LT3751_15 Datasheet, PDF (14/34 Pages) Linear Technology – High Voltage Capacitor Charger Controller with Regulation

LT3751
Operation
Periodic Refresh
When the LT3751 enters regulation, the internal circuitry
deactivates switching when the internal one-shot clock
is high. The clock operates at a 1/20th duty cycle with a
minimum blank time of 1.5µs. This reset pulse is timed to
drastically reduce switching frequency content within the
audio spectrum and is active during all loading conditions.
Each reset pulse guarantees at least one energy cycle. A
minimum load is required to prevent the LT3751 from
entering no-load operation.
Heavy Load Operation
The LT3751 enters peak current mode control at higher
output load conditions. The control loop maximizes the
number of switch cycles between each reset pulse. Since
the control scheme operates in boundary mode, the reso-
nant boundary-mode period changes with varying peak
primary current:
Period = I_PK
• L_PRI
•
₁


V_TRANS
+
N
V_OUT



and the power output is proportional to the peak primary
current:
P_OUT
=



1/ 2
1
V_TRANS
• I_PK
+^N
V_OUT



Noise becomes an issue at very low load currents. The
LT3751 remedies this problem by setting the lower peak
current limit to one tenth the maximum level and begins
to employ duty-cycle control.
Light Load Operation
The LT3751 uses duty cycle control to drastically reduce
audible noise in both the transformer (mechanical) and
the ceramic capacitors (piezoelectric effects). Internal
control circuitry forces a one-shot condition at a periodic
rate greater than 20kHz and out of the audio spectrum.
The regulation loop then determines the number of pulses
that are required to maintain the correct output voltage.
Figure 5 shows the use of duty-cycle control.
No-Load Operation
The LT3751 can remain in low noise regulation at very low
loading conditions. Below a certain load current threshold
(Light Load Operation), the output voltage would continue
to increase and a runaway condition could occur. This is
due to the periodic one-shot forced by the periodic refresh
circuitry. By design, the LT3751 has built-in overvoltage
protection associated with the FB pin.
When the FB pin voltage exceeds 1.34V (±20mV), the
LT3751 enters no-load operation. No-load operation does
not reset with the one-shot clock. Instead, the pulse train
is completely load-dependent. These bursts are asynchro-
nous and can contain long periods of inactivity. This allows
regulation at a no-load condition but with the increase of
audible noise and voltage ripple. Note that when operating
with no-load, the output voltage will increase 10% above
the nominal output voltage.
3751fc
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