Evaluation Board User Guide
UG-439
Rev. A | Page 7 of 13
Measuring Dropout Voltage of LDO Regulators
Dropout voltage is defined as the input-to-output voltage differen-
tial when the input voltage is set to the nominal output voltage.
One way to measure dropout voltage is to obtain the output voltage
(V
OUT
nominal) with V
IN
initially set to V
OUT
n 0.5 V; set
the output load to 100 µA. Then, force the input voltage equal to
V
OUT
nominal, and measure the output voltage accordingly (V
OUT
dropout). Dropout voltage is then calculated as V
OUT
nominal −
V
OUT
dropout. This calculation applies only for output voltages
greater than 1.7 V.
Dropout voltage increases with larger loads. For more accurate
measurements, use a second voltmeter to monitor the input
voltage across the input capacitor. The input supply voltage may
need to be adjusted to account for IR drops, especially if large
load currents are used.
Cascading an LDO Regulator from the Buck Regulator
For certain applications such as analog circuit supplies, the
LDO regulators are preferred over the buck regulators because
of improved noise performance. When none of the buck
regulator outputs are being used, the input supply of the LDO
regulators can be taken from these outputs. An example
evaluation board connection is shown in Figure 15, wherein
VOUT1 is tied to VIN3, which is the supply of LDO1, by shunting
Jumper JV3V1. Subsequently, VIN4, which is the supply of
LDO2 can also be connected to VOUT2 through Jumper JV4V2.
In this configuration, the output voltage of the buck regulator has
enough headroom with the desired output voltage of the LDO
regulator to guarantee that the LDO regulator operates within
specifications.
Figure 15. Cascading an LDO Regulator from the Buck Regulator
+
–
VOLTAGE
SOURCE
+
–
1.99711
VOLTMETER
10895-
012
