Datasheet

LTC3838-1
34
38381f
For more information www.linear.com/3838-1
APPLICATIONS INFORMATION
related to the stability of the closed-loop system. However,
it is better to look at the filtered and compensated feedback
loop response at the ITH pin.
The gain of the loop increases with the R
ITH
and the band-
width of the loop increases with decreasing C
ITH1
. If R
ITH
is increased by the same factor that C
ITH1
is decreased,
the zero frequency
will be kept the same, thereby keeping
the phase the same in the most critical frequency range
of the feedback loop. In addition, a feedforward capacitor,
C
FF
, can be added to improve the high frequency response,
as used in the typical application at the last page of this
data sheet. Feedback capacitor C
FF
provides phase lead by
creating a high frequency zero with R
FB2
which improves
the phase margin.
A more severe transient can be caused by switching in
loads with large supply bypass capacitors. The discharged
bypass capacitors of the load
are effectively put in parallel
with the converter’s C
OUT
, causing a rapid drop in V
OUT
.
No regulator can deliver current quick enough to prevent
this sudden step change in output voltage, if the switch
connecting the C
OUT
to the load has low resistance and is
driven quickly. The solution is to limit the turn-on speed
of the load switch driver. Hot Swap™ controllers are de-
signed specifically for this purpose and usually incorporate
current limiting, short-circuit protection and soft
starting.
Load-Release Transient Detection
As the output voltage requirement of step-down switching
regulators becomes lower, V
IN
to V
OUT
step-down ratio
increases, and load transients become faster, a major
challenge is to limit the overshoot in V
OUT
during a fast
load current drop, or “load-release” transient.
Inductor current slew rate di
L
/dt = V
L
/L is proportional
to voltage across the inductor V
L
= V
SW
– V
OUT
. When
the top MOSFET is turned on, V
L
= V
IN
– V
OUT
, inductor
current ramps up. When bottom MOSFET turns on, V
L
=
V
SW
– V
OUT
= –V
OUT
, inductor current ramps down. At
very low V
OUT
, the low differential voltage, V
L
, across the
inductor during the ramp down makes the slew rate of the
inductor current much slower than needed to follow the
load
current change. The excess inductor current charges
up the output capacitor, which causes overshoot at V
OUT
.
If the bottom MOSFET could be turned off during the load-
release transient, the inductor current would flow through
the body diode of the bottom MOSFET, and the equation
can be modified to include the bottom MOSFET body
diode drop to become V
L
= –(V
OUT
+ V
BD
). Obviously the
benefit increases
as the output voltage gets lower, since
V
BD
would increase the sum significantly, compared to a
single V
OUT
only.
The load-release overshoot at V
OUT
causes the error ampli-
fier output, ITH, to drop quickly. ITH voltage is proportional
to the inductor current setpoint. A load transient will
result in a quick change of this load current setpoint, i.e.,
a negative spike of the first
derivative of the ITH voltage.
The LTC3838-1 uses a detect transient (DTR) pin to
monitor the first derivative of the ITH voltage, and detect
the load-release transient. Referring to the Functional
Diagram, the DTR pin is the input of a DTR comparator,
and the internal reference voltage for the DTR comparator
is half of INTV
CC
. To use this pin for transient detection,
ITH compensation needs
an additional R
ITH
resistor tied
to INTV
CC
, and connects the junction point of ITH com-
pensation components C
ITH1
, R
ITH1
and R
ITH2
to the DTR
pin as shown in the Functional Diagram. The DTR pin is
now proportional to the first derivative of the inductor
current setpoint, through the highpass filter of C
ITH1
and
(R
ITH1
//R
ITH2
).
The two R
ITH
resistors establish a voltage divider from
INTV
CC
to SGND, and bias the DC voltage on DTR pin (at
steady-state load or ITH voltage) slightly above half of
INTV
CC
. Compensation performance will be identical by
using the same C
ITH1
and make R
ITH1
//R
ITH2
equal the
R
ITH
as used in conventional single resistor OPTI-LOOP
compensation. This will also provide the R-C time
constant
needed for the DTR duration. The DTR sensitivity can be
adjusted by the DC bias voltage difference between DTR
and half INTV
CC
. This difference could be set as low as
200mV, as long as the ITH ripple voltage with DC load
current does not trigger the DTR.