Diodes ZLDO1117 - Manual (Page 7)

ZLDO1117
Document number: DS32018 Rev. 6 - 2
7 of 14
www.diodes.com
July 2012
© Diodes Incorporated
ZLDO1117
A Product Line of
Diodes Incorporated
Application Information
The ZLDO1117 family of quasi-LDO regulators is easy to use. They are protected against short circuit and thermal overloads. (see block
diagram).
Thermal protection circuitry will shut down the regulator should the junction temperature exceed +150°C at the sense point. The ZLDO1117 is
pin compatible with similar ‘1117 regulators and offers extended temperature range and improved regulation specifications.
Operation
The ZLDO1117 develops a 1.25V reference voltage between the output and the adjust terminal (see block diagram). By placing a resistor
between these two terminals, a constant current is caused to flow through R1 and down through R2. For fixed output variants Resistors R1 and
R2 are internal.
Stability
The ZLDO1117 requires an output capacitor as part of the device frequency compensation. As part of its improved performance over industry
standard 1117 the ZLDO1117 is suitable for use with MLCC (Multi Layer Ceramic Chip) capacitors. A minimum of 4.7µF ceramic X7R, 4.7µF
tantalum, or 47 µF of aluminum electrolytic is required. The ESR of the output capacitor should be less than 0.5 Ω. Surface mount tantalum
capacitors, which have very low ESR, are available from several manufacturers. When using MLCC capacitors avoid the use of Y5V dielectrics.
Load Regulation
For improved load regulation the ZLDO1117-ADJ should have the upper feedback resistor, R
1
, connected as close as possible to V
OUT
and the
lower resistor, R2, connected as close as possible to the load GND return. This helps reduce any parasitic resistance in series with the load.
Thermal Considerations
ZLDO1117 series regulators have internal thermal limiting circuitry designed to protect the device during overload conditions. For continuous
normal load conditions however, the maximum junction temperature rating of +125°C must not be exceeded.
It is important to give careful consideration to all sources of thermal resistance from junction to ambient. For the SOT223-3L and TO252-3L
packages, which are designed to be surface mounted, additional heat sources mounted near the device must also be considered. Heat sinking is
accomplished using the heat spreading capability of the PCB and its copper traces. The θ
JC
(junction to tab)of the TO252-3L and SOT223-3L are
+12°C/W and +16°C/W respectively.
Thermal resistances from tab to ambient can be as low as +30°C/W. The total thermal resistance from junction to ambient can be as low as
+42 to +46°C/W. This requires a reasonable sized PCB with at least one layer of copper to spread the heat across the board and couple it into
the surrounding air. Datasheet specifications using 2 oz copper and a 5mmx5mm pad with T
A
= +27°C, no air flow yielded
θ
JA
(junction to tab) of
+73°C/W and +107°C/W for TO252-3L and SOT223-3L respectively.
The thermal resistance for each application will be affected by thermal interactions with other components on the board. Some experimentation
will be necessary to determine the actual value.
Ripple Rejection
When using the ZLDO1117 adjustable device the adjust terminal can be bypassed to improve ripple rejection. When the adjust terminal is
bypassed the required value of the output capacitor increases.
The device will require an output capacitor of 22µF tantalum or 150µF aluminum electrolytic when the adjust pin is bypassed. Normally, capacitor
values on the order of 100µF are used in the output of many regulators to ensure good load transient response with large load current changes.
Output capacitance can be increased without limit and larger values of output capacitance further improve stability and transient response.
The curves for Ripple Rejection were generated using an adjustable device with the adjust pin bypassed. These curves will hold true for all
values of output voltage. For proper bypassing, and ripple rejection approaching the values shown, the impedance of the adjust pin capacitor, at
the ripple frequency, should be < R1. R1 is normally in the range of 100 Ω to 200Ω. The size of the required adjust pin capacitor is a function of
the input ripple frequency. At 120Hz, with R1 = 100 Ω, the adjust pin capacitor should be >13µF. At 10kHz only 0.16µF is needed.
For fixed voltage devices, and adjustable devices without an adjust pin capacitor, the output ripple will increase as the ratio of the output voltage
to the reference voltage (V
OUT
/V
REF
). For example, with the output voltage equal to 5V, the output ripple will be increased by the ratio of 5V/1.25V.
It will increase by a factor of four. Ripple rejection will be degraded by 12dB from the value shown on the curve.