Datasheet

R
FF
=
-1
V
RAMP
VIN
MIN
(1-
F
OSC
x C
FF
x In
)
VIN
RFF
CFF
LM5046
SLOPE
PROPORTIONAL
TO VIN
COMP
Gate Drive
1V
CLK
VIN
R
R
5V
5k
1:1
RAMP
LM5046
www.ti.com SNVS703G –FEBRUARY 2011–REVISED MARCH 2013
phases results in flux imbalance that causes a dc buildup in the transformer. This continual dc buildup may
eventually push the transformer into saturation. The volt-second asymmetry can be corrected by employing
current mode control. In current mode control, a signal representative of the primary current is compared against
an error signal to control the duty cycle. In steady-state, this results in each phase being terminated at the same
peak current by adjusting the pulse-width and thus applying equal volt-seconds to both the phases.
Current mode control can be susceptible to noise and sub-harmonic oscillation, while voltage mode control
employs a larger ramp for PWM and is usually less susceptible. Voltage-mode control with input line feed-
forward also has excellent line transient response. When configuring for voltage mode control, a dc blocking
capacitor can be placed in series with the primary winding of the power transformer to avoid any flux imbalance
that may cause transformer core saturation.
VOLTAGE MODE CONTROL USING THE LM5046
To configure the LM5046 for voltage mode control, an external resistor (R
FF
) and capacitor (C
FF
) connected to
VIN, AGND, and the RAMP pins is required to create a saw-tooth modulation ramp signal shown in Figure 22.
The slope of the signal at RAMP will vary in proportion to the input line voltage. The varying slope provides line
feed-forward information necessary to improve line transient response with voltage mode control. With a constant
error signal, the on-time (T
ON
) varies inversely with the input voltage (VIN) to stabilize the Volt- Second product
of the transformer primary. Using a line feed-forward ramp for PWM control requires very little change in the
voltage regulation loop to compensate for changes in input voltage, as compared to a fixed slope oscillator ramp.
Furthermore, voltage mode control is less susceptible to noise and does not require leading edge filtering.
Therefore, it is a good choice for wide input range power converters. Voltage mode control requires a Type-III
compensation network, due to the complex-conjugate poles of the L-C output filter.
Figure 22. Feed-Forward Voltage Mode Configuration
The recommended capacitor value range for C
FF
is from 100pF to 1800pF. Referring to Figure 22, 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
(6)
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.
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