Datasheet

LTC3838
36
3838fa
APPLICATIONS INFORMATION
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 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 con-
nects the junction point of ITH compensation 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.
Note the internal 2.5µA pull-up current from the DTR pin
will generate an additional offset on top of the resistor
divider itself, making the total difference between the DC
bias voltage on the DTR pin and half INTV
CC
:
V
DTR
– 0.5V
INTVCC
=
R
ITH1
(R
ITH1
+R
ITH2
)
– 0.5
⎡
⎣
⎢
⎤
⎦
⎥
•5.3V
+2.5µA • R
ITH1
//R
ITH2
()
As illustrated in Figure 12, when load current suddenly
drops, V
OUT
overshoots, and ITH drops quickly. The voltage
on the DTR pin will also drop quickly, since it is coupled
to the ITH pin through a capacitor. If the load transient
is fast enough that the DTR voltage drops below half of
INTV
CC
, a load release event is detected. The bottom gate
(BG) will be turned off, so that the inductor current flows
through the body diode in the bottom MOSFET. This al-
lows the SW node to drop below PGND by a voltage of
a forward-conducted silicon diode. This creates a more
negative differential voltage (V
SW
– V
OUT
) across the
inductor, allowing the inductor current to drop at a faster
rate to zero, therefore creating less overshoot on V
OUT
.
The DTR comparator output is overridden by reverse
inductor current detection (I
REV
) and overvoltage (OV)
condition. This means BG will be turned off when SENSE
+
is higher than SENSE
–
(i.e., inductor current is positive),
Figure 12. Comparison of V
OUT
Overshoot with Detect Transient (DTR) Feature Enabled and Disabled
(12a) DTR Enabled
3838 F12
(12b) DTR Disabled
BG
5V/DIV
DTR
1V/DIV
I
L
10A/DIV
DTR DETECTS LOAD
RELEASE, TURNS OFF BG
FOR FASTER INDUCTOR
CURRENT (I
L
) DECAY
5µs/DIV
LOAD RELEASE = 15A TO 0A
V
IN
= 5V
V
OUT
= 0.6V
BG TURNS BACK ON, INDUCTOR
CURRENT (I
L
) GOES NEGATIVE
SW
5V/DIV
SW
5V/DIV
BG
5V/DIV
ITH
1V/DIV
I
L
10A/DIV
5µs/DIV
LOAD RELEASE = 15A TO 0A
V
IN
= 5V
V
OUT
= 0.6V
BG REMAINS ON
DURING THE LOAD
RELEASE EVENT