YB7002 Datasheet, PDF (6/8 Pages) YOBON TECHNOLOGIES,INC. – Photoflash Capacitor Charger with Automatic Refresh

YB7002
Photoflash Capacitor Charger with Automatic Refresh
Application Information
Hysteresis voltage is setup by connecting a
resistor Rhyst from VHYST to ground. A
resistor divider of 13.5k (internal resistor)
and Rhyst from bandgap voltage will
determine heysteresis voltage. Once VFB
drops below VHYST voltage, charging
cycle is activated again. To calculate the
refresh time, consider the followings:
C _OUT
⋅ ^∂VOUT
∂t
+
V_OUT
R1+ R2
+
I _{COUT ,LEAKAGE}
=0
As the capacitor leakage current is a
function of output voltage, it can be
considered to be constant within the tiny
range of voltage from Vout to
VHYST ⋅ ^{R1+ R2 , where R1 and R2 are the}
R2
resistor divider at the secondary side output.
By solving the above equation, we can find
the time it takes for capacitor voltage to
drop from Vout to _{VHYST â‹} ^{R1+ R2}
R2
For the convenience of the user, two tables
have been setup for reference. Table 1 is for
Limit Pin while Table 2 is for VHYST.
Please check the tables to pick the resistor
values for peak primary inductor current and
recharge voltage.
Table4
RLIMIT
10
KΩ
15
KΩ
20
KΩ
25
KΩ
Peak
Current
0.8A
1A
1.2A 1.3A
Table5 (V_OUT=300V)
RHYST
Hysteresis
ratio
Recharge
Voltage
436.5 KΩ
3%
291V
211.5 KΩ
6%
282V
136.5 KΩ
9%
273V
99 KΩ
12%
264V
Once capacitor reaches the designated
voltage, DONE pin will be pulled low to
indicate the completion of charging
process.When output, voltage drops below
secondary capacitance and series resistance
will improve system efficiency and
performance.
Primary side inductance will affect the charging
time. Higher primary side inductance will raise
the average discharging current into the output
capacitance, resulting a smaller charging time.
Yet, thermal effect will become adversely
affecting the system. So always pay attention to
the amount of dissipated _{VHYST â‹} ^{R1+ R2} , DONE
R2
is released to signal the re-start of charging
process.
User always has the freedom to shut off the
operation of the chip by pulling the ENABLE
pin lower than 0.4V. Also to activate the IC,
ENABLE pin NEED needs to be above 1.8V. In
shutdown mode, the chip is only drawing about
2uA current.
Component Selection:
The rectifying diode must be of low capacitance
type while being able to withstand peak reverse
voltage and heavy forward current.
The peak reverse voltage is:
V_R,PEAK = V_OUT − (−N ⋅V_BAT ) = V_OUT + N ⋅V_BAT
I _{SEC ,MAX}
=
I _{PRI ,MAX}
N
As I_{PRI, MAX} is being set by RLIMIT, its value
can reach as high as 1.5A.
Since the switching voltage across the rectifying
diode is very large, any parasitic capacitance
across the diode will translate to huge loss for
the system. Moreover, it will slow down the
transition of the Switch pin, inducing further loss
of efficiency.
Input Capacitor:
Even though the chip is not drawing much
current, still a good decoupling capacitor is
needed for VIN pin.
It is very important to have a solid VBAT
voltage. A low ESR ceramic capacitor is
recommended.
Transformer:
A number of issues must be considered before
selecting a transformer.
YB7002 MRev1.0
www.yobon.com.tw
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