lm25119psqx National Semiconductor Corporation, lm25119psqx Datasheet - Page 13

• Manufacturer Part Number: lm25119psqx
• Description: Lm25119 Wide Input Range Dual Synchronous Buck Controller
• Manufacturer: National Semiconductor Corporation

The sample-and-hold DC level is derived from a measure-
ment of the recirculating current flowing through the current
sense resistor. The voltage across the sense resistor is sam-
pled and held just prior to the onset of the next conduction
interval of the buck switch. The current sensing and sample-
and-hold provide the DC level of the reconstructed current
signal. The positive slope inductor current ramp is emulated
by an external capacitor connected from RAMP pin to AGND
and a series resistor connected between SW and RAMP. The
ramp resistor should not be connected to VIN directly be-
cause the RAMP pin voltage rating could be exceeded under
high VIN conditions. The ramp created by the external resistor
and capacitor will have a slope proportional to the rising in-
ductor current plus some additional slope required for slope
compensation. Connecting the RAMP pin resistor to SW pro-
vides optimum slope compensation with a RAMP capacitor
slope that is proportional to VIN. This “adaptive slope com-
pensation” 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 em-
ulated ramp signal is approximately linear and the ramp slope
is given by:
The factor of 10 in equation (2) corresponds to the internal
current sense amplifier gain of the LM25119. 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
resistor value.
The voltage on the ramp capacitor is given by:
The approximation is the first order term in a Taylor Series
expansion of the exponential and is valid since t
relative to the RAMP pin R-C time constant.
Multiplying (2) by t
age, and then equating (2) with (4) allows us to solve for
C
RAMP
:
PERIOD
to convert the slope to a peak volt-
S
is the external sense
FIGURE 4. Current Limit and Ramp Circuit
PERIOD
is small
(2)
(3)
(4)
13
Choose either C
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 rip-
ple 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 com-
pensation is easily added by selecting a lower value for
R
Current Limit
The LM25119 contains a current limit monitoring scheme to
protect the regulator from possible over-current conditions.
When set correctly, the emulated current signal is proportion-
al to the buck switch current with a scale factor determined by
the current limit sense resistor, R
fier 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
plified 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 sam-
ple-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.
RAMP
or C
Figure 4
RAMP
.
RAMP
is the current limit comparator and a sim-
or R
30126213
RAMP
and use (5) to calculate the
S
, and current sense ampli-
www.national.com
(5)