Datasheet

CSG
CS
I
L
TRACK
and HOLD
R
G
RAMP
1k
R
G
1k
R
S
+
-
10k
10k
0.2V
+
-
RAMP
RESET
Vth
C
RAMP
CURRENT LIMIT
COMPARATOR
I
RAMP
CURRENT SENSE
AMPLIFIER
1 +
VOUT
Iout x
VIN
LM5118, LM5118-Q1
SNVS566G –APRIL 2008–REVISED FEBRUARY 2013 www.ti.com
Current Limit
In the buck mode the average inductor current is equal to the output current (IOUT). In buck-boost mode the
average inductor current is approximately equal to:
(6)
Consequently, the inductor current in buck-boost mode is much larger especially when VOUT is large relative to
VIN. The LM5118 provides a current monitoring scheme to protect the circuit from possible over-current
conditions. When set correctly, the emulated current sense signal is proportional to the buck switch current with a
scale factor determined by the current sense resistor. The emulated ramp signal is applied to the current limit
comparator. If the peak of the emulated ramp signal exceeds 1.25V when operating in the buck mode, the PWM
cycle is immediately terminated (cycle-by-cycle current limiting). In buck-boost mode the current limit threshold is
increased to 2.50V to allow higher peak inductor current. To further protect the external switches during
prolonged overload conditions, an internal counter detects consecutive cycles of current limiting. If the counter
detects 256 consecutive current limited PWM cycles, the LM5118 enters a low power dissipation hiccup mode. In
the hiccup mode, the output drivers are disabled, the UVLO pin is momentarily pulled low, and the soft-start
capacitor is discharged. The regulator is restarted with a normal soft-start sequence once the UVLO pin charges
back to 1.23V. The hiccup mode off-time can be programmed by an external capacitor connected from UVLO pin
to ground. This hiccup cycle will repeat until the output overload condition is removed.
In applications with low output inductance and high input voltage, the switch current may overshoot due to the
propagation delay of the current limit comparator and control circuitry. If an overshoot should occur, the sample-
and-hold circuit will detect the excess recirculating diode current. If the sample-and-hold pedestal level exceeds
the internal current limit threshold, the buck switch will be disabled and will skip PWM cycles until the inductor
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 before the buck switch is
turned on again.
Figure 18. Current Limit and Ramp Circuit
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Product Folder Links: LM5118 LM5118-Q1