5A Low-Voltage Low-Dropout Regulator
Application Information
LM39500/39501/39502
The LM39500/1 is a high-performance low-dropout voltage regulator suitable for moderate to high-current voltage
regu-lator applications. Its 400mV dropout voltage at full load makes it especially valuable in battery-powered
systems and as a high-efficiency noise filter in post-regulator applications. Unlike older NPN-pass transistor
designs, where the mini-mum dropout voltage is limited by the base-to-emitter voltage drop and collector-to-
emitter saturation voltage, dropout per-formance of the PNP output of these devices is limited only by the low V
CE
saturation voltage.A trade-off for the low dropout voltage is a varying base drive requirement.
The LM39500/1/2 regulator is fully protected from damage due to fault conditions. Current limiting is provided. This
limiting is linear output current during overload conditions is constant. Thermal shutdown disables the device when
the die temperature exceeds the maximum safe operating tem-perature. Transient protection allows device (and
load) sur-vival even when the input voltage spikes above and below nominal. The output structure of these regulators
allows voltages in excess of the desired output voltage to be applied without reverse current flow.
Thermal Design
Linear regulators are simple to use. The most complicated design parameters to consider are thermal
characteristics.Thermal design requires four application-specific param-eters:
•Maximum ambient temperature (T
A
)
•Output Current (I
OUT
)
•Output Voltage (V
OUT
)
•Input Voltage (V
IN
)
•Ground Current (I
GND
)
Calculate the power dissipation of the regulator from these numbers and the device parameters from this
datasheet,where the ground current is taken from the data sheet.
PD = (V
IN
– V
OUT
) I
OUT
+ V
IN
·I
GND
The heat sink thermal resistance is determined by:
θ
SA=
(T
JMAX
-T
A
)/P
D
-(θ
JC
+θ
CS
)
where TJ (max) 125 ℃ and
θ
CS
is between 0℃ and 2℃/W.
The heat sink may be significantly reduced in applications where the minimum input voltage is known and is large
compared with the dropout voltage. Use a series input resistor to drop excessive voltage and distribute the heat
between this resistor and the regulator. The low dropout properties of Taejin regulators allow signifi-cant reductions
in regulator power dissipation and the asso-ciated heat sink without compromising performance. When this
technique is employed, a capacitor of at least 1.0F is needed directly between the input and regulator ground.Refer
to Application Note 9 for further details and examples on thermal design and heat sink specification.
Output Capacitor
The LM39500/1/2 requires an output capacitor to maintain stability and improve transient response. Proper capacitor
selection is important to ensure proper operation. The LM39500/1/2 output capacitor selection is dependent upon
the ESR (equivalent series resistance) of the output capacitor to maintain stability. When the output capacitor is 47F
or greater, the output capacitor should have less than 1 of ESR. This will improve transient response as well as
promote stability. Ultralow ESR capacitors, such as ceramic chip capacitors may promote instability. These very low
ESR levels may cause an oscillation and/or underdamped tran-sient response. A low-ESR solid tantalum capacitor
works extremely well and provides good transient response and stability over temperature. Aluminum electrolytics can
also be used, as long as the ESR of the capacitor is < 1.The value of the output capacitor can be increased without
limit. Higher capacitance values help to improve transient response and ripple rejection and reduce output noise.
Input Capacitor
An input capacitor of 1uF or greater is recommended when the device is more than 4 inches away from the bulk ac
supply capacitance, or when the supply is a battery. Small, surface-mount, ceramic chip capacitors can be used for
the bypass-ing. Larger values will help to improve ripple rejection by bypassing the input to the regulator, further
improving the integrity of the output voltage.Transient Response and 3.3V.
Fig 1. Capacitor Requirements
HTC
5
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
