Datasheet

L
10 x R
S
x K x R
RAMP
C
RAMP
=
V
IN
x t
PERIOD
V
RAMP

R
RAMP
x C
RAMP
t
PERIOD
V
RAMP
= V
IN
x (1 - e
R
RAMP
x C
RAMP
)
=
L
10 x K x V
IN
x R
S
dt
dV
RAMP
LM5119/LM5119Q
SNVS676F –AUGUST 2010–REVISED FEBRUARY 2013 www.ti.com
inductor current plus some additional slope required for slope compensation. Connecting the RAMP pin resistor
to SW provides optimum slope compensation with a RAMP capacitor slope that is proportional to VIN. This
“adaptive slope compensation” eliminates the requirement for additional slope compensation circuitry with high
output voltage set points and frees the user from additional concerns in this area. The emulated ramp signal is
approximately linear and the ramp slope is given by:
(2)
The factor of 10 in Equation 2 corresponds to the internal current sense amplifier gain of the LM5119. The K
factor is a constant which adds additional slope for robust pulse-width modulation control at lower input voltages.
In practice this constant can be varied from 1 to 3. R
S
is the external sense resistor value.
The voltage on the ramp capacitor is given by:
(3)
(4)
The approximation is the first order term in a Taylor Series expansion of the exponential and is valid since t
PERIOD
is small relative to the RAMP pin R-C time constant.
Multiplying Equation 2 by t
PERIOD
to convert the slope to a peak voltage, and then equating Equation 2 with
Equation 4 allows us to solve for C
RAMP
:
(5)
Choose either C
RAMP
or R
RAMP
and use Equation 5 to calculate the other component.
The difference between the average inductor current and the DC value of the sampled inductor current can
cause instability for certain operating conditions. This instability is known as sub-harmonic oscillation, which
occurs when the inductor ripple current does not return to its initial value by the start of next switching cycle.
Sub-harmonic oscillation is normally characterized by alternating wide and narrow pulses at the switch node. The
ramp equation above contains the optimum amount of slope compensation, however extra slope compensation is
easily added by selecting a lower value for R
RAMP
or C
RAMP
.
Current Limit
The LM5119 contains a current limit monitoring scheme to protect the regulator from possible over-current
conditions. When set correctly, the emulated current signal is proportional to the buck switch current with a scale
factor determined by the current limit sense resistor, R
S
, and current sense amplifier gain. The emulated signal is
applied to the current limit comparator. If the emulated ramp signal exceeds 1.2V, the present cycle is terminated
(cycle-by-cycle current limiting). Shown in Figure 5 is the current limit comparator and a simplified current
measurement schematic. In applications with small output inductance and high input voltage, the switch current
may overshoot due to the propagation delay of the current limit comparator. If an overshoot should occur, the
sample-and-hold circuit will detect the excess recirculating current before the buck switch is turned on again. If
the sample-and-hold DC level exceeds the internal current limit threshold, the buck switch will be disabled and
skip pulses until the current has decayed below the current limit threshold. This approach prevents current
runaway conditions due to propagation delays or inductor saturation since the inductor current is forced to decay
to a controlled level following any current overshoot.
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