Datasheet

Vout
D =
Vin + Vout
0
100 200 300 400 500 600
FREQUENCY (kHz)
0.75
0.8
0.85
0.9
0.95
1
MAX DUTY CYCLE
LM5118, LM5118-Q1
SNVS566H –APRIL 2008–REVISED JANUARY 2014 www.ti.com
Maximum Duty Cycle
Each conduction cycle of the buck switch is followed by a forced minimum off-time of 400 ns to allow sufficient
time for the recirculating diode current to be sampled. This forced off-time limits the maximum duty cycle of the
controller. The actual maximum duty cycle will vary with the operating frequency as follows:
D
MAX
= 1 - f x 400 x 10
-9
(7)
where
• f is the oscillator frequency in Hz
Figure 18. Maximum Duty Cycle vs Frequency
Limiting the maximum duty cycle will limit the maximum boost ratio (VOUT/VIN) while operating in buck-boost
mode. For example, from Figure 18, at an operating frequency of 500 kHz, D
MAX
is 80%. Using the buck-boost
transfer function.
(8)
With D = 80%, solving for VOUT results in:
• VOUT = 4 x VIN
With a minimum input voltage of 5 V, the maximum possible output voltage is 20 V at f = 500 kHz. The buck-
boost step-up ratio can be increased by reducing the operating frequency which increases the maximum duty
cycle.
Soft-Start
The soft-start feature allows the regulator to gradually reach the initial steady state operating point, thus reducing
start-up stresses and surges. The internal 10 µA soft-start current source gradually charges an external soft-start
capacitor connected to the SS pin. The SS pin is connected to the positive input of the internal error amplifier.
The error amplifier controls the pulse-width modulator such that the FB pin approximately equals the SS pin as
the SS capacitor is charged. Once the SS pin voltage exceeds the internal 1.23 V reference voltage, the error
amp is controlled by the reference instead of the SS pin. The SS pin voltage is clamped by an internal amplifier
at a level of 150 mV above the FB pin voltage. This feature provides a soft-start controlled recovery in the event
a severe overload pulls the output voltage (and FB pin) well below normal regulation but doesn’t persist for 256
clock cycles.
Various sequencing and tracking schemes can be implemented using external circuits that limit or clamp the
voltage level of the SS pin. The SS pin acts as a non-inverting input to the error amplifier anytime SS voltage is
less than the 1.23 V reference. In the event a fault is detected (over-temperature, VCC under-voltage, hiccup
current limit), the soft-start capacitor will be discharged. When the fault condition is no longer present, a new soft-
start sequence will begin.
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