lm5037mtx National Semiconductor Corporation, lm5037mtx Datasheet - Page 16

• Manufacturer Part Number: lm5037mtx
• Description: Dual-mode Pwm Controller With Alternating Outputs
• Manufacturer: National Semiconductor Corporation

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CURRENT MODE CONTROL USING THE LM5037
The LM5037 can be configured in current mode control by
applying the primary current signal into the RAMP pin. One
way to achieve this is shown in Figure 9, which depicts a sim-
plified push-pull converter. The primary current is sensed
using a sense resistor and the current information is then fil-
Current mode control inherently provides line voltage feed-
forward, cycle-by-cycle current limiting and ease of loop com-
pensation 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 character-
ized by observing alternating wide and narrow duty cycles at
the controller output. Adding an artificial ramp (slope com-
pensation) to the current sense signal will eliminate this po-
tential oscillation. Current mode control is also susceptible to
noise and layout considerations. It is recommended that
C
avoid any noise pickup and trace inductance. When the con-
verter 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 con-
trol 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
slope
be placed as close to the IC as possible to
Filter
and C
Filter
as seen from C
FIGURE 9. Current Mode Configuration
slope
at rela-
16
tered and applied to the RAMP pin through capacitor C
for use as the modulation ramp. It can be seen that the signal
applied to the RAMP pin consists of the primary current in-
formation from the CS pin plus an additional ramp for slope
compensation, added by R
tively low values, so that the slope compensation is primarily
dictated by R
filtering time (R
lected for 50 ns and if the value selected for R
Resulting in a value of C
standard value). In general, the amount of slope compensa-
tion 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 oscil-
lation 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 using a
resistor, the amount of slope compensation for dead-beat
control required can be calculated from:
Where, Turns-Ratio is referred with respect to the primary.
For example, for a 5V output converter with a turns ratio be-
tween secondary and primary of 1:2, an oscillator frequency
slope
Filter
and C
and C
slope
Filter
Filter
slope
components. For example, if the
) for leading edge noise is se-
= 680 pF (approximated to a
and C
30069430
slope
.
Filter
= 25Ω, then
slope
,