accessi-
ble from the pins Vcomp and Vref The compensa-
tion network is highly dependant of the operating
conditions (switching frequency, external compo-
nents R, L, C, MOSFET, output capacitor, etc...).
3. Current Control
The Current control loop can be set by the sense
resistor that has to be placed in series on the out-
put positive line. The output side of the Sense re-
sistor has to be to Ictrl pin,and the common point
between Rsense and the choke has to be con-
nected to the Vs pin. If Ilim is the value of the
charging current limit The value of Rsense should
verify:
❑ eq2: V
s
= R
sense
x Il
im
In Constant Current Control mode, the output cur-
rent is fixed by to the Rsense resistor (under out-
put short circuit conditions, please refer to this cor-
responding section).
The wattage calibration (W) of the sense resistor
should be chosen according to:
❑ eq2a: W > R
sense
x Il
im
2
The current compensation loop is directly accessi-
ble from Icomp and Ictrl pin.
The compensation network is highly dependant on
the operating conditions.
4. PWM frequency
The internal oscillator frequency is a saw tooth
waveform that can be adjusted. In accessories like
battery chargers, it is suggested to set the switch-
ing frequency at a typical 100kHz in order to ob-
tain a good compromise between the ripple cur-
rent and the choke size. An external capacitor
connected between Osc pin and ground set the
switching frequency
The maximum duty cycle of the PWM function is
limited to 95% in order to ensure safe driving of
the MOSFET.
TSM108
PRINCIPLE OF OPERATION AND APPLICATION HINTS
