Datasheet

R
FF
=
-1
V
RAMP
VIN
MIN
(1-
F
OSC
x C
FF
x In
)
VIN
RFF
CFF
LM5045
SLOPE
PROPORTIONAL
TO VIN
COMP
Gate Drive
1V
CLK
VIN
R
R
5V
5k
1:1
RAMP
LM5045
SNVS699G –FEBRUARY 2011–REVISED MARCH 2013 www.ti.com
Figure 19. Feed-Forward Voltage Mode Configuration
The recommended capacitor value range for C
FF
is from 100pF to 1800pF. Referring to Figure 19, it can be seen
that C
FF
value must be small enough to be discharged with in the clock pulse-width which is typically within 50ns.
The R
DS(ON)
of the internal discharge FET is 5.5Ω.
The value of R
FF
required can be calculated from
(4)
For example, assuming a V
RAMP
of 1.5V (a good compromise of signal range and noise immunity), at VIN
MIN
of
36V (oscillator frequency of 400 kHz and C
FF
= 470pF results in a value for R
FF
of 125 kΩ.
Current Mode Control Using the LM5045
The LM5045 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 20. 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:
where
• N
TR
is the turns-ratio with respect to the secondary (5)
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.
22 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: LM5045