Texas Instruments LM5019 Isolated Bias Supply Evaluation Board LM5019ISOEVAL/NOPB LM5019ISOEVAL/NOPB 데이터 시트
제품 코드
LM5019ISOEVAL/NOPB
0.07 x V
IN
T
OFF(ILIM)
=
V
FB
+ 0.2V
P
s
10
-10
x R
ON
T
ON
=
V
IN
FB
SW
L1
C
OUT
R
FB2
VOUT
R
C
LM5019
+
R
FB1
VOUT
(low ripple)
(low ripple)
SNVS788E – JANUARY 2012 – REVISED DECEMBER 2013
At high input voltages, the power dissipated in the high voltage regulator is significant and can limit the overall
achievable output power. As an example, with the input at 48 V and switching at high frequency, the V
achievable output power. As an example, with the input at 48 V and switching at high frequency, the V
CC
regulator may supply up to 7 mA of current resulting in 48 V x 7 mA = 336 mW of power dissipation. If the V
CC
voltage is driven externally by an alternate voltage source, between 8 V and 13 V, the internal regulator is
disabled. This reduces the power dissipation in the IC.
disabled. This reduces the power dissipation in the IC.
Figure 13. Low Ripple Output Configuration
Regulation Comparator
The feedback voltage at FB is compared to an internal 1.225 V reference. In normal operation, when the output
voltage is in regulation, an on-time period is initiated when the voltage at FB falls below 1.225 V. The high side
switch will stay on for the on-time, causing the FB voltage to rise above 1.225 V. After the on-time period, the
high side switch will stay off until the FB voltage again falls below 1.225 V. During start-up, the FB voltage will be
below 1.225 V at the end of each on-time, causing the high side switch to turn on immediately after the minimum
forced off-time of 144 ns. The high side switch can be turned off before the on-time is over, if the peak current in
the inductor reaches the current limit threshold.
voltage is in regulation, an on-time period is initiated when the voltage at FB falls below 1.225 V. The high side
switch will stay on for the on-time, causing the FB voltage to rise above 1.225 V. After the on-time period, the
high side switch will stay off until the FB voltage again falls below 1.225 V. During start-up, the FB voltage will be
below 1.225 V at the end of each on-time, causing the high side switch to turn on immediately after the minimum
forced off-time of 144 ns. The high side switch can be turned off before the on-time is over, if the peak current in
the inductor reaches the current limit threshold.
Overvoltage Comparator
The feedback voltage at FB is compared to an internal 1.62 V reference. If the voltage at FB rises above 1.62 V
the on-time pulse is immediately terminated. This condition can occur if the input voltage and/or the output load
changes suddenly. The high side switch will not turn on again until the voltage at FB falls below 1.225 V.
the on-time pulse is immediately terminated. This condition can occur if the input voltage and/or the output load
changes suddenly. The high side switch will not turn on again until the voltage at FB falls below 1.225 V.
On-Time Generator
The on-time for the LM5019 is determined by the R
ON
resistor, and is inversely proportional to the input voltage
(V
IN
), resulting in a nearly constant frequency as V
IN
is varied over its range. The on-time equation for the
LM5019 is:
(4)
See
. R
ON
should be selected for a minimum on-time (at maximum V
IN
) greater than 100ns, for proper
operation. This requirement limits the maximum switching frequency for high V
IN
.
Current Limit
The LM5019 contains an intelligent current limit off-timer. If the current in the buck switch exceeds 240mA the
present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of off-time is
controlled by the FB voltage and the input voltage V
present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of off-time is
controlled by the FB voltage and the input voltage V
IN
. As an example, when FB = 0V and V
IN
= 48 V, the
maximum off-time is set to 16
μs. This condition occurs when the output is shorted, and during the initial part of
start-up. This amount of time ensures safe short circuit operation up to the maximum input voltage of 100 V.
In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time is
reduced. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and
start-up time. The off-time is calculated from the following equation:
reduced. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and
start-up time. The off-time is calculated from the following equation:
(5)
The current limit protection feature is peak limited. The maximum average output will be less than the peak.
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