LMH6629_1011 Datasheet, PDF (17/28 Pages) National Semiconductor (TI) – Ultra-Low Noise, High-Speed Operational Amplifier with Shutdown

Application Section
INTRODUCTION
The LMH6629 is a high gain bandwidth, ultra low-noise volt-
age feedback operational amplifier. The excellent noise and
bandwidth enables applications such as medical diagnostic
ultrasound, magnetic tape & disk storage and fiberoptics to
achieve maximum high frequency signal-to-noise ratios. The
following discussion will enable the proper selection of exter-
nal components to achieve optimum system performance.
The LMH6629 (LLP-8 package only) has some additional
features to allow maximum flexibility. As shown in Figure 2
there are provisions for low-power shutdown and two internal
compensation settings, which are further discussed below
under the COMPENSATION heading. Also provided is a
feedback (FB) pin which allows the placement of the feedback
resistor directly adjacent to the inverting input (IN-) pin. This
pin simplifies printed circuit board layout and minimizes the
possibility of unwanted interaction between the feedback path
and other circuit elements.
30068061
FIGURE 2. 8-Pin LLP Pinout Diagram
The LLP-8 package requires the bottom-side Die Attach Pad-
dle (DAP) to be soldered to the circuit board for proper thermal
dissipation and to get the thermal resistance number speci-
fied. The DAP is tied to the V^- potential within the LMH6629
package. Thus, the circuit board copper area devoted to DAP
heatsinking connection should be at the V- potential as well.
Please refer to the package drawing for the recommended
land pattern and recommended DAP connection dimensions.
30068052
FIGURE 3. LLP–8 DAP(Top View)
LLP-8 CONTROL PINS & SOT23-5 COMPARISON
The LMH6629 LLP-8 package has two digital control pins;
PD and COMP pins. The PD pin, used for powerdown, floats
high (device on) when not driven. When the PD pin is pulled
low, the amplifier is disabled and the amplifier output stage
goes into a high impedance state so the feedback and gain
set resistors determine the output impedance of the circuit.
The other control pin, the COMP pin, allows control of the
internal compensation and defaults to the lower gain mode or
logic 0.
The SOT23-5 package has the following differences relative
to the LLP-8 package:
1. No power down (shutdown) capability.
2. No COMP pin to set the minimum stable gain. SOT23–5
package minimum stable gain is internally fixed to be
10V/V.
3. No feedback (FB) pin.
From a performance point of view, the LLP-8 and the
SOT23-5 packages perform very similarly except in the fol-
lowing areas:
1. SSBW, Peaking, and 0.1 dB Bandwidth: These
differences are highlighted in the Typical Performance
Characteristics section and the Electrical Characteristics
tables. Most notable differences are with small signal (0.2
Vpp) and close to the minimum stable gain of 10V/V.
2. Distortion: It is possible to get slightly different distortion
performance. The board layout, decoupling capacitor
return current routing strongly influences this
3. Output Current: In heavy current applications, there will
be differences between these package types because of
the difference in their respective Thermal Resistances
(θ_JA).
COMPENSATION
The LMH6629 has two compensation settings that can be
controlled by the COMP pin (LLP-8 package only). The de-
fault setting is set through an internal pull down resistor and
places the COMP pin at the logic 0 state. In this configuration
the on-chip compensation is set to the maximum and band-
width is reduced to enable stability at gains as low as 4V/V.
When this pin is driven to the logic 1 state, the internal com-
pensation is decreased to allow higher bandwidth at higher
gains. In this state, the minimum stable gain is 10V/V. Due to
the reduced compensation, slew rate and large signal band-
width are significantly enhanced for the higher gains.
As mentioned earlier, the SOT23-5 package does not offer
the two compensation settings that the LLP-8 offers. The
SOT23-5 is internally set for a minimum gain of 10 V/V.
It is possible to externally compensate the LMH6629 for any
of the following reasons, as shown in Figure 4:
• To operate the SOT23-5 package (which does not offer
the COMP pin) at closed loop gains < 10V/V.
• To operate the LLP-8 package at gains below the
minimum stable gain of 4V /V when the COMP pin is LO.
Note: In this case, Figure 4 “Constraint 1” may be changed
to ≥ 4 V/V instead of ≥ 10 V/V.
• To operate either package at low gain and need maximum
slew rate (COMP pin HI).
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