LTC3856_15 Datasheet, PDF (25/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Applications Information
error amplifier. The amplifier has an output swing range
of 0V to 3.6V. The output uses an NPN emitter follower
with 80k feedback resistance.
Active Voltage Positioning (AVP)
In an application, the AVP scheme modifies the regulated
output voltage depending on its current loading. AVP
can improve overall transient response and save power
consumption.
The LTC3856 senses inductor current information by moni-
toring voltage drops across the sense resistors R_SENSE or
the DCR sensing network of the two channels. The voltage
drops are added together and applied as V_PRE-AVP between
the AVP and DIFFP pins, which are connected through
resistor R_{PRE-AVP }. Then V_PRE-AVP is scaled through R_AVP
and added to output voltage as the compensation for the
load voltage drop.
Let:
∆V = V_SENSE1+ – V_SENSE1–
∆V = V_SENSE2+ – V_SENSE2–
then:
∆V_DIFFP,VOUT
=
2
•

∆V _
R_AVP
R_PRE-AVP


The final load slope is defined by the inductor current
sense resistors and the two external resistors previously
mentioned.
In summary, the load slope is:


R_SENSE
•
R_AVP
R_PRE-AVP


V/A
The recommended value for R_AVP is 90Ω to 100Ω. The
maximum output voltage at AVP is 2.5V. Therefore, for
outputs higher than 2.5V, the AVP function is not supported.
The DIFFP pin, however, should always be connected to the
output even when AVP or diffamp functions are not used.
Programmable Stage Shedding Mode
When the MODE pin is tied to INTV_CC, the LTC3856 en-
ters Stage Shedding mode. This means that the second
channel will stop switching when I_TH is below a certain
programmed threshold. This threshold voltage on I_TH is
programmed according to the following formula:
( ) ^V_SHED
=
0.5
+


5
3 
•
0.5 − V_ISET
The valid range of V_ISET is between 0V to 0.5V, where V_ISET
is the voltage on the ISET pin. There is a precision 7.5µA
flowing out of the ISET pin. Connecting a resistor to SGND
sets the V_ISET voltage. When left floating, V_ISET voltage will
be at INTV_CC. The Stage Shedding mode threshold voltage
in this case will be 0.5V. There is a 50mV hysteresis for
the Stage Shedding mode threshold comparator.
Programmable Burst Mode Operation
When the MODE pin is floating, the LTC3856 enters Burst
Mode operation. This means that both channels will stop
switching when I_TH is below a certain threshold.
The Burst Mode clamp, which sets the current limit when
bursting, can be programmed through V_ISET according to
the following formula:
V_CLAMP = 0.7 + 0.62 (0.5 – V_ISET)
The valid range of V_ISET is between 0.3V to 0.5V and
V_ISET is the voltage on the ISET pin. There is a precision
7.5µA flowing out of ISET. Connecting a resistor to SGND
sets the V_ISET voltage. When left floating, V_ISET will be at
INTV_CC. The Burst Mode clamp voltage in this case will
be 0.7V. There is a 50mV hysteresis for the Burst Mode
comparator.
3856f
25