LM36010 Datasheet, PDF (23/37 Pages) Texas Instruments – Synchronous-Boost, Single-LED Flash Driver With 1.5-A High-Side Current Source

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LM36010
SNVSAN4A – APRIL 2017 – REVISED JULY 2017
8.2.2 Detailed Design Procedure
8.2.2.1 Thermal Performance
Output power is limited by three things: the peak current limit, the ambient temperature, and the maximum power
dissipation in the package. If the die temperature of the device is below the absolute maximum rating of 125°C,
the maximum output power can be over 6 W. However, any appreciable output current causes the internal power
dissipation to increase and therefore increase the die temperature. Any circuit configuration must ensure that the
die temperature remains below 125°C taking into account the ambient temperature derating. The thermal scale-
back protection (TSB) helps ensure that temperature requirement is held valid. If the TSB feature is disabled,
thermal shutdown (TSD) is the next level of protection for the device, which is set to 150°C. This mechanism
cannot be disabled, and operation of the device above 125°C is not ensured by the electrical specification.
In boost mode, where V_IN < V_LED + V_HR, the power dissipation can be approximated by Equation 1:
ªª§
P_DISS | ««¬««¬¨¨©
V_OUT
V_IN
V_IN²
u
V_OUT
·
¸¸¹
u I_LED²
º
u
R_NFET
»
»¼
ª§
«¬¨©
V_OUT
V_IN
·
¸¹
u I_LED²
u R_PFET
º
»
¼
º
V_HR u I_LED
»
»¼
(1)
When the device is in pass mode, where V_IN > V_LED + V_HR, the power dissipation equals:
P_DISS
ª¬ª¬ _{VIN VLED u ILED º¼}
I_LED² u R_INDUCTOR
º
¼
(2)
Use Equation 3 to calculate the junction temperature (T_J) of the device:
T_J R_TJA u P_DISS
(3)
Note that these equations only provide approximation of the junction temperature and do not take into account
thermal time constants, which play a large role in determining maximum deliverable output power and flash
durations.
8.2.2.2 Output Capacitor Selection
The LM36010 is designed to operate with a 10-µF ceramic output capacitor. When the boost converter is
running, the output capacitor supplies the load current during the boost converter on-time. When the NMOS
switch turns off, the inductor energy is discharged through the internal PMOS switch, supplying power to the load
and restoring charge to the output capacitor. This causes a sag in the output voltage during the on-time and a
rise in the output voltage during the off-time. Therefore, choose the output capacitor to limit the output ripple to
an acceptable level depending on load current and input or output voltage differentials and also to ensure the
converter remains stable.
Larger capacitors such as a 22-µF or capacitors in parallel can be used if lower output voltage ripple is desired.
To estimate the output voltage ripple considering the ripple due to capacitor discharge (ΔV_Q) and the ripple due
to the capacitors ESR (ΔV_ESR), use Equation 4 and Equation 5:
For continuous conduction mode, the output voltage ripple due to the capacitor discharge is:
' V_Q
I_LED u V_OUT V_IN
f_SW u V_OUT u C_OUT
(4)
The output voltage ripple due to the output capacitors ESR is found by:
' V_ESR
R_ESR
u
§
¨¨©
§
¨
©
I_LED
u V_OUT
V_IN
·
¸
¹
·
'_{IL ¸¸¹'IL}
V_IN u V_OUT V_IN
2 u f_SW u L u V_OUT
(5)
In ceramic capacitors, the ESR is very low so the assumption is that 80% of the output voltage ripple is due to
capacitor discharge and 20% from ESR. Table 2 lists different manufacturers for various output capacitors and
their case sizes suitable for use with the LM36010.
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