LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 36

• Manufacturer Part Number: LTC3855
• Description: Dual, Fast, Accurate Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3838
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
To use this pin for transient detection, ITH compensation
needs an additional R
nects the junction point of ITH compensation components
C
Functional Diagram. The DTR pin is now proportional to
the first derivative of the inductor current setpoint, through
the highpass filter of C
The two R
INTV
steady-state load or ITH voltage) slightly above half of
INTV
using the same C
R
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
200mV, as long as the ITH ripple voltage with DC load
current does not trigger the DTR.
36
ITH1
ITH
CC
CC
as used in conventional single resistor OPTI-LOOP
, R
. Compensation performance will be identical by
to SGND, and bias the DC voltage on DTR pin (at
ITH1
10A/DIV
5V/DIV
5V/DIV
1V/DIV
RELEASE, TURNS OFF BG
ITH
FOR FASTER INDUCTOR
DTR
SW
CURRENT (I
BG
I
DTR DETECTS LOAD
and R
L
CC
resistors establish a voltage divider from
Figure 12. Comparison of V
. This difference could be set as low as
ITH1
ITH2
L
) DECAY
ITH
ITH1
and make R
to the DTR pin as shown in the
resistor tied to INTV
and (R
(12a) DTR Enabled
BG TURNS BACK ON, INDUCTOR
CURRENT (I
5μs/DIV
ITH1
ITH1
LOAD RELEASE = 15A TO 0A
V
V
//R
IN
OUT
OUT
//R
L
= 5V
) GOES NEGATIVE
ITH2
= 0.6V
ITH2
Overshoot with Detect Transient (DTR) Feature Enabled and Disabled
CC
).
, and con-
equal the
CC
.
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
As illustrated in Figure 12, when load current suddenly
drops, V
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
(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
inductor, allowing the inductor current to drop at a faster
rate to zero, therefore creating less overshoot on V
The DTR comparator output is overridden by reverse
inductor current detection (I
condition. This means BG will be turned off when SENSE
is higher than SENSE
10A/DIV
5V/DIV
5V/DIV
1V/DIV
V
SW
ITH
DTR
BG
CC
I
L
LOAD RELEASE = 15A TO 0A
V
V
, a load release event is detected. The bottom gate
IN
OUT
OUT
– 0.5V
= 5V
= 0.6V
overshoots, and ITH drops quickly. The voltage
INTVCC
(12b) DTR Disabled
BG REMAINS ON
DURING THE LOAD
RELEASE EVENT
–
=
(i.e., inductor current is positive),
⎡
⎢
⎣
+ 2.5µA • R
5μs/DIV
(R
ITH1
REV
R
ITH1
SW
+R
) and overvoltage (OV)
(
ITH2
– V
ITH1
)
OUT
/ /R
– 0.5
CC
) across the
3838 F12
:
ITH2
⎤
⎥ • 5.3V
⎦
)
OUT
3838fa
.
+