Kingbor Technology Co.,Ltd
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KB3426
SIMPLIFIED BLOC DIAGRAM
SLOPE
COMP
OSC
OSC
0.65V
FREQ
SHIFT
V
FB
/V
OUT
5
R1
FB
R2
0.6V
4 V
IN
–
+
–
+
EA
0.4V
–
+
BURST
S
R
Q
Q
SLEEP
–
I
COMP
+
5
RS LATCH
V
IN
RUN
1
0.6V REF
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTI-
SHOOT-
THRU
3 SW
SHUTDOWN
OPERATION
Main Control Loop
(Refer to Functional Diagram)
Burst Mode Operation
The KB3426 is capable of Burst Mode operation in which
the internal power MOSFETs operate intermittently based
on load demand.
In Burst Mode operation, the peak current of the inductor
is set to approximately 200mA regardless of the output
load. Each burst event can last from a few cycles at light
loads to almost continuously cycling with short sleep
intervals at moderate loads. In between these burst events,
the power MOSFETs and any unneeded circuitry are turned
off, reducing the quiescent current to 20µA. In this sleep
state, the load current is being supplied solely from the
output capacitor. As the output voltage droops, the EA
amplifier’s output rises above the sleep threshold signal-
ing the BURST comparator to trip and turn the top MOSFET
on. This process repeats at a rate that is dependent on the
load demand.
The KB3426 uses a constant frequency, current mode
step-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET) switches
are internal. During normal operation, the internal top
power MOSFET is turned on each cycle when the oscillator
sets the RS latch, and turned off when the current com-
parator, I
COMP
, resets the RS latch. The peak inductor
current at which I
COMP
resets the RS latch, is controlled by
the output of error amplifier EA. When the load current
increases, it causes a slight decrease in the feedback
voltage, FB, relative to the 0.6V reference, which in turn,
causes the EA amplifier’s output voltage to increase until
the average inductor current matches the new load cur-
rent. While the top MOSFET is off, the bottom MOSFET is
turned on until either the inductor current starts to reverse,
as indicated by the current reversal comparator I
RCMP
, or
the beginning of the next clock cycle.
6
–
I
RCMP
+
2 GND
KINGBOR [ KINGBOR TECHNOLOGY CO ]
