LTC3839 Datasheet, PDF (12/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
OPERATION (Refer to Functional Diagram)
Main Control Loop
The LTC3839 is a controlled on-time, valley current mode
step-down DC/DC single-output controller with two chan-
nels operating out of phase. Each channel drives both main
and synchronous N-channel MOSFETs.
The top MOSFET is turned on for a time interval determined
by a one-shot timer. The duration of the one-shot timer is
controlled to maintain a fixed switching frequency. As the
top MOSFET is turned off, the bottom MOSFET is turned
on after a small delay. The delay, or dead time, is to avoid
both top and bottom MOSFETs being on at the same time,
causing shoot-through current from V_IN directly to power
ground. The next switching cycle is initiated when the cur-
rent comparator, I_CMP, senses that inductor current falls
below the trip level set by voltages at the ITH and V_RNG
pins. The bottom MOSFET is turned off immediately and
the top MOSFET on again, restarting the one-shot timer
and repeating the cycle. In order to avoid shoot-through
current, there is also a small dead-time delay before the
top MOSFET turns on. At this moment, the inductor cur-
rent hits its “valley” and starts to rise again.
Inductor current is determined by sensing the voltage
between SENSE⁺ and SENSE^–, either by using an explicit
resistor connected in series with the inductor or by implic-
itly sensing the inductor’s DC resistive (DCR) voltage drop
through an RC filter connected across the inductor. The
trip level of the current comparator, I_CMP, is proportional
to the voltage at the ITH pin, with a zero-current threshold
corresponding to an ITH voltage of around 0.8V.
The error amplifier (EA) adjusts this ITH voltage by com-
paring the feedback signal to the internal 0.6V reference
voltage. The difference amplifier (DIFFAMP) converts the
differential feedback signal (V_OUTSENSE+ – V_{OUTSENSE–}) to
a single-ended input for the EA. Output voltage is regu-
lated so that the feedback voltage is equal to the internal
0.6V reference. If the load current increases/decreases, it
causes a momentary drop/rise in the differential feedback
voltage relative to the reference. The EA then moves ITH
voltage, or inductor valley current setpoint, higher/lower
until the average inductor current again matches the load
current, so that the output voltage comes back to the
regulated voltage.
The LTC3839 features a detect transient (DTR) pin to detect
“load-release”, or a transient where the load current sud-
denly drops, by monitoring the first derivative of the ITH
voltage. When detected, the bottom gate (BG) is turned
off and inductor current flows through the body diode
in the bottom MOSFET, allowing the SW node voltage to
drop below PGND by the body diode’s forward-conduction
voltage. This creates a more negative differential voltage
(V_SW – V_OUT) across the inductor, allowing the inductor
current to drop faster to zero, thus creating less overshoot
on V_OUT. See Load-Release Transient Detection in Applica-
tions Information for details.
Differential Output Sensing
The LT3839 features differential output voltage sensing. The
output voltage is resistively divided externally to create a
feedback voltage for the controller. The internal difference
amplifier (DIFFAMP) senses this feedback voltage with
respect to the output’s remote ground reference to create a
differential feedback voltage. This scheme eliminates any
ground offsets between local ground and remote output
ground, resulting in a more accurate output voltage. It al-
lows remote output ground to deviate as much as ±500mV
with respect to local ground (SGND).
DRV_CC/EXTV_CC/INTV_CC Power
DRV_CC1,2 are the power for the bottom MOSFET drivers.
Normally the two DRV_CC pins are shorted together on
the PCB, and decoupled to PGND with a minimum 4.7μF
ceramic capacitor, C_DRVCC. The top MOSFET drivers are
biased from the floating bootstrap capacitors (C_B1,2)
which are recharged during each cycle through an external
Schottky diode when the top MOSFET turns off and the
SW pin swings down.
The DRV_CC can be powered on two ways: an internal low-
dropout (LDO) linear voltage regulator that is powered
from V_IN and can output 5.3V to DRV_CC1. Alternatively,
an internal EXTV_CC switch (with on-resistance of around
2Ω) can short the EXTV_CC pin to DRV_CC2.
If the EXTV_CC pin is below the EXTV_CC switchover voltage
(typically 4.6V with 200mV hysteresis, see the Electrical
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