7 Detailed Description
SNVS037G – SEPTEMBER 1999 – REVISED JANUARY 2015
A basic functional diagram for the LM1084-ADJ (excluding protection circuitry) is shown in Figure 11. The
topology is basically that of the LM317 except for the pass transistor. Instead of a Darlington NPN with its two
diode voltage drop, the LM1084 uses a single NPN. This results in a lower dropout voltage. The structure of the
pass transistor is also known as a quasi LDO. The advantage of a quasi LDO over a PNP LDO is its inherently
lower quiescent current. The LM1084 is ensured to provide a minimum dropout voltage of 1.5-V over
temperature, at full load.
Figure 11. Basic Functional Diagram for the LM1084, Excluding Protection Circuitry
7.2 Functional Block Diagram
7.3 Feature Description
7.3.1 Ripple Rejection
Ripple rejection is a function of the open loop gain within the feed-back loop (refer to Figure 11 and Figure 14).
The LM1084 exhibits 75dB of ripple rejection (typ.). When adjusted for voltages higher than VREF, the ripple
rejection decreases as a function of adjustment gain: (1+R1/R2) or VO/VREF. Therefore a 5-V adjustment
decreases ripple rejection by a factor of four (−12dB); Output ripple increases as adjustment voltage increases.
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