ZLDO1117

Issue 1

7 of 12

June 2009

www.diodes.com

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 42~46°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

TO-252 and SOT223 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 (VOUT/VREF). 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.