LTC3816_15 Datasheet, PDF (19/44 Pages) Linear Technology – Single-Phase Wide VIN Range DC/DC Controller for Intel IMVP-6/IMVP-6.5 CPUs

LTC3816
APPLICATIONS INFORMATION
achieve accurate current sensing. Figure 4 shows a real
current sensing resistor, R_SENSE, which can be modeled
with an ideal resistance, R_SEN, in series with its parasitic
ESL. As shown in Figure 4, the voltage across the sense
resistor includes the voltage across the parasitic induc-
tor which is a strong function of inductor ripple current
and the switching frequency. This effectively reduces the
current limit threshold, typically by more than 30%. The
voltage across the sense resistor can be extracted from
a lowpass filter placed close to the controller input sense
pins as shown in Figure 4. The voltage across the sensing
capacitor, C_ISR, is:
V_CISR
=
I_L
• R_SEN




1+
1+
sESL
R_SEN
sR_ISR • C_ISR






In the frequency domain, the second term in the above
equation must be equal to 1 to ensure that the voltage
across the filter capacitor is independent of operating
frequency. To meet this requirement, the value of the RC
filter should fulfill the following condition:
R_ISR
•
C_ISR
=
ESL
R_SEN
The ESL value can be obtained from the manufacturer ’s
data sheet or estimated with an oscilloscope, as shown in
the Figure 4 waveform, using the following equation:
ESL = ^{VESL(ON) + VESL(OFF)}
∆I_L


1
t_ON
+
1
t_OFF


where t_ON is the TG on time and t_OFF is the TG off time.
For high efficiency applications, the inductor DCR provides
a method of sensing the inductor current without incurring
additional power loss from a sense resistor. The DCR of
the inductor represents the small amount of resistance
in the copper winding, which can be less than 1mΩ for
today ’s low value, high current inductors. Figure 5 shows
a simplified inductor model, which can be modeled with an
ideal inductor, L, in series with its parasitic DCR. The DCR
value can be obtained from the inductor manufacturer ’s
V_IN
LTC3816
I_L
TG
Q_T
L
ESL R_SEN
SW
BG
BSOURCE
Q_B
D
SENSE RESISTOR ₊
V_OUT
C_OUT
R_ISR
I_SENP
I_SENN
I_MAX
C_ISR
R_IMAX
V_ISR
V_ISR = V_RSEN + V_ESL
V_ESL(ON)
V_RSEN = I_L • R_SEN
V_ESL(OFF)
3816 F04
Figure 4. Current Limit Sensing Using
a Low Value Sense Resistor
LTC3816
TG
SW
BG
V_IN
Q_T
Q_B
D
BSOURCE
I_SENP
I_SENN
I_MAX
R_IDCR
C_IDCR
R_IMAX
I_L
L DCR
INDUCTOR
V_OUT
+
C_OUT
3916 F05
Figure 5. Current Limit Sensing Using Inductor DCR
data sheet. Similar to the sense resistor application circuit,
the voltage across the inductor DCR can be extracted from
a lowpass filter and the current limit threshold is given by
the following equation:
I_L(PEAK)
=
I_IMAX •R_IMAX
R_DCR
I_LOAD(MAX)
< I_LIMIT
= I_L(PEAK)
–
∆I_L
2
=
I_IMAX •R_IMAX
R_DCR
–
∆I_L
2
if
R_IDCR
• C_IDCR
=
L
R_DCR
3816f
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