Advanced Power-Save Mode for Light-Load Efficiency
Buck-Boost Mode Operation
Inherent Excellent Line-Transient Regulation
I
p
t
clock
A
pos
neg
V
L
+
in
V
L
M0116-01
o
outp
outn
V = V
+ V
o
in
o
V
D =
V
h +
g
SLVS831A – APRIL 2008 – REVISED JULY 2008 ............................................................................................................................................................. www.ti.com
In order to maintain high efficiency over the entire load-current range, the converter reduces its switching
frequency as the load current decreases. The advanced power-save mode controls the switching frequency
using a voltage-controlled oscillator (VCO). The VCO frequency is proportional to the inductor peak current, with
a lower frequency limit of 20 kHz. This avoids disturbance of the audio band and minimizes audible noise coming
from the ceramic input and output capacitors. By maintaining a controlled switching frequency, possible EMI is
minimized. This is especially important when using the device in mobile phones. See
Figure 8 for typical
switching frequency versus load current.
Buck-boost mode operation allows the input voltage to be higher than the output voltage. This mode allows the
use of batteries and supply voltages that are above the fixed 4.6-V output voltage of OUTP.
The SIMO regulator achieves inherent superior line-transient regulation when operating in discontinuous
conduction mode, shown in
Figure 5 and
Figure 6. In discontinuous conduction mode, the current delivered to
the output is given by the inductor peak current and falling slope of the inductor current. This is shown in
Figure 14, where the output current, given by the area A, is the same for different input voltages. Because the
converter uses peak-current-mode control, the peak current is fixed as long as the load current is fixed. The
falling slope of the inductor current is given by the sum of the output voltage and inductor value. This is also a
fixed value and independent of the input voltage. Because of this, any change in input voltage changes the
converter duty cycle but does not change the inductor peak current or the falling slope of the inductor current.
Therefore, the output current, given by the area A
(Figure 14), remains constant over any input voltage variation.
Because the area A is constant, the converter has an inherently perfect line regulation when operating in
discontinuous conduction mode. Entering continuous conduction mode (CCM) linearly decreases the
line-transient performance. However, the line-transient response in CCM is still as good as for any standard
current-mode-controlled switching converter. The following formulas detail the relations of the TPS65136
converter topology operating in CCM.
Figure 14. Inherently Perfect Line-Transient Regulation
The converter always sees the sum of the negative and positive output voltage, which is calculated as:
The converter duty cycle is calculated using the efficiency estimation from the data sheet curves or from real
application measurements. A 70% efficiency value is a good value to go through the calculations.
The output current for entering continuous conduction mode can be calculated. The switching frequency can be
obtained from the data sheet graphs. A frequency of 1.5 MHz is usually sufficient for these types of calculations.
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Copyright 2008, Texas Instruments Incorporated