Texas Instruments LM5017 Isolated Bias Supply Evaluation Board LM5017ISOEVAL/NOPB LM5017ISOEVAL/NOPB 데이터 시트
제품 코드
LM5017ISOEVAL/NOPB
R
FB1
x R
FB2
R
2
x (R
FB1
+ R
FB2
) + R
FB1
x R
FB2
V
FB
= (VCC - V
D
) x
û
I
L(MIN)
25 mV
R
C
g
sw
(R
FB2
||R
FB1
)
>
5
C >
C
r
= 3300 pF
R
r
C
r
<
C
ac
= 100 nF
(V
IN(MIN)
- V
OUT
) x T
ON
25 mV
25 mV
R
C
û
I
L(MIN)
V
OUT
V
REF
x
>
GND
To FB
L1
C
OUT
R
FB2
R
FB1
V
OUT
R
C
GND
To FB
L1
C
OUT
R
FB2
R
FB1
V
OUT
R
C
C
ac
C
OUT
V
OUT
GND
R
r
C
ac
C
r
To FB
R
FB2
R
FB1
L1
SNVS783G – JANUARY 2012 – REVISED DECEMBER 2013
Ripple ConfiguratioN
LM5017 uses Constant-On-Time (COT) control scheme, in which the on-time is terminated by an on-timer, and
the off-time is terminated by the feedback voltage (V
the off-time is terminated by the feedback voltage (V
FB
) falling below the reference voltage (V
REF
). Therefore, for
stable operation, the feedback voltage must decrease monotonically, in phase with the inductor current during
the off-time. Furthermore, this change in feedback voltage (V
the off-time. Furthermore, this change in feedback voltage (V
FB
) during off-time must be large enough to
suppress any noise component present at the feedback node.
shows three different methods for generating appropriate voltage ripple at the feedback node. Type 1
and Type 2 ripple circuits couple the ripple at the output of the converter to the feedback node (FB). The output
voltage ripple has two components:
voltage ripple has two components:
1. Capacitive ripple caused by the inductor current ripple charging/discharging the output capacitor.
2. Resistive ripple caused by the inductor current ripple flowing through the ESR of the output capacitor.
The capacitive ripple is not in phase with the inductor current. As a result, the capacitive ripple does not
decrease monotonically during the off-time. The resistive ripple is in phase with the inductor current and
decreases monotonically during the off-time. The resistive ripple must exceed the capacitive ripple at the output
node (V
decrease monotonically during the off-time. The resistive ripple is in phase with the inductor current and
decreases monotonically during the off-time. The resistive ripple must exceed the capacitive ripple at the output
node (V
OUT
) for stable operation. If this condition is not satisfied unstable switching behavior is observed in COT
converters, with multiple on-time bursts in close succession followed by a long off-time.
Type 3 ripple method uses R
r
and C
r
and the switch node (SW) voltage to generate a triangular ramp. This
triangular ramp is ac coupled using C
ac
to the feedback node (FB). Since this circuit does not use the output
voltage ripple, it is ideally suited for applications where low output voltage ripple is required. See application note
AN-1481 (
AN-1481 (
) for more details for each ripple generation method.
Table 1.
Type 1
Type 2
Type 3
Lowest Cost Configuration
Reduced Ripple Configuration
Minimum Ripple Configuration
Soft Start
A soft-start feature can be implemented to the LM5017 using an external circuit. As shown in
, the soft-
start circuit consists of one capacitor, C
1
, two resistors, R
1
and R
2
, and a diode, D. During the initial start-up, the
VCC voltage is established prior to the V
OUT
voltage. D is thereby forward biased and the FB voltage is pulled up
above the reference voltage (1.225 V). The switcher is disabled. With the charging of the capacitor C
1
, the
voltage at node B gradually decreases. Due to the action of the control circuit, V
OUT
will gradually rise to maintain
the FB voltage at the reference voltage. Once the voltage at node B is lower than the FB voltage, plus the
voltage drop of D, the soft-start is finished and D is reverse biased.
voltage drop of D, the soft-start is finished and D is reverse biased.
During the initial part of the start-up, the FB voltage can be approximated as follows. Please note that the effect
of R
of R
1
has been ignored to simplify the calculation:
16
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