MAX1513 Datasheet, PDF (19/28 Pages) Maxim Integrated Products – TFT-LCD Power-Supply Controllers

TFT-LCD Power-Supply Controllers
5) If V_SENSE is greater than 100mV, the current-feed-
back signal is too high and can trip the current limit
before the full load current is delivered. Use the cur-
rent-sense configuration in Figure 9 to attenuate the
sense signal. Define the scale factor (SF) as:
SF = ^100mV
V_SENSE
Calculate R_S1 and R_S2:
R_S1
=
R_S
SF
R_S2
=
R_S1 × SF
1 - SF
6) If V_SENSE is less than 80mV, the current-feedback
signal is low relative to the current-limit threshold. Use
the Figure 8 configuration, or, if good current-limit
accuracy is desired, use the optional current-sense
configuration in Figure 10 to increase the amplitude of
the sense signal. Calculate R_S3 and R_S4:
R_S3
=
V_MAIN - V_IN(MIN)
V_MAIN - V_IN(MIN) -100mV + V_SENSE
×
R_S
R_S4 = R_S - R_S3
If the 2.2µH inductor used in the typical operating cir-
cuit (Figures 1 and 2) had a typical DCR of 24mΩ and a
maximum DCR of 30mΩ, the RC time constant of the
sense network would be:
τ = ^2.2µH = 91.7µs
24mΩ
Select C_S = 0.1µF and calculate R_S:
R_S
=
91.7µs
0.1µF
=
917Ω
Assuming ∆T is 40°C and TC is 0.5%, the worst-case
high sense voltage over temperature is:
V_SENSE = 2.6A × 30mΩ (1 + 0.005 × 40°C) = 93.6mV
Because V_SENSE would be between 80mV and 100mV,
the circuit in Figure 8 should be used. The closest 1%
standard value for R_S is 909Ω.
If the 2.2µH inductor used in the typical operating cir-
cuit (Figures 1 and 2) has a typical DCR of 45mΩ and a
maximum DCR of 56mΩ, the RC time constant of the
sense network is:
τ = ^2.2µH = 48.9µs
45mΩ
Select C_S = 0.1µF and calculate R_S:
R_S
=
48.9µs
0.1µF
=
489Ω
Assuming ∆T is 40°C and TC is 0.5%, the worst-case
high sense voltage over temperature is:
L
V_IN
R_S
C_S
C_S+
C_S-
GATE
MAX1513
GND
MAX1514
FB
V_MAIN
Figure 8. Lossless Current Sensing with 80mV < V_SENSE < 100mV
L
V_IN
R_S1
R_S2
C_S
V_MAIN
C_S+
C_S-
GATE
MAX1513
MAX1514
GND
FB
Figure 9. Lossless Current Sensing with V_SENSE > 100mV
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