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BF1550
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Reference voltage for over voltage protection
VREF_OV
2.4
V
Power Transistor
Collector base breakdown voltage
BVcbo
Ic=1mA
Collector emitter saturation voltage
Vcesat
Ic=500mA, Ib=100mA
DC current gain
hFE
Vce=5V, Ic=100mA
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Datasheet
WI-D06-J-0094
800
V
0.5
20
Rev.A/1
V
30
Page 3 of 7
Documents
BYD Microelectronics Co., Ltd.
BF1550
Operation Description
● CC/CV Operation
The BF1550 uses PFM control mode in CC mode while PWM+PFM mode in CV mode. In charger applications, a discharged
battery starts charging in the CC mode, the IC switches to CV mode until the battery is nearly full charged. In order to achieve
constant output current and voltage, the working frequency and primary-side peak current will be changed with the feedback
voltage of VFB pin. The output voltage and current is given by:
Profile
VOUT
V REF_EA
Where V
D
I OUT
Logout
R6
R7
R7 N S
NA
(1)
VD
indicates the drop voltage of output Diode, N /N indicates the transformer secondary-auxiliary turns ratio.
S
A
1 3 NP
I
2 7 NS P
(2)
Where I indicates the maximum peak current of primary winding, N /N indicates the primary-secondary turns ratio.
P
P
S
●Cycle-by-Cycle Current Limiting
The primary-side current flow through the sense-resistor and make the voltage on IFB pin ramp up. When the voltage exceeds
the internal threshold voltage value, the power BJT will be turned off immediately. The threshold voltage is changing gradually
in CV mode while constant in CC mode.
●Soft Start
A built-in soft start circuit is applied at start-up state. Once the VDD reaches UVLO(ON), the primary-side peak current voltage
threshold will ramp up from 0.275V to 0.9V gradually because of the soft start circuit.
●Voltage Protection Function
The BF1550 includes such a function that protect against output over-voltage and under-voltage, which could be monitored by
VFB pin and VDD pin. If the voltage at VFB pin exceeds the over-voltage threshold, the external power BJT will be turned off
immediately and the controller will restart. Once VDD drops below the UVLO(OFF) threshold, the controller will reset itself and
go into a new start cycle. The controller will continue the start cycle until the error condition is removed.
●Output Cable Drop Compensation
The BF1550 includes programmable output cable compensation to achieve good load regulation. The cable drop
compensation is programmable by changing the value of the resistance R6. The maximum compensation voltage VCOMP is
given by:
VCOMP
Where I
I COMP R6
COMP
NS
NA
(3)
34uA is the maximum compensation current which generated by an internal current source.
●Built-in line-voltage compensation
Datasheet
WI-D06-J-0094
Rev.A/1
Page 4 of 7
4 of 7
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