LT3797
20
3797fa
For more information www.linear.com/LT3797
applicaTions inForMaTion
Switch Current Sense Resistors Selection
The LT3797 measures each channel’s power N-channel
MOSFET current by using a sense resistor (see R
SW_SEN
in
Figure 1) between GND and the MOSFET source. Figure 7
shows a typical waveform of the sense voltage (V
SW_SENSE
)
across the sense resistor in CCM. The placement of the
sense resistor R
SW_SEN
should be close to the source of the
MOSFET and GND. The SENSEP and SENSEN sense node
traces should run parallel to each other to a Kelvin con
-
nection on the positive and negative terminals of R
SW_SEN
.
Due to the current limit function of the power switch cur-
rent control, R
SW_SEN
should be selected to guarantee that
the peak current sense voltage V
SW_SENSE(PEAK)
during
steady-state normal operation is lower than the SENSE
current limit threshold (100mV minimum). It is recom
-
mended to give a 20% margin and set V
SW_SENSE(PEAK)
to be 80mV. Then, the switch current sense resistor value
can be calculated as:
R
SW _SEN
=
I
SW(PEAK)
where I
SW(PEAK)
is the peak switch current. I
SW(PEAK)
of
the boost, buck mode and buck-boost mode LED driver is:
I
SW(PEAK)
= I
L(PEAK)
I
SW(PEAK)
of the SEPIC LED driver is:
I
SW(PEAK)
= I
L1(PEAK)
+ I
L2(PEAK)
Sense Voltage Ripple Verification
After the inductor ripple current and the switch current
sense resistor value have been selected according to the
previous sections, the sense voltage ripple ∆V
SW_SENSE
(refer to Figure 7) of the boost, buck, and buck-boost LED
drivers can be determined using the following equation:
∆V
SW_SENSE
= ∆I
L
•R
SW_SEN
∆V
SW_SENSE
of the SEPIC LED driver can be determined
using the following equation:
∆V
SW_SENSE
=2•∆I
L
•R
SW_SEN
The LT3797 has internal slope compensation to stabilize
the control loop against sub-harmonic oscillation. When
the LT3797 operates at a duty cycle greater than 0.66
in CCM, the sense voltage ripple, ∆V
SW_SENSE
(refer to
Figure7), needs to be limited to ensure the internal slope
compensation is sufficient to stabilize the control loop.
Figure 8 shows the maximum ∆V
SW_SENSE
over the duty
cycle. It is recommended to check and ensure ∆V
SW_SENSE
is below this curve at the highest duty cycle. If ∆V
SW_SENSE
is above the maximum ∆V
SW_SENSE
curve at the highest
duty cycle, the ∆I
L
needs to be reduced and the parameters
in the previous two sections need to be recalculated until
the optimized values are obtained.
Figure 7. The Sense Voltage Across the Sense Resistor in CCM
∆V
SW_SENSE
V
SW_SENSE(PEAK)
t
V
SW_SENSE
D/f
3797 F07
1/f
Figure 8. The Maximum Sense Voltage Ripple
vs Duty Cycle for CCM
DUTY CYCLE
0.5
MAX ∆V
SW_SENSE
(mV)
30
90
100
110
3797 F08
10
70
50
20
80
0
60
40
0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95
