10

REV A.02

www.3peakic.com.cn

LMV321TP/LMV358TP/LMV324TP

40μA, 1.27MHz, Micro-Power Rail-to-Rail I/O Op Amps

10k

Ω

5pF

10k

Ω

Figure 2

Driving Large Capacitive Load

The LMV321TP/358TP/324TP of OPA is designed to drive large capacitive loads. Refer to Typical Performance

Characteristics for “Phase Margin vs. Load Capacitance”. As always, larger load capacitance decreases overall

phase margin in a feedback system where internal frequency compensation is utilized.

As the load capacitance

increases, the feedback loop’s phase margin decreases, and the closed-loop bandwidth is reduced. This

produces gain peaking in the frequency response, with overshoot and ringing in output step response. The

unity-gain buffer (G = +1V/V) is the most sensitive to large capacitive loads.

When driving large capacitive loads with the LMV321TP/358TP/324TP (e.g., > 200 pF when G = +1V/V), a small

) improves the feedback loop’s phase margin and stability by making

the output load resistive at higher frequencies.

Figure 3

Power Supply Layout and Bypass

The LMV321TP/358TP/324TP OPA

capacitor (i.e., 0.01

μF to 0.1μF) within 2mm for good high frequency performance. It can also use a bulk capacitor

(i.e., 1μF or larger) within 100mm to provide large, slow currents. This bulk capacitor can be shared with other

analog parts.

Ground layout improves performance by decreasing the amount of stray capacitance and noise at the OPA

’s

inputs and outputs. To decrease stray capacitance, minimize PC board lengths and resistor leads, and place

external components

as close to the op amps’ pins as possible.

Proper Board Layout

To ensure optimum performance at the PCB level, care must be taken in the design of the board layout. To avoid

leakage currents, the surface of the board should be kept clean and free of moisture. Coating the surface creates

a barrier to moisture accumulation and helps reduce parasitic resistance on the board.

Keeping supply traces short and properly bypassing the power supplies minimizes power supply disturbances

due to output current variation, such as when driving an ac signal into a heavy load. Bypass capacitors should be

connected as closely as possible to the device supply pins. Stray capacitances are a concern at the outputs and

the inputs of the amplifier. It is recommended that signal traces be kept at least 5mm from supply lines to minimize

coupling.