Diodes AUR9716 - Manual (Page 11)

Data Sheet
1.0MHz to 1.4MHz, 2A, STEP DOWN DC-DC CONVERTER AUR9716
Apr. 2012 Rev. 1. 0 BCD Semiconductor Manufacturing Limited
11
Application Information (Continued)
5. Slope Compensation
The slope compensation of AUR9716 provides stability
in constant frequency construction by preventing
oscillations at duty cycle more than 50%. It’s
accomplished externally by adding a series of capacitor
and resistor, as shown in
Figure 24.
Figure 24. Stability Compensation Components
The DC loop gain of the system is determined by the
following equation:
CS V
OUT
FB
VDC
G A
I
V
A =
,
Where A
V
is error amplifier voltage gain and G
CS
is
current sense transconductance.
The dominant pole P1 is due to C
COMP
:
COMP V
EA
P C A
G
f
π 2 1
=
,
Where
EA
G is error amplifier transconductance.
The output pole P2 is due to C
OUT
:
OUT OUT
OUT
P C V
I
f
π
=
2 2
The zero Z1 is due to C
COMP
and R
COMP
:
COMP COMP
Z C R
f
π 2
1
1
=
If C
COMP2
is used, the third pole is due to R
COMP
and
C
COMP2
:
2
3 2
1
COMP COMP
P C R
f
π
=
Then the cross over frequency often sets at 1/5 to 1/10
of the switching frequency.
Table 1 shows some calculated results based on stability
compensation equations above.
Switching Frequency
(MHz)
1.4 1.2 1.0
R
COMP
(kΩ) 4.3 4.3 3.6
C
COMP
(nF) 1 1 1.5
L (µH) 1.5 1.5 2.2
C
OUT
(µF) 10 10 10
Table 1. Stability Compensation Components
To optimize the components for stability compensation
listed in Table 1, we will introduce the selection value of
R
COMP
and C
COMP
as detail as possible.
1. R
COMP
: determine this resistor value according to
the desired crossover frequency is f
C
, default as 1.4MHz.
FB
OUT
CS EA
C OUT
COMP V
V
G G
f C
R
π 2
=
2. C
COMP
: determine this capacitor value according to
the desired phase margin. We often choose this
compensation.
Zero point below one forth of the crossover frequency to
ensure the loop stability.
C COMP
COMP f R
C
π 2
4
>
6. Short-Circuit Protection
When AUR9716 output node is shorted to GND, as
V
FB
drops under 0.4V, chip will enter soft-start to
protect itself, when short circuit is removed, and V
FB
rises over 0.4V, the AUR9716 enters normal
operation again. If AUR9716 reaches OCP threshold
while short circuit, it will enter soft-start cycle until
the current under OCP threshold. When AUR9716 is
used to transfer V
IN
=5V to V
OUT
=2.5V, shorting V
OUT
to GND makes big current which enables SCP
protection. The waveform is shown in Figure 25.