Texas Instruments LM5019 Isolated Bias Supply Evaluation Board LM5019ISOEVAL/NOPB LM5019ISOEVAL/NOPB Datenbogen
Produktcode
LM5019ISOEVAL/NOPB
=
1.225V
R
FB1
V
OUT
- 1.225V
R
FB2
R
FB2
+ R
FB1
V
OUT
= 1.225V x
R
FB1
V
OUT
g
sw
=
10
-10
x R
ON
SNVS788E – JANUARY 2012 – REVISED DECEMBER 2013
Functional Description
The LM5019 step-down switching regulator features all the functions needed to implement a low cost, efficient,
buck converter capable of supplying up to 100 mA to the load. This high voltage regulator contains 100 V, N-
channel buck and synchronous switches, is easy to implement, and is provided in thermally enhanced SO Power
PAD-8 and WSON-8 packages. The regulator operation is based on a constant on-time control scheme using an
on-time inversely proportional to V
buck converter capable of supplying up to 100 mA to the load. This high voltage regulator contains 100 V, N-
channel buck and synchronous switches, is easy to implement, and is provided in thermally enhanced SO Power
PAD-8 and WSON-8 packages. The regulator operation is based on a constant on-time control scheme using an
on-time inversely proportional to V
IN
. This control scheme does not require loop compensation. The current limit
is implemented with a forced off-time inversely proportional to V
OUT
. This scheme ensures short circuit protection
while providing minimum foldback. The simplified block diagram of the LM5019 is shown in
.
The LM5019 can be applied in numerous applications to efficiently regulate down higher voltages. This regulator
is well suited for 48 V telecom and 42 V automotive power bus ranges. Protection features include: thermal
shutdown, undervoltage lockout, minimum forced off-time, and an intelligent current limit.
is well suited for 48 V telecom and 42 V automotive power bus ranges. Protection features include: thermal
shutdown, undervoltage lockout, minimum forced off-time, and an intelligent current limit.
Control Overview
The LM5019 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with the
output voltage feedback (FB) compared to an internal reference (1.225 V). If the FB voltage is below the
reference the internal buck switch is turned on for the one-shot timer period, which is a function of the input
voltage and the programming resistor (R
output voltage feedback (FB) compared to an internal reference (1.225 V). If the FB voltage is below the
reference the internal buck switch is turned on for the one-shot timer period, which is a function of the input
voltage and the programming resistor (R
ON
). Following the on-time the switch remains off until the FB voltage
falls below the reference, but never before the minimum off-time forced by the minimum off-time one-shot timer.
When the FB pin voltage falls below the reference and the minimum off-time one-shot period expires, the buck
switch is turned on for another on-time one-shot period. This will continue until regulation is achieved and the FB
voltage is approximately equal to 1.225 V (typ).
When the FB pin voltage falls below the reference and the minimum off-time one-shot period expires, the buck
switch is turned on for another on-time one-shot period. This will continue until regulation is achieved and the FB
voltage is approximately equal to 1.225 V (typ).
In a synchronous buck converter, the low side (sync) FET is ‘on’ when the high side (buck) FET is ‘off’. The
inductor current ramps up when the high side switch is ‘on’ and ramps down when the high side switch is ‘off’.
There is no diode emulation feature in this IC, and therefore, the inductor current may ramp in the negative
direction at light load. This causes the converter to operate in continuous conduction mode (CCM) regardless of
the output loading. The operating frequency remains relatively constant with load and line variations. The
operating frequency can be calculated as:
inductor current ramps up when the high side switch is ‘on’ and ramps down when the high side switch is ‘off’.
There is no diode emulation feature in this IC, and therefore, the inductor current may ramp in the negative
direction at light load. This causes the converter to operate in continuous conduction mode (CCM) regardless of
the output loading. The operating frequency remains relatively constant with load and line variations. The
operating frequency can be calculated as:
(1)
The output voltage (V
OUT
) is set by two external resistors (R
FB1
, R
FB2
). The regulated output voltage is calculated
as:
(2)
(3)
This regulator regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum
amount of ESR for the output capacitor (C
amount of ESR for the output capacitor (C
OUT
). A minimum of 25 mV of ripple voltage at the feedback pin (FB) is
required for the LM5019. In cases where the capacitor ESR is too small, additional series resistance may be
required (R
required (R
C
in
).
For applications where lower output voltage ripple is required the output can be taken directly from a low ESR
output capacitor, as shown in
output capacitor, as shown in
. However, R
C
slightly degrades the load regulation.
V
CC
Regulator
The LM5019 contains an internal high voltage linear regulator with a nominal output of 7.6 V. The input pin (V
IN
)
can be connected directly to the line voltages up to 100 V. The V
CC
regulator is internally current limited to 30
mA. The regulator sources current into the external capacitor at V
CC
. This regulator supplies current to internal
circuit blocks including the synchronous MOSFET driver and the logic circuits. When the voltage on the V
CC
pin
reaches the undervoltage lockout threshold of 4.5 V, the IC is enabled.
The V
CC
regulator contains an internal diode connection to the BST pin to replenish the charge in the gate drive
boot capacitor when SW pin is low.
8
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