Datasheet

A =
10k
1k + R
G
g
m
x L
C
RAMP
R
S
x A =
g
m
x L
C
RAMP
=
A x R
S
LM5118, LM5118-Q1
SNVS566H –APRIL 2008–REVISED JANUARY 2014 www.ti.com
(4)
where
• g
m
is the ramp generator transconductance (5 µA/V)
• A is the current sense amplifier gain (10V/V)
The ramp capacitor should be located very close to the device and connected directly to the RAMP and AGND
pins.
The relationship between the average inductor current and the pedestal value of the sampled inductor current
can cause instability in 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 the next switching cycle.
Sub-harmonic oscillation is normally characterized by observing alternating wide and narrow pulses at the switch
node. Adding a fixed slope voltage ramp (slope compensation) to the current sense signal prevents this
oscillation. The 50 µA of offset current provided from the emulated current source adds enough slope
compensation to the ramp signal for output voltages less than or equal to 12 V. For higher output voltages,
additional slope compensation may be required. In such applications, the ramp capacitor can be decreased from
the nominal calculated value to increase the ramp slope compensation.
The pedestal current sample is obtained from the current sense resistor (Rs) connected to the CS and CSG pins.
It is sometimes helpful to adjust the internal current sense amplifier gain (A) to a lower value in order to obtain
the higher current limit threshold. Adding a pair of external resistors RG in a series with CS and CSG as in
Figure 17 reduces the current sense amplifier gain A according to Equation 5:
(5)
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