STR-A6100 View Datasheet(PDF) - Sanken Electric co.,ltd.
Part Name
Description
Manufacturer
STR-A6100 Datasheet PDF : 26 Pages
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STR-A6100 Series
controls on-time with fixed off-time. In addition, the IC
uses the peak-current-mode control method, which
enhances the response speed and provides the stable
operation.
D
Timer reset
OFF
OFF signal
output
Timer
SQ
Drive
Gate
control
7,8
Circuit
R
ON/OFF PRC latch circuit
- VSC
+ VOCPM
FB Comp.
Buffer
+
-
OCP Comp.
VOCP(TH)
U1
S/OCP
1
ID
3 GND
DZ1
4 FB/OLP
IFB ROCP
VROCP
C4
C3 PC1
Figure 9-4 FB/OLP pin peripheral circuit
FB comparator
-
+
VSC
VOCPM
Drain current, ID
Figure 9-5 The waveform of ID and FB comparator input
The internal fixed off-time, tOFF is made from internal
off timer circuit, the turn-on timing of power MOSFET
depends on tOFF.
Turn-on
After the period of tOFF, OFF signal output becomes
High, Q of PRC latch circuit is latched to Low. As a
result, turn-on signal is input to the gate control circuit,
and power MOSFET turns on.
Tuen-off
When the OCP comparator or the FB comparator
resets the PRC latch circuit, Q of PRC latch circuit
is latched to High. As a result, turn-off signal is input
to the gate control circuit, and power MOSFET turns
off.
The IC controls the peak value of VOCPM voltage to be
close to target voltage (VSC), comparing VOCPM with VSC
by internal FB comparator.
VOCPM is amplified VROCP voltage that is a detection
voltage by current detection resistor, ROCP.
Light load conditions
When load conditions become lighter, the output
voltage, VOUT, increases. Thus, the feedback current
from the error amplifier on the secondary-side also
increases. The feedback current is sunk at the FB/OLP
pin, transferred through a photo-coupler, PC1, and the
FB/OLP pin voltage decreases. Thus, VSC decreases,
and the peak value of VOCPM is controlled to be low,
and the peak drain current of ID decreases.
This control prevents the output voltage from
increasing.
Heavy load conditions
When load conditions become greater, the IC
performs the inverse operation to that described above.
Thus, VSC increases and the peak drain current of ID
increases.
This control prevents the output voltage from
decreasing.
9.4 Leading Edge Blanking Function
The constant voltage control of output of the IC uses
the peak-current-mode control method.
In peak-current-mode control method, there is a case
that the power MOSFET turns off due to unexpected
response of FB comparator or overcurrent protection
circuit (OCP) to the steep surge current in turning on a
power MOSFET.
In order to prevent this operation, Leading Edge
Blanking Time, tBW = 320 ns is built-in.
In the period of tBW, the IC does not respond to the
surge voltage in turning on the power MOSFET.
9.5 Auto Standby Function
Automatic standby mode is activated automatically
when the drain current, ID, reduces under light load
conditions, at which ID is less than 25% of the maximum
drain current (it is in the Overcurrent Protection state).
The operation mode becomes burst oscillation, as shown
in Figure 9-6. The 25% of the maximum drain current
corresponds to the Burst Threshold Voltage of FB/OLP
pin, VBURST = 0.79 V (0.75 V for STR-A61××M and
STR-A6153E).
Burst oscillation mode reduces switching losses and
improves power supply efficiency because of periodic
non-switching intervals.
Generally, to improve efficiency under light load
conditions, the frequency of the burst mode becomes
STR-A6100 - DS Rev.2.1
SANKEN ELECTRIC CO.,LTD.
15
Jun. 05, 2014
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