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LT3800EFE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3800
10
3800fc
APPLICATIONS INFORMATION
Burst Mode
The LT3800 employs low current Burst Mode functional-
ity to maximize effi ciency during no load and low load
conditions. Burst Mode operation is enabled by shorting
the BURST_EN pin to SGND. Burst Mode operation can
be disabled by shorting BURST_EN to either V
FB
or V
CC
.
When the required switch current, sensed via the V
C
pin voltage, is below 15% of maximum, the Burst Mode
operation is employed and that level of sense current is
latched onto the IC control path. If the output load requires
less than this latched current level, the converter will
overdrive the output slightly during each switch cycle.
This overdrive condition is sensed internally and forces
the voltage on the V
C
pin to continue to drop. When the
voltage on V
C
drops 150mV below the 15% load level,
switching is disabled and the LT3800 shuts down most
of its internal circuitry, reducing total quiescent current
to 100µA. When the converter output begins to fall, the
V
C
pin voltage begins to climb. When the voltage on the
V
C
pin climbs back to the 15% load level, the IC returns
to normal operation and switching resumes. An internal
clamp on the V
C
pin is set at 100mV below the switch
disable threshold, which limits the negative excursion of
the pin voltage, minimizing the converter output ripple
during Burst Mode operation.
During Burst Mode operation, V
IN
pin current is 20µA
and V
CC
current is reduced to 80µA. If no external drive is
provided for V
CC
, all V
CC
bias currents originate from the
V
IN
pin, giving a total V
IN
current of 100µA. Burst current
can be reduced further when V
CC
is driven using an output
derived source, as the V
CC
component of V
IN
current is
then reduced by the converter buck ratio.
Reverse-Current Inhibit
The LT3800 contains a reverse-current inhibit feature to
maximize effi ciency during light load conditions. This
mode of operation allows discontinuous operation, and
is sometimes referred to as “pulse-skipping” mode. Refer
to Figure 2.
This feature is enabled with Burst Mode operation, and can
also be enabled while Burst Mode operation is disabled
by shorting the BURST_EN pin to V
FB
.
When reverse-current inhibit is enabled, the LT3800 sense
amplifi er detects inductor currents approaching zero and
disables the synchronous switch for the remainder of
the switch cycle. If the inductor current is allowed to go
negative before the synchronous switch is disabled, the
switch node could inductively kick positive with a high
dv/dt. The LT3800 prevents this by incorporating a 10mV
positive offset at the sense inputs.
Figure 2. Inductor Current vs Mode
PULSE SKIP MODE
I
L
I
L
FORCED CONTINUOUS
DECREASING
LOAD
CURRENT
3800 F02
LT3800
11
3800fc
APPLICATIONS INFORMATION
With the reverse-current inhibit feature enabled, an LT3800
converter will operate much like a nonsynchronous
converter during light loads. Reverse-current inhibit
reduces resistive losses associated with inductor ripple
currents, which improves operating effi ciencies during
light-load conditions.
An LT3800 DC/DC converter that is operating in reverse-
inhibit mode has a minimum load requirement of 1mA
(BURST_EN = V
FB
). Since most applications use output-
generated power for the LT3800, this requirement is met
by the bias currents of the IC, however, for applications
that do not derive power from the output, this require-
ment is easily accomplished by using a 1.2k resistor
connected from V
FB
to ground as one of the converter
output voltage programming resistors (R1). There are no
minimum load restrictions when in Burst Mode operation
(BURST_EN < 0.5V) or continuous conduction mode
(BURST_EN > 2.5V).
Soft-Start
The LT3800 incorporates a programmable soft-start
function to control start-up surge currents, limit output
overshoot and for use in supply sequencing. The soft-start
function directly monitors and controls output voltage slew
rate during converter start-up.
As the output voltage of the converter rises, the soft-start
circuit monitors δV/δt current through a coupling capaci-
tor and adjusts the voltage on the V
C
pin to maintain an
average value of 2µA. The soft-start function forces the
programmed slew rate while the converter output rises to
95% regulation, which corresponds to 1.185V on the V
FB
pin. Once 95% regulation is achieved, the soft-start circuit
is disabled. The soft-start circuit will re-enable when the
V
FB
pin drops below 70% regulation, which corresponds to
300mV of control hysteresis on the V
FB
pin, which allows
for a controlled recovery from a ‘brown-out’ condition.
