LTC3788-1_1 LINER [Linear Technology], LTC3788-1_1 Datasheet - Page 15

• Manufacturer Part Number: LTC3788-1_1
• Description: 2-Phase, Dual Output Synchronous Boost Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Sense Resistor Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 2a. R
output current.
The current comparator has a maximum threshold
V
peak of the inductor current, yielding a maximum average
output current, I
peak-to-peak ripple current, ΔI
resistor value, use the equation:
When using the controller in low V
output applications, the maximum output current level will
be reduced due to the internal compensation required to
meet stability criterion for boost regulators operating at
greater than 50% duty factor. A curve is provided in the
Typical Performance Characteristics section to estimate
this reduction in peak output current level depending upon
the operating duty factor.
Inductor DCR Sensing
For applications requiring the highest possible effi ciency
at high load currents, the LTC3788-1 is capable of sensing
the voltage drop across the inductor DCR, as shown in
Figure 2b. The DCR of the inductor can be less than 1mΩ
for high current inductors. In a high current application
requiring such an inductor, conduction loss through a
sense resistor could reduce the effi ciency by a few percent
compared to DCR sensing.
If the external R1||R2 • C1 time constant is chosen to be
exactly equal to the L/DCR time constant, the voltage drop
across the external capacitor is equal to the drop across
the inductor DCR multiplied by R2/(R1 + R2). R2 scales the
voltage across the sense terminals for applications where
the DCR is greater than the target sense resistor value.
To properly dimension the external fi lter components, the
DCR of the inductor must be known. It can be measured
using a good RLC meter, but the DCR tolerance is not
SENSE(MAX)
R
SENSE
=
. The current comparator threshold sets the
V
I
MAX
SENSE MAX
SENSE
MAX
, equal to the peak value less half the
+
(
is chosen based on the required
Δ
2
I
L
)
L
. To calculate the sense
IN
and very high voltage
always the same and varies with temperature. Consult the
manufacturer’s data sheets for detailed information.
Using the inductor ripple current value from the inductor
value calculation section, the target sense resistor value
is:
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
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
coeffi cient of resistance, which is approximately 0.4%/°C. A
conservative value for the maximum inductor temperature
(T
To scale the maximum inductor DCR to the desired sense
resistor value, use the divider ratio:
C1 is usually selected to be in the range of 0.1μF to 0.47μF .
This forces R1|| R2 to around 2k, reducing error that might
have been caused by the SENSE
The equivalent resistance R1|| R2 is scaled to the room
temperature inductance and maximum DCR:
The sense resistor values are:
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at V
L(MAX)
SENSE(MAX)
R
R
R
R
P
LOSS
1
1
D
SENSE EQUIV
||
=
=
R
R
) is 100°C.
DCR
2
1
R
R
(
=
||
1
R
D
R
=
).
(
SENSE EQUIV
MAX
DCR at
2
(
V
;
)
OUT
=
at T
R
(
2
V
I
−
MAX
SENSE MAX
L MAX
L
=
20
R
(
V
R R
1
1
IN
°
)
1
C C
−
+
) •
•
) •
(
)
R
Δ
D
V
2
D
I
IN
L
1
+
)
pin’s ±1μA current.
LTC3788-1
IN
= 1/2 V
OUT
15
37881fa
: