LTC3829 Datasheet, PDF (18/40 Pages) Linear Technology – 3-Phase, Single Output Synchronous Step-Down DC/DC Controller with Diffamp

LTC3829
Applications Information
A conservative value for T_L(MAX) is 100°C. To scale the
maximum inductor DCR to the desired sense resistor
value, use the divider ratio:
R_D
=
R_SENSE(EQUIV)
DCR_(MAX) at T_L(MAX)
C1 is usually selected to be in the range of 0.047µF to
0.47µF. This forces R1|| R2 to around 2k, reducing error
that might have been caused by the SENSE⁺ pins’ ±1µA
current. T_L(MAX) is the maximum inductor temperature.
The equivalent resistance R1|| R2 is scaled to the room
temperature inductance and maximum DCR:
R1||R2 =
L
(DCR at 20°C) • C1
The sense resistor values are:
R1
=
R1|| R2
R_D
;
R2
=
R1• R_D
1− R_D
The LTC3829 also features a DCR temperature compen-
sation circuit by using a NTC temperature sensor. See
the Inductor DCR Sensing Temperature Compensation
section for details.
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
( ) _{PLOSS R1=}
V_IN(MAX) − V_OUT
R1
• V_OUT
Ensure that R1 has a power rating higher than this value.
If high efficiency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
sense resistors. Light load power loss can be modestly
higher with a DCR network than with a sense resistor, due
to the extra switching losses incurred through R1. However,
DCR sensing eliminates a sense resistor, reduces conduc-
tion losses and provides higher efficiency at heavy loads.
Peak efficiency is about the same with either method. To
maintain a good signal-to-noise ratio for the current sense
signal, use a minimum ∆V_SENSE of 10mV for duty cycles
18
less than 40%. For a DCR sensing application, the actual
ripple voltage will be determined by the equation:
∆V_SENSE
=
V_IN − V_OUT
R1• C1
V_OUT
V_IN • f_OSC
Inductor DCR Sensing Temperature Compensation
and the ITEMP Pin
Inductor DCR current sensing provides a lossless method
of sensing the instantaneous current. Therefore, it can
provide higher efficiency for applications of high output
currents. However, the DCR of the inductor, which is the
small amount of DC winding resistance of the copper,
typically has a positive temperature coefficient. As the
temperature of the inductor rises, its DCR value increases.
The current limit of the controller is therefore reduced.
The LTC3829 offers a method to counter this inaccuracy
by allowing the user to place an NTC temperature sensing
resistor near the inductor to actively correct this error. The
ITEMP pin, when left floating, is at a voltage around 5V and
DCR temperature compensation is disabled. The ITEMP
pin has a constant 10µA precision current flowing out the
pin. By connecting an NTC resistor from the ITEMP pin to
SGND, the maximum current sense threshold can be varied
over temperature according the following equation:
V_{SENSEMAX(ADJ)}
=
V_SENSE(MAX)
•
1.8
– V_ITEMP
1.3
where:
V_{SENSEMAX(ADJ)} is the maximum adjusted current sense
threshold.
V_SENSE(MAX) is the maximum current sense threshold
specified in the Electrical Characteristics table. It is
typically 75mV, 50mV or 30mV depending on the set-
ting I_LIM pins.
V_ITEMP is the voltage of the ITEMP pin.
The valid voltage range for DCR temperature compensa-
tion on the ITEMP pin is between 0.5V to 0.2V, with 0.5V
or above being no DCR temperature correction and 0.2V
the maximum correction. However, if the duty cycle of the
controller is less than 25%, the ITEMP range is extended
from 0.5V to 0V.
3829f