AN1025 datasheet(2/8 Pages) MICROCHIP

2 / 8 page

AN1025

DS01025A-page 2

FIGURE 1:

Basic LDO System Schematic.

Understanding LDO IGND Specifications

There are three current elements, IIN, IOUT and IGND,

labeled in Figure 1. IGND is the current used by the LDO

to perform the regulating operation and is often referred

to as the quiescent current (Iq) for no load conditions.

Since the specified Iq varies greatly depending on the

specific LDO or particular manufacture, it is important

to understand how this one specification impacts the

system performance.

An LDO can form a very efficient step-down regulator.

When the LDO output current is much greater than the

device quiescent current, the system efficiency is found

by dividing the output voltage by the input voltage. This

is shown in Equation 1.

EQUATION 1:

System efficiency at lighter load currents is one of the

impacts Iq has on the system performance. In basic

terms, an LDO with a low Iq will only be more efficient

at lighter loads. This is because as the load current

increases, the Iq is only a small percentage of the total

IIN. The efficiency of two Microchip LDOs, the

MCP1700 and TC1017, is shown in Figure 2. Notice

how the efficiency of the MCP1700 is vastly greater

than the TC1017 at light loads since the TC1017 has a

higher IQ.

FIGURE 2:

LDO Efficiency Comparison.

System line and load step performance is greatly

improved on LDOs that have higher Iq. Since the Iq is

used by the LDO to preform the regulating operation, it

can respond quicker to a sudden change in load

requirements or line voltage.

VREF

IGND

IOUT

IIN

CIN

COUT

VIN

Efficiency

VOUT

VIN

----------------

When: IGND << IOUT

0.01

0.10

1.00

10.00

100.00

Output Current (mA)

MCP1700

TC1017

VIN = 5.0V

VOUT = 3.0V

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