The desired soft-start rise time (t
SS
) is programmed via
a programming capacitor C
SS1
, using a value that cor-
responds to 2µA average current during the soft-start
interval. This capacitor value follows the relation:
C
SS1
=
2E
–6
•t
SS
V
OUT
R
SS
is typically set to 200k for most applications.
Considerations for Low Voltage Output Applications
The LT3800 C
SS
pin biases to 220mV during the soft-start
cycle, and this voltage is increased at network node “A” by
the 2µA signal current through R
SS
, so the output has to
reach this value before the soft-start function is engaged.
The value of this output soft-start start-up voltage offset
(V
OUT(SS)
) follows the relation:
V
OUT(SS)
= 220mV + R
SS
• 2E
–6
which is typically 0.64V for R
SS
= 200k.
In some low voltage output applications, it may be desir-
able to reduce the value of this soft-start start-up voltage
offset. This is possible by reducing the value of R
SS
. With
reduced values of R
SS
, the signal component caused by
voltage ripple on the output must be minimized for proper
soft-start operation.
Peak-to-peak output voltage ripple (∆V
OUT
) will be imposed
on node “A” through the capacitor C
SS1
. The value of R
SS
can be set using the following equation:
R
SS
=
V
OUT
1.3E
–6
It is important to use low ESR output capacitors for LT3800
voltage converter designs to minimize this ripple voltage
component. A design with an excessive ripple component
can be evidenced by observing the V
C
pin during the start
cycle.
C
SS
V
OUT
C
SS1
R
SS
A
3800 AI06
LT3800
LT3800
12
3800fc
APPLICATIONS INFORMATION
The soft-start cycle should be evaluated to verify that the
reduced R
SS
value allows operation without excessive
modulation of the V
C
pin before fi nalizing the design.
If the V
C
pin has an excessive ripple component during the
soft-start cycle, converter output ripple should be reduced
or R
SS
increased. Reduction in converter output ripple is
typically accomplished by increasing output capacitance
and/or reducing output capacitor ESR.
External Current Limit Foldback Circuit
An additional start-up voltage offset can occur during the
period before the LT3800 soft-start circuit becomes active.
Before the soft-start circuit throttles back the V
C
pin in
response to the rising output voltage, current as high as
the peak programmed current limit (I
MAX
) can fl ow in the
inductor. Switching will stop once the soft-start circuit takes
hold and reduces the voltage on the V
C
pin, but the output
voltage will continue to increase as the stored energy in
the inductor is transferred to the output capacitor. With
I
MAX
fl owing in the inductor, the resulting leading-edge
rise on V
OUT
due to energy stored in the inductor follows
the relationship:
V
OUT
= I
MAX
•
L
C
OUT
1/ 2
Inductor current typically doesn’t reach I
MAX
in the few
cycles that occur before soft-start becomes active, but can
with high input voltages or small inductors, so the above
relation is useful as a worst-case scenario.
This energy transfer increase in output voltage is typically
small, but for some low voltage applications with relatively
small output capacitors, it can become signifi cant. The volt-
age rise can be reduced by increasing output capacitance,
which puts additional limitations on C
OUT
for these low
voltage supplies. Another approach is to add an external
current limit foldback circuit which reduces the value of
I
MAX
during start-up.
An external current limit foldback circuit can be easily
incorporated into an LT3800 DC/DC converter application
by placing a 1N4148 diode and a 47k resistor from the
converter output (V
OUT
) to the LT3800’s V
C
pin. This limits
the peak current to 0.25 • I
MAX
when V
OUT
= 0V. A current
limit foldback circuit also has the added advantage of
providing a reduced output current in the DC/DC converter
during short-circuit fault conditions, so a foldback circuit
may be useful even if the soft-start function is disabled.
If the soft-start circuit is disabled by shorting the C
SS
pin
to ground, the external current limit fold-back circuit must
be modifi ed by adding an additional diode and resistor.
The 2-diode, 2-resistor network shown also provides
0.25 • I
MAX
when V
OUT
= 0V.
Soft-Start Characteristic Showing Excessive Ripple Component Desirable Soft-Start Characteristic
V
OUT
V
OUT(SS)
V(V
C
)
250µs/DIV
3800 AI07
V
OUT
V
OUT(SS)
V(V
C
)
250µs/DIV
3800 AI08
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LT3800EFE#TRPBF

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Buy LT3800EFE#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators High-Voltage Synch Step-Down Controller
Lifecycle:
New from this manufacturer.
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