LTC3807_15 Datasheet, PDF (16/32 Pages) Linear Technology – Low IQ, Synchronous Step-Down Controller with 24V Output Voltage Capability

LTC3807
APPLICATIONS INFORMATION
Low Value Resistor Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 2a. R_SENSE is chosen based on the required
output current.
The current comparator has a maximum threshold
V_SENSE(MAX) determined by the I_LIM setting. The current
comparator threshold voltage sets the peak of the induc-
tor current, yielding a maximum average output current,
I_MAX, equal to the peak value less half the peak-to-peak
ripple current, ΔI_L. To calculate the sense resistor value,
use the equation:
R_SENSE
=
V_{SENSE(MAX )}
I_MAX
+
∆I_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 (30mV,
50mV or 75mV, depending on the state of the I_LIM pin).
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due
to the internal compensation required to meet stability
criterion for buck regulators operating at greater than
50% duty factor. A curve is provided in the Typical Perfor-
mance Characteristics section to estimate this reduction
in peak inductor current depending upon the operating
duty factor.
Inductor DCR Sensing
For applications requiring the highest possible efficiency
at high load currents, the LTC3807 is capable of sensing
the voltage drop across the inductor DCR, as shown in
Figure 2b. The DCR of the inductor represents the small
amount of DC resistance of the copper wire, which can be
less than 1mΩ for today’s low value, high current inductors.
In a high current application requiring such an inductor,
power loss through a sense resistor would cost several
points of efficiency compared to inductor 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 filter components, the
DCR of the inductor must be known. It can be measured
using a good RLC meter, but the DCR tolerance is not
always the same and varies with temperature; consult the
manufacturers’ data sheets for detailed information.
Using the inductor ripple current value from the Inductor
Value Calculation section, the target sense resistor
value is:
R_{SENSE(EQUIV )}
=
V_{SENSE(MAX )}
I_MAX
+
∆I_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 (30mV,
50mV or 75mV, depending on the state of the I_LIM pin).
Next, determine the DCR of the inductor. When provided,
use the manufacturer’s maximum value, usually given at
20°C. Increase this value to account for the temperature
coefficient of copper 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 (R_D), 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 0.1μF to 0.47μF.
This forces R1 || R2 to around 2k, reducing error that might
have been caused by the SENSE⁺ pin’s ±1μA current.
3807f
16
For more information www.linear.com/LTC3807