Datasheet
R
slope
=
-1
F
OSC
x C
slope
x In
Slope-Comp
1 -
V
REF
- R
filter
Slope-Comp =
Turns-Ratio x Vout x R
CS
F
OSC
x L
filter
C
Filter
=
50 x 10
-9
3 x 25:
LM25037, LM25037-Q1
SNVS572D –JULY 2008–REVISED MARCH 2013
www.ti.com
Current mode control inherently provides line voltage feed-forward, cycle-by-cycle current limiting and ease of
loop compensation as it removes the additional pole due to output inductor. Also, in push-pull and full-bridge
converters, current mode control inherently balances volt-second product in both the phases by varying the duty
cycle as needed to terminate the cycle at the same peak current for each output phase. For duty cycles greater
than 50% (25% for each phase), peak current mode controlled circuits are subject to sub-harmonic oscillation.
Sub-harmonic oscillation is normally characterized by observing alternating wide and narrow duty cycles at the
controller output. Adding an artificial ramp (slope compensation) to the current sense signal will eliminate this
potential oscillation. Current mode control is also susceptible to noise and layout considerations. It is
recommended that C
Filter
and C
slope
be placed as close to the IC as possible to avoid any noise pickup and trace
inductance. When the converter is operating at low duty cycles and light load, the primary current amplitude is
small and is susceptible to noise. The artificial ramp, added to avoid sub-harmonic oscillations, provides
additional benefits by improving the noise immunity of the converter.
Configuration and component selection for current mode control is recommended as follows: 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 220 mV. It is recommended to set the impedances of R
Filter
and C
Filter
as seen from
C
slope
at relatively low values, so that the slope compensation is primarily dictated by R
slope
and C
slope
components. For example, if the filtering time (R
Filter
and C
Filter
) for leading edge noise is selected for 50 ns and if
the value selected for R
Filter
= 25Ω, then
(4)
Resulting in a value of C
Filter
= 680 pF (approximated to a standard value). 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
transformed to primary. This is known as deadbeat control. For circuits where primary current is sensed, the
amount of slope compensation for dead-beat control can be calculated from:
(5)
Where, Turns-Ratio is referred with respect to the primary. For example, for a 5V output converter with a turns
ratio between secondary and primary of 1:2, an oscillator frequency (F
OSC
) of 250 kHz, a filter inductance of 4 µH
(L
Filter
) and a current sense resistor (R
CS
) of 32 mΩ, slope compensation of 80 mV will suffice. The slope
compensation "volts" that results from the above expression is the maximum voltage of the artificial ramp added
linearly to the RAMP pin till the end of maximum switching period. For circuits where a current sense
tramsformer is used for primary current sensing, the turns-ratio of the current sense transformer has to be taken
into account.
C
slope
should be selected such that it can be fully discharged by the internal RAMP discharge FET. Capacitor
values ranging from 100 pF to 1500 pF are recommended. The value must be small enough such that the
capacitor can be discharged within the clock (CLK) pulse width each cycle.
R
slope
can be selected from the following formula:
(6)
For example, with a C
slope
of 1500 pF, F
OSC
of 250 kHz, reference voltage of 5V (V
REF
), slope compensation of
80 mV and R
filter
= 25Ω results in R
slope
value of 165 kΩ.
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