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TSM105 Datasheet(PDF) 5 Page - STMicroelectronics

Part No. TSM105
Description  CONSTANT VOLTAGE AND CONSTANT CURRENT CONTROLLER FOR BATTERY CHARGERS AND ADAPTORS
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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TSM105 Datasheet(HTML) 5 Page - STMicroelectronics

   
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1. Voltage and Current Control
1.1. Voltage Control
The voltage loop is controlled via a first transcon-
ductance operational amplifier, the resistor bridge
R1, R2, and the optocoupler which is directly con-
nected to the output.
The relation between the values of R1 and R2
should be chosen as writen in Equation 1.
R1 = R2 x Vref / (Vout - Vref)
Eq1
where Vout is the desired output voltage.
To avoid the discharge of the load, the resistor
bridge R1, R2 should be highly resistive. For this
type of application, a total value of 100K
Ω (or
more) would be appropriate for the resistors R1
and R2.
As an example, with R2 = 100K
Ω, Vout = 4.10V,
Vref = 1.210V, then R1 = 41.9K
Ω.
Note that if the low drop diode should be inserted
between the load and the voltage regulation resis-
tor bridge to avoid current flowing from the load
through the resistor bridge, this drop should be
taken into account in the above calculations by re-
placing Vout by (Vout + Vdrop).
1.2. Current Control
The current loop is controlled via the second
trans-conductance
operational
amplifier,
the
sense resistor Rsense, and the optocoupler.
The control equation verifies:
Rsense x Ilim = Vsense
eq2
Rsense = Vsense / Ilim
eq2’
where Ilim is the desired limited current, and
Vsense is the threshold voltage for the current
control loop.
As an example, with Ilim = 1A, Vsense = -200mV,
then Rsense = 200m
Ω.
Note that the Rsense resistor should be chosen
taking into account the maximum dissipation
(Plim) through it during full load operation.
Plim = Vsense x Ilim.
eq3
As an example, with Ilim = 1A, and Vsense =
200mV, Plim = 200mW.
Therefore, for most adapter and battery charger
applications, a quarter-watt, or half-watt resistor to
make the current sensing function is sufficient.
Vsense threshold is achieved internally by a re-
sistor bridge tied to the Vref voltage reference. Its
middle point is tied to the positive input of the cur-
rent control operational amplifier, and its foot is to
be connected to lower potential point of the sense
resistor as shown on the following figure. The re-
sistors of this bridge are matched to provide the
best precision possible
The current sinking outputs of the two trans-con-
nuctance operational amplifiers are common (to
the output of the IC). This makes an ORing func-
tion which ensures that whenever the current or
the voltage reaches too high values, the optocou-
pler is activated.
The relation between the controlled current and
the controlled output voltage can be described
with a square characteristic as shown in the fol-
lowing V/I output-power graph.
Figure 3 : Output voltage versus output current
2. Compensation
The voltage-control trans-conductance operation-
al amplifier can be fully compensated. Both its out-
put and the negative input are directly accessible
for external compensation components.
Vout
Iout
Voltage regulation
TSM105 Vcc : independent power supply
0
Secondary current regulation
TSM105 Vcc : On power output
Primary current regulation
TSM105
PRINCIPLE OF OPERATION AND APPLICATION HINTS


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