LTC3882_15 Datasheet, PDF (48/104 Pages) Linear Technology – Dual Output PolyPhase Step-Down DC/DC Voltage Mode Controller with Digital Power System Management

LTC3882
APPLICATIONS INFORMATION
chosen to be exactly equal to the L/DCR time constant of
the inductor, the voltage drop across the external capaci-
tor is equal to the voltage drop across the inductor DCR.
Check the manufacturer’s data sheet for specifications
regarding the inductor DCR in order to properly dimension
the external filter components. The DCR of the inductor
can also be measured using a good RLC meter.
Use the nominal or measured value of DCR to program
IOUT_CAL_GAIN (in mΩ). The temperature coefficient
of the inductor’s DCR is typically high, like copper. Again,
consult the manufacturer’s data sheet. The LTC3882 can
adjust for this non-ideality if the correct MFR_IOUT_CAL_
GAIN_TC value is programmed. Typically this coefficient
is around 3900ppm/°C.
Resistor R1 should be placed close to the switch node,
to prevent noise from coupling into sensitive small-signal
nodes. Capacitor C1 should be placed close to the IC pins.
An example of discrete resistor sensing of output current is
shown in Figure 26. Previously, the parasitic inductance of
the sense resistor could represent a relatively small error.
New high current density solutions may utilize low sense
resistor values producing sense voltages less than 20mV.
In addition, inductor ripple currents greater than 50%
with operation up to 1MHz are becoming more common.
Under these conditions, the voltage drop across the sense
resistor’s parasitic inductance is no longer negligible. An
RC filter can be used to extract the resistive component
of the current sense signal in the presence of parasitic
48
VINSNS
5V
LTC3882
V_CC
V_DD33
PWM
GND ^ISENSE– ^ISENSE+
V_CC BOOST
V_LOGIC
TG
LTC4449
IN
TS
BG
GND
C1*
V_IN
12V
INDUCTOR
L
DCR
R1*
V_OUT
R1 • C1 =
L
DCR
*PLACE R1 NEAR INDUCTOR
PLACE C1 NEAR I_SENSE+, I_SENSE– PINS
3882 F25
Figure 25. Inductor DCR Output Current Sense
VINSNS
5V
LTC3882
V_CC
V_DD33
PWM
GND ^ISENSE– ^ISENSE+
V_CC BOOST
V_LOGIC
TG
LTC4449
IN
TS
BG
GND
C_F
R_F
R_F
V_IN
12V
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
L
R_S ESL
C_F • 2R_F ≤ ESL/R_S
POLE-ZERO
CANCELLATION
V_OUT
3882 F26
FILTER COMPONENTS PLACED NEAR SENSE PINS
Figure 26. Discrete Resistor Output Current Sense
For more information www.linear.com/LTC3882
3882f