LM3687 Datasheet, PDF (26/35 Pages) National Semiconductor (TI) – Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode

LM3687
SNVS473A – DECEMBER 2007 – REVISED JULY 2008
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POWER DISSIPATION AND DEVICE OPERATION
The permissible power dissipation for any package is a measure of the capability of the device to pass heat from
the power source, the junctions of the IC, to the ultimate heat sink, the ambient environment. Thus the power
dissipation is dependent on the ambient temperature and the thermal resistance across the various interfaces
between the die and ambient air.
As stated in ⁽¹⁾ in the electrical specification section, the allowable power dissipation for the device in a given
package can be calculated using the equation:
P_{D_SYS} = (T_J(MAX) - T_A) / θ_JA
For the LM3687 there are two different main sources contributing to the systems power dissipation (P_{D_SYS}): the
DC-DC converter (P_{D_DCDC}) and the linear regulator (P_{D_LIN}). Neglecting switching losses and quiescent currents
these two main contributors can be estimated by the following equations:
• P_{D_LIN} = (V_{IN_LIN} - V_{OUT_LIN}) * I_{OUT_LIN}
• P_{D_DCDC} = I_{OUT_DCDC2} * [(R_DSON(P) * D) + (R_DSON(N) * (1-D))]
with duty cycle D = V_{OUT_DCDC} / V_BATT.
As an example, assuming the typical post regulation application, the conversion from V_BATT = 3.6V to V_{OUT_DCDC}
= 1.8V and further to V_{OUT_LIN} = 1.5V, at maximum load currents, results in following power dissipations:
P_{D_DCDC} = (0.75A)² * (0.38Ω * 1.8V / 3.6V + 0.25Ω * (1 - 1.8V / 3.6V)) = 177mW and
P_{D_LIN} = (1.8V - 1.5V) * 0.35A = 105mW.
P_{D_SYS} = 282mW.
With a θ_JA = 70°C/W for the micro SMD 9 package this P_{D_SYS} will cause a rise of the junction temperature T_J of:
ΔT_J = P_{D_SYS} * θ_JA = 20K.
For the same conditions but the linear regulator biased from V_BATT, this results in a P_{D_LIN} of 735mW, P_{D_DCDC} =
50mW (because I_{OUT_DCDC} = 400mA) and therefore an increase of T_J of 55K.
As lower total power dissipation translates to higher efficiency this example highlights the advantage of the post
regulation setup.
NO-LOAD STABILITY
Both outputs of the LM3687 will remain stable and in regulation with no external load. This is an important
consideration in some circuits, for example CMOS RAM keep-alive applications.
ENABLE OPERATION
The outputs of LM3687 may be switched ON or OFF by a logic input at the Enable pins, V_{EN_DCDC} and V_{EN_LIN}. A
logic high (related to V_BATT) at these pins will turn the outputs on (for information on startup sequence please
refer to 'Operation Description').
When both enable pins are low, the outputs are off (pins SW and V_{OUT_LIN} are high impedance) and the device
typically consumes 0.1µA.
If the application does not require the Enable switching feature, the enable pins should be tied to V_BATT to keep
the outputs permanently on.
(1) In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may
have to be derated. Maximum ambient temperature (T_A-MAX) is dependent on the maximum operating junction temperature (T_J-MAX-OP =
125°C), the maximum power dissipation of the device in the application (P_D-MAX), and the junction-to ambient thermal resistance of the
part/package in the application (θ_JA), as given by the following equation: T_A-MAX = T_J-MAX-OP – (θ_JA × P_D-MAX).
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