Datasheet

Switching
frequency
Peak Inductor
Current
I
PEAK-MAX
Dummy Load
0
Load current
Const Frequency
(2)
Const Peak Current
(3)
Ton-min
(1)
F
SW-MIN
|V
OUT
|
0
0
0
Load current
Load current
Load current
LMR70503
www.ti.com SNVS850A –JUNE 2012–REVISED APRIL 2013
Figure 31. The LMR70503 Operation Modes vs. Load Current
Minimum Switching Frequency Operation
In a typical peak current mode controlled DC-DC converter, the peak current limit is constant and the switching
frequency decreases when load current reduces. To maintain low noise operation and avoid audio frequency
interference, the minimum switching frequency of the LMR70503 is limited at 500 kHz typically. At heavy load,
the peak current limit remains constant and the switching frequency varies with the load to regulate the output
voltage. With reduced loading, the absolute output voltage is going to be charged higher than regulation if the
switching frequency cannot decrease accordingly. Therefore, to regulate the output voltage with minimum
frequency at light load, the peak current limit is reduced, in proportional to the output voltage offset.
In this mode, as shown in Figure 31, the switching frequency is fixed to the minimum switching frequency, the
peak inductor current increases with load current, and the output voltage magnitude has a small offset above
regulation.
Minimum ON-Time and Dummy Load
When load current is near zero, the peak inductor current can not reduce further due to T
ON-MIN
of the high side
switch. Under such conditions, an internal dummy load is turned on by sensing excessive output voltage offset,
which removes the extra charge from the output capacitor(s). In this condition, the switching frequency is fixed to
the minimum value. The peak inductor current value is at its minimum value, as shown in Figure 31. The dummy
load current is zero when the LMR70503 operates with on time higher than T
ON-MIN
.
The minimum peak inductor current is determined by
I
PEAK-MIN
= T
ON-MIN
× V
IN
/ L
where
• V
IN
is the supply voltage
• L is the inductance value (1)
The peak inductor current is higher with higher V
IN
. The inductor current falling slew rate is determined by
SR
FALLING
= (|V
OUT
| + V
F
) / L
where
• |V
OUT
| is the absolute value of the output voltage
• V
F
is the forward voltage drop of the power diode (2)
At lower |V
OUT
|, it takes longer time to discharge the inductor current to zero. Therefore, there is more energy to
charge the output capacitor(s). The output voltage will have more offset at higher V
IN
and lower V
OUT
. The
dummy load current is a function of the FB voltage: the more the offset at the FB node, the higher the dummy
load current, as shown in Figure 32.
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