Datasheet

C
RAMP
=
R
S
x A
g
m
x L
T
ON
Sample and Hold
DC Level
V/A10 x R
S
RAMP
T
ON
(5 PA/V x (VIN ± VOUT) + 25 PA) x
C
RAMP
LM5088, LM5088-Q1
www.ti.com SNVS600H –DECEMBER 2008–REVISED MARCH 2013
relatively large in comparison to the output voltage, controlling small pulse widths and duty cycles is necessary
for regulation. The LM5088 utilizes a unique ramp generator which does not actually measure the buck switch
current but rather reconstructs or emulates the signal. Emulating the inductor current provides a ramp signal that
is free of leading edge spikes and measurement or filtering delays. The current reconstruction is comprised of
two elements; a sample & hold DC level and an emulated current ramp.
Figure 17. Composition of Current Sense Signal
The sample & hold DC level illustrated in Figure 17 is derived from a measurement of the re-circulating (or free-
wheeling) diode current. The diode current flows through the current sense resistor connected between the CS
and CSG pins. The voltage across the sense resistor is sampled and held just prior to the onset of the next
conduction interval of the buck switch. The diode current sensing and sample & hold provide the DC level for the
reconstructed current signal. The positive slope inductor current ramp is emulated by an external capacitor
connected from the RAMP pin to GND and an internal voltage controlled current source. The ramp current
source that emulates the inductor current is a function of the VIN and VOUT voltages per the following equation:
I
RAMP
= 5 µA/V x (VIN x VOUT) + 25 µA (2)
Proper selection of the RAMP capacitor depends upon the selected value of the output inductor and the current
sense resistor (R
S
). For proper current emulation, the DC sample & hold value and the ramp amplitude must
have the same dependence on the load current. That is:
where
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