LP38691_15 Datasheet, PDF (15/34 Pages) Texas Instruments – LP3869x/-Q1 500-mA Low-Dropout CMOS Linear Regulators Stable With Ceramic Output Capacitors

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LP38691, LP38693, LP38691-Q1, LP38693-Q1
SNVS321N – JANUARY 2005 – REVISED MARCH 2015
9.2.1 Design Requirements
Table 1. Design Parameters
DESIGN PARAMETERS
Input voltage range
Output range
Output current
Output capacitor range
Input and output capacitor ESR range
EXAMPLE VALUE
2.7 V to 10 V
1.8 V
1A
1 µF
5 mΩ to 500 mΩ
9.2.2 Detailed Design Procedure
9.2.2.1 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 junction and ambient air.
The maximum allowable power dissipation for the device in a given package can be calculated using Equation 1:
P_D-MAX = ((T_J-MAX – T_A) / R_θJA)
(1)
The actual power being dissipated in the device can be represented by Equation 2:
P_D = (V_IN – V_OUT) x I_OUT
(2)
These two equations establish the relationship between the maximum power dissipation allowed due to thermal
consideration, the voltage drop across the device, and the continuous current capability of the device. These two
equations should be used to determine the optimum operating conditions for the device in the application.
In applications where lower power dissipation (P_D) and/or excellent package thermal resistance (R_θJA) is present,
the maximum ambient temperature (T_A-MAX) may be increased.
In applications where high power dissipation and/or poor package thermal resistance is present, the maximum
ambient temperature (T_A-MAX) may have to be derated. T_A-MAX is dependent on the maximum operating junction
temperature (T_J-MAX-OP = 125°C), the maximum allowable power dissipation in the device package in the
application (P_D-MAX), and the junction-to ambient thermal resistance of the part/package in the application (R_θJA),
as given by Equation 3:
T_A-MAX = (T_J-MAX-OP – (R_θJA × P_D-MAX))
(3)
Alternately, if T_A-MAX can not be derated, the P_D value must be reduced. This can be accomplished by reducing
V_IN in the 'V_IN – V_OUT' term as long as the minimum V_IN is met, or by reducing the I_OUT term, or by some
combination of the two.
9.2.2.2 External Capacitors
In common with most regulators, the LP38691 and LP38693 require an external capacitors for regulator stability.
The devices are specifically designed for portable applications requiring minimum board space and smallest
components. These capacitors must be correctly selected for good performance.
9.2.2.3 Input Capacitor
An input capacitor is required for stability. It is recommended that a 1-µF capacitor be connected between the
devices' IN pin and GND pin (this capacitance value may be increased without limit).
This capacitor must be located a distance of not more than 1 cm from the IN pin and returned to a clean
analogue ground. Any good quality ceramic, tantalum, or film capacitor may be used at the input.
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Product Folder Links: LP38691 LP38693 LP38691-Q1 LP38693-Q1