LTC3865-1_15 Datasheet, PDF (17/38 Pages) Linear Technology – Dual, 2-Phase Synchronous DC/DC Controller with Pin Selectable Outputs

LTC3865/LTC3865-1
APPLICATIONS INFORMATION
always the same and varies with temperature; consult the
manufacturers’ data sheets for detailed information.
Using the inductor ripple current value from the Induc-
tor Value Calculation section, the target sense resistor
value is:
R_SENSE(EQUIV)
=
V_SENSE(MAX)
I_(MAX)
+
ΔI_L
2
To ensure that the application will deliver full load current
over the full operating temperature range, choose the
minimum value for the Maximum Current Sense Threshold
(V_SENSE(MAX)) in the Electrical Characteristics table (24mV,
44mV or 68mV, depending on the state of the I_LIM pin).
Next, determine the DCR of the inductor. Where provided,
use the manufacturer’s maximum value, usually given at
20°C. Increase this value to account for the temperature
coefficient of resistance, which is approximately 0.4%/°C.
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 2kΩ, reducing error
that might have been caused by the SENSE pins’ ±1μA
current.
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} ; R2 = ^{R1• RD}
R_D
1− R_D
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 ΔV_SENSE of 10mV to 15mV.
For a DCR sensing application, the actual ripple voltage
will be determined by the equation:
ΔV_SENSE
=
V_IN − V_OUT
R1• C1
V_OUT
V_IN • f_OSC
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant-
frequency architectures by preventing subharmonic oscil-
lations at high duty cycles. It is accomplished internally
by adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this
results in a reduction of maximum inductor peak current
for duty cycles >40%. However, the LTC3865/LTC3865-1
use a patented scheme that counteracts this compensating
ramp, which allows the maximum inductor peak current
to remain unaffected throughout all duty cycles.
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency f_OSC directly determine the
inductor’s peak-to-peak ripple current:
I_RIPPLE
=
V_OUT
V_IN
⎛
⎝⎜
V_IN – V_OUT
f_OSC •L
⎞
⎠⎟
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