LTC3838-2_15 Datasheet, PDF (15/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

LTC3838-2
OPERATION (Refer to Functional Diagram)
Both channels adjust outputs through system feedback
loops so that the internally-generated single-ended feed-
back voltages (V_FB1,2 in the Functional Diagram) equal
either the internal 0.6V reference (for channel 1) or the
EXTV_REF2 = V_REF2+ (for channel 2) when in regulation.
Therefore, the differential V_OUT1 is regulated to 0.6V • (R_FB1
+ R_FB2)/R_FB1, and the differential V_OUT2 is regulated to
(V_REF2+ – V_REF2–) • (R_DFB1 + R_DFB2)/R_DFB1. Such schemes
eliminate any ground offsets between local ground and
remote output ground, resulting in a more accurate output
voltage. Channel 1 allows remote output ground to deviate
as much as ±500mV, and channel 2 allows as much as
±200mV, with respect to local ground (SGND).
Channel 2 senses the external reference voltage V_REF2 dif-
ferentially. The positive terminal of the external reference
V_REF2+ is fed into the EXTV_REF2 pin. The remote ground
of the external reference V_REF2– is connected to the third
resistor in the external output voltage feedback network.
The IC itself does not see the V_REF2– directly, but to keep
the regulation accuracy, the common mode voltage of the
external reference is practically limited by the voltages on
EXTV_REF2 pin as specified in the Electrical Characteristic
table, and the V_DFB2+ pin (≥ –0.2V), etc.
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 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 Electri-
cal Characteristics Table), then the internal 5.3V LDO is
enabled. If the EXTV_CC pin is tied to an external voltage
source greater than this EXTV_CC switchover voltage, then
the LDO is shut down and the internal EXTV_CC switch
shorts the EXTV_CC pin to the DRV_CC2 pin. In this case,
DRV_CC and INTV_CC draw power from the external voltage
source which helps to increase overall efficiency and de-
crease internal self heating from power dissipated in the
LDO. If the output voltage of the converter itself is above
the upper switchover voltage limit (4.8V), it can provide
power for EXTV_CC. The V_IN pin still needs to be powered
up but now draws minimum current.
Power for most internal control circuitry other than gate
drivers is derived from the INTV_CC pin. INTV_CC can be pow-
ered from the combined DRV_CC pins through an external
RC filter to SGND to filter out noises due to switching.
Shutdown and Start-Up
Each of the RUN1 and RUN2 pins has an internal proportion-
al-to-absolute-temperature (PTAT) current source (around
1.2µA at 25°C) to pull up the pins. Taking both RUN1 and
RUN2 pins below a certain threshold voltage (around 0.8V
at 25°C) shuts down all bias of INTV_CC and DRV_CC and
places the LTC3838-2 into micropower shutdown mode
with a minimum I_Q at the V_IN pin. The LTC3838-2’s DRV_CC
(through the internal 5.3V LDO regulator or EXTV_CC) and
the corresponding channel’s internal circuitry off INTV_CC
will be biased up when either or both RUN pins are pulled
up above the 0.8V threshold, either by the internal pull-up
current or driven directly by an external voltage source
such as a logic gate output.
A channel of the LTC3838-2 will not start switching until
the RUN pin of the respective channel is pulled up to 1.2V.
When a RUN pin rises above 1.2V, the corresponding
channel’s TG and BG drivers are enabled, and TRACK/
SS released. An additional 5µA temperature-independent
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