Datasheet

5 PA
V
x VIN + 50 PA
I
RAMP
(buck - boost) =
5 PA
V
x (VIN - VOUT) + 50 PA
I
RAMP
(buck) =
Emulated
Ramp
Ton
Buck:
Buck - Boost:
t
v
Pedestal Level = (volts/amp)
Rsx10
Cramp
ton
x)
PA
50+)- VoutVin(x
PA
5(
Cramp
ton
x)
PA
50+Vinx
PA
5(
LM5118, LM5118-Q1
www.ti.com SNVS566H –APRIL 2008–REVISED JANUARY 2014
Figure 16. Composition of Emulated Current Signal
Ramp Generator
The ramp signal of a pulse-width modulator with current mode control is typically derived directly from the buck
switch drain current. This switch current corresponds to the positive slope portion of the inductor current signal.
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 buck switch current
signal for PWM control is the large leading edge spike due to circuit parasitics. The leading edge spike must be
filtered or blanked to avoid early termination of the PWM pulse. Also, the current measurement may introduce
significant propagation delays. The filtering, blanking time and propagation delay limit the minimal achievable
pulse width. In applications where the input voltage may be relatively large in comparison to the output voltage,
controlling a small pulse width is necessary for regulation. The LM5118 utilizes a unique ramp generator which
does not actually measure the buck switch current but instead creates a signal representing or emulating the
inductor current. The emulated ramp provides signal to the PWM comparator that is free of leading edge spikes
and measurement or filtering delays. The current reconstruction is comprised of two elements, a sample-and-
hold pedestal level and a ramp capacitor which is charged by a controlled current source. Refer to Figure 16 for
details.
The sample-and-hold pedestal level is derived from a measurement of the recirculating current through a current
sense resistor in series with the recirculating diode of the buck regulator stage. A small value current sensing
resistor is required between the recirculating diode anode and ground. The CS and CSG pins should be Kelvin
connected directly to the sense resistor. The voltage level across the sense resistor is sampled 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 sample and hold of the recirculating diode current is
valid for both buck and buck-boost modes. The positive slope inductor current ramp is emulated by an external
capacitor connected from the RAMP pin to the AGND and an internal voltage controlled current source. In buck
mode, the ramp current source that emulates the inductor current is a function of the VIN and VOUT voltages per
Equation 2:
(2)
In buck-boost mode, the ramp current source is a function of the input voltage VIN, per Equation 3:
(3)
Proper selection of the RAMP capacitor (C
RAMP
) depends upon the value of the output inductor (L) and the
current sense resistor (R
S
). For proper current emulation, the sample and hold pedestal value and the ramp
amplitude must have the same relative relationship to the actual inductor current. That is:
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