SE1051 SEAWARD [Seaward Electronics Inc.], SE1051 Datasheet - Page 4

• Manufacturer Part Number: SE1051
• Description: Constant Voltage and Constant Current Controller
• Manufacturer: SEAWARD [Seaward Electronics Inc.]

Revision 12/4/2008
Application Hints
Voltage Control
The voltage loop is controlled via a first
transconductance
resistor bridge R1, R2, and the optocoupler which
is directly connected to the output.
The relation between the values of R1 and R2
should be chosen as written in Equation 1.
R1 = R2 x Vref / (Vout - Vref)
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 100KΩ (or more)
would be appropriate for the resistors R1 and R2.
As an example, with R2 = 100KΩ, Vout = 4.10V,
Vref = 1.210V, then R1 = 41.9KΩ.
Note that if the low drop diode should be inserted
between the load and the voltage regulation
resistor bridge to avoid current flowing from the
load through the resistor bridge, this drop should
be taken into account in the above calculations by
replacing Vout by (Vout + Vdrop).
Current Control
The current loop is controlled via the second
trans-conductance
sense resistor Rsense, and the optocoupler.
The control equation is:
Rsense x I-limit = Vsense
Rsense = Vsense / I-limit
where I-limit is the desired current limit, and
Vsense is the threshold voltage for the current
control loop.
As an example, with I-limit = 1A, Vsense =
-240mV, then Rsense = 240mΩ.
Note that the Rsense resistor should be selected
with the consideration of the Maximum Power in
full load operations (P-limit).
P-limit = Vsense x I-limit.
As an example, with I-limit = 1A, and Vsense
=-240mV, P-limit = 240mW.
Consequently, for most adapter and battery
charger applications, a quarter-watt resistor to
make the current sensing function is sufficient.
Vsense threshold is achieved internally by a
resistor bridge tied to the Vref voltage reference.
Its middle point is tied to the positive input of the
current 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 resistors of this bridge are matched in layout
to provide the best precision possible.
Preliminary and all contents are subject to change without prior notice.
operational
operational
© Seaward Electronics, Inc., 2006. • www.seawardinc.com.cn • Page 4
Eq2
Eq3
Eq4
Eq1
amplifier,
amplifier,
the
the
The
trans-conductance operational amplifiers are
connected together. This makes an ORing
function which ensures that whenever the current
or the voltage reaches too high values, the
optocoupler is activated.
The relation between the controlled current and
the controlled output voltage can be described
with a square characteristic as shown in the
following V/I output-power graph.
Fig.2 Output voltage versus output current
Compensation
The
operational amplifier can be fully compensated.
Both of its output and negative input are directly
accessible
components.
An example of a suitable compensation network
is shown in Fig.1. It consists of a capacitor
Cvc1=2.2nF and a resistor Rcv1=470KΩ in
series, connected in parallel with another
capacitor Cvc2=22pF.
The
operational
compensated. Both of its output and negative
input
compensation components.
An example of a suitable compensation network
is shown in Fig.1. It consists of a capacitor
Cic1=2.2nF and a resistor Ric1=22KΩ in series.
When the Vcc voltage reaches 12V it could be
interesting to limit the current coming through the
output in the aim to reduce the dissipation of the
device and increase the stability performances of
the whole application.
An example of a suitable Rout value could be
330Ω in series with the opto-coupler in case
Vcc=12V.
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