Datasheet

CS
R
FILTER
R
CS
LM5046
RAMP
CLK + LEB
CLK
Current
Sense
C
FILTER
SLOPE
100 PA
0
R
SLOPE
OUT CS
SLOPE
FILTER OSC SLOPE TR
V R
R
L F I N
´
=
´ ´ ´
LM5046
SNVS703G –FEBRUARY 2011–REVISED MARCH 2013 www.ti.com
CURRENT MODE CONTROL USING THE LM5046
The LM5046 can be configured for current mode control by applying a signal proportional to the primary current
to the RAMP pin. One way to achieve this is shown in Figure 23. The primary current can be sensed using a
current transformer or sense resistor, the resulting signal is filtered and applied to the RAMP pin through a
resistor used for slope compensation. It can be seen that the signal applied to the RAMP pin consists of the
primary current information from the CS pin plus an additional ramp for slope compensation, added by the
resistor R
SLOPE
.
The current sense resistor is selected such that during over current condition, the voltage across the current
sense resistor is above the minimum CS threshold of 728mV.
In general, the amount of slope compensation required to avoid sub-harmonic oscillation is equal to at least one-
half the down-slope of the output inductor current, transformed to the primary. To mitigate sub-harmonic
oscillation after one switching period, the slope compensation has to be equal to one times the down slope of the
filter inductor current transposed to primary. This is known as deadbeat control. The slope compensation resistor
required to implement dead-beat control can be calculated as follows:
(7)
Where N
TR
is the turns-ratio with respect to the secondary. For example, for a 3.3V output converter with a turns-
ratio between primary and secondary of 9:1, an output filter inductance (L
FILTER
) of 800nH and a current sense
resistor (R
SENSE
) of 150mΩ, R
SLOPE
of 1.67kΩ will suffice.
Figure 23. Current Mode Configuration
VIN and VCC
The voltage applied to the VIN pin, which may be the same as the system voltage applied to the power
transformer’s primary (V
PWR
), can vary in the range of the 14 to 100V. It is recommended that the filter shown in
Figure 24 be used to suppress the transients that may occur at the input supply. This is particularly important
when VIN is operated close to the maximum operating rating of the LM5046. The current into VIN depends
primarily on the LM5046’s operating current, the switching frequency, and any external loads on the VCC pin,
that typically include the gate capacitances of the power MOSFETs. In typical applications, an auxiliary
transformer winding is connected through a diode to the VCC pin. This pin must raise VCC voltage above 8V to
shut off the internal start-up regulator.
After the outputs are enabled and the external VCC supply voltage has begun supplying power to the IC, the
current into the VIN pin drops below 1mA. VIN should remain at a voltage equal to or above the VCC voltage to
avoid reverse current through the internal body diode of the internal VCC regulator.
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