LTC3839 Datasheet, PDF (33/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
APPLICATIONS INFORMATION
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 LTC3839 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 5μA pull-up
current from the DTR pin will generate an additional offset
on top of the resistor divider itself, making the total dif-
ference between the DC bias voltage on the DTR pin and
half INTV_CC:
V_DTR(DC) – 0.5 • V_INTVCC = [R_ITH1/(R_ITH1 + R_ITH2) – 0.5]
• 5.3V + 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.
SW
5V/DIV
BG
5V/DIV
DTR
1V/DIV
I_L
10A/DIV
BG TURNS BACK ON, INDUCTOR
CURRENT (I_L) GOES NEGATIVE
SW
5V/DIV
BG
5V/DIV
ITH
1V/DIV
I_L
10A/DIV
BG REMAINS ON
DURING THE LOAD
RELEASE EVENT
DTR DETECTS LOAD
RELEASE, TURNS OFF BG
FOR FASTER INDUCTOR
CURRENT (I_L) DECAY
5μs/DIV
V_IN = 5V
V_OUT = 0.6V
(12a) DTR Enabled
V_IN = 5V
V_OUT = 0.6V
5μs/DIV
(12b) DTR Disabled
Figure 12. Comparison of Detect Transient Load-Release (DTR) Feature Enabled and Disabled
3839 F12
3839fa
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