LED2001PHR Datasheet, PDF (28/42 Pages) STMicroelectronics – 4 A monolithic step-down current source with synchronous rectification

Application information
LED2001
V_IN=12 V, V_{FW_LED}=3.5 V, n_LED=2, I_LED=700 mA
The typical output voltage is:
Equation 36
V_OUT = n_LED ⋅ V_{FW_LED} + V_FB = 7.1V
R_{DSON_HS} has a typical value of 95 mΩ and R_{DSON_LS} is 69 mΩ @ 25 °C.
For the calculation we can estimate R_{DSON_HS} = 140 mΩ and R_{DSON_LS}= 100 mΩ as a
consequence of Tj increase during the operation.
T_{SW_EQ} is approximately 12 ns.
I_Q has a typical value of 1.5 mA @ V_IN = 12 V.
The overall losses are:
Equation 37
P_TOT = R_{DSON_HS} ⋅ (I_OUT)² ⋅ D + R_{DSON_LS} ⋅ (I_OUT)² ⋅ (1 – D) + V_IN ⋅ I_OUT ⋅ f_SW ⋅ T_SW + V_IN ⋅ I_Q
2
Equation 38
P_TOT = 0.14 ⋅ 0.7² ⋅ 0.6 + 0.1 ⋅ 0.7² ⋅ 0.4 + 12 ⋅ 0.7 ⋅ 12 ⋅ 10^–9 ⋅ 850 ⋅ 10³ + 12 ⋅ 1.5 ⋅ 10^–3 ≅ 205mW
The junction temperature of the device is:
Equation 39
T_J = T_A + Rth_{J – A} ⋅ P_TOT
where T_A is the ambient temperature and Rth_J-A is the thermal resistance junction-to-
ambient. The junction-to-ambient (Rth_J-A) thermal resistance of the device assembled in the
HSO8 package and mounted on the board is about 40 °C/W.
Assuming the ambient temperature is around 40 °C, the estimated junction temperature is:
T_J = 60 + 0.205 ⋅ 40 ≅ 68°C
7.4
Short-circuit protection
In overcurrent protection mode, when the peak current reaches the current limit threshold,
the device disables the power element and it is able to reduce the conduction time down to
the minimum value (approximately 100 nsec typical) to keep the inductor current limited.
This is the pulse-by-pulse current limitation to implement the constant current protection
feature.
In overcurrent condition, the duty cycle is strongly reduced and, in most applications, this is
enough to limit the switch current to the current threshold.
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