Datasheet

HO_ENABLE
SW
CS
RAMP
CSG
+
-
+
-
A
S
=10
R
RAMP
C
RAMP
R
S
I
L
10 xV
CS(TH)
Current Limit
Comparator
Current Sense
Amplifier
LM5117
Sample and Hold
DC Level
t
ON
I
LO
x R
S
x 10
Additional Slope
V
IN
x t
ON
R
RAMP
x C
RAMP
RAMP
PK
=
LM5117, LM5117-Q1
www.ti.com SNVS698E –APRIL 2011–REVISED MARCH 2013
Ramp Generator and Emulated Current Sense
The ramp signal used in the pulse width modulator for traditional current mode control is typically derived directly
from the high-side switch current. This switch current corresponds to the positive slope portion of the inductor
current. Using this signal for the PWM ramp simplifies the control loop transfer function to a single pole response
and provides inherent input voltage feed-forward compensation.
The disadvantage of using the high-side switch current signal for PWM control is the large leading edge spike
due to circuit parasitics that must be filtered or blanked. Minimum achievable pulse width is limited by the
filtering, blanking time and propagation delay with a high-side current sensing scheme.
In the applications where the input voltage may be relatively large in comparison to the output voltage, controlling
small pulse widths and duty cycles are necessary for regulation. The LM5117 utilizes a unique ramp generator
which does not actually measure the high-side switch current but rather reconstructs the signal. Representing or
emulating the inductor current provides a ramp signal to the PWM comparator that is free of leading edge spikes
and measurement or filtering delays, while maintaining the advantages of traditional peak current mode control.
The current reconstruction is comprised of two elements: a sample-and-hold DC level and the emulated inductor
current ramp as shown in Figure 23. The sample-and-hold DC level is derived from a measurement of the
recirculating current flowing through the current sense resistor. The voltage across the sense resistor is sampled
and held just prior to the onset of the next conduction interval of the high-side switch. The current sense amplifier
with a gain of 10 and sample-and-hold circuit provide the DC level of the reconstructed current signal as shown
in Figure 24.
Figure 23. Composition of Emulated Current Sense Signal
Figure 24. RAMP Generator and Current Limit Circuit
The positive slope inductor current ramp is emulated by C
RAMP
connected between RAMP and AGND and R
RAMP
connected between SW and RAMP. R
RAMP
should not be connected to VIN directly because the RAMP pin
absolute maximum voltage rating could be exceeded under high input voltage conditions. C
RAMP
is discharged by
an internal switch during the off-time and must be fully discharged during the minimum off-time. This limits the
ramp capacitor to be less than 2nF. A good quality, thermally stable ceramic capacitor is recommended for
C
RAMP
.
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