L6726A
Application details
8
8.1
Application details
Output voltage selection
L6726A is capable to precisely regulate an output voltage as low as 0.8 V. In fact, the device
comes with a fixed 0.8 V internal reference that guarantees the output regulated voltage to
be within ±1% tolerance over line and temperature variations between 0 °C and 70 °C
(excluding output resistor divider tolerance, when present).
Output voltage higher than 0.8 V can be achieved by adding a resistor R
OS
between FB pin
and ground. Referring to
the steady state DC output voltage will be:
R
FB
V
OUT
=
V
REF
⋅ ⎛
1
+ ----------
⎞
-
⎝
R
OS
⎠
where V
REF
is 0.8 V.
8.2
Compensation network
The control loop shown in
is a voltage mode control loop. The error amplifier is a
transconductance type with fixed gain (3.3 ms typ.). The FB voltage is regulated to the
internal reference, thus the output voltage is fixed accordingly to the output resistor divider
(when present).
Transconductance error amplifier output current generates a voltage across Z
F
, which is
compared to oscillator saw-tooth waveform to provide PWM signal to the driver section.
PWM signal is then transferred to the switching node with V
IN
amplitude. This waveform is
filtered by the output filter.
Figure 8.
PWM control loop
V
IN
OSC
ΔV
OSC
_
+
PWM
COMPARATOR
L
R
C
OUT
ESR
V
OUT
OTA
COMP
C
F
R
F
C
P
+
_
V
REF
FB
R
FB
R
OS
OUTPUT
DIVIDER
Z
F
The converter transfer function is the small signal transfer function between the voltage at
the output node of the EA (COMP) and V
OUT
. This function has a double pole (complex
conjugate) at frequency F
LC
depending on the L-C
OUT
resonance and a zero at F
ESR
Doc ID 12754 Rev 4
15/35
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