LMH6644-MIL Datasheet, PDF (24/28 Pages) Texas Instruments – Low-Power, 130-MHz, 75-mA Rail-to-Rail Output Amplifier

LMH6644-MIL
SNOSD65 – JUNE 2017
www.ti.com
Typical Application (continued)
9.2.2 Detailed Design Procedure
No matter how low an Rf is selected, there is a need for C_f in order to stabilize the circuit. The reason for this is
that the op amp input capacitance and Q1 equivalent collector capacitance together (C_IN) will cause additional
phase shift to the signal fed back to the inverting node. C_f will function as a zero in the feedback path
counteracting the effect of the C_IN and acting to stabilized the circuit. By proper selection of C_f such that the Op
Amp open loop gain is equal to the inverse of the feedback factor at that frequency, the response is optimized
with a theoretical 45° phase margin where GBWP is the Gain Bandwidth Product of the Op Amp, optimized as
such, the I-V converter will have a theoretical pole, fp, at: (2) With Op Amp input capacitance of 3pF and an
estimate for Q1 output capacitance of about 3 pF as well, C_IN = 6 pF. From the typical performance plots, GBWP
is approximately 57 MHz. Therefore, with Rf = 1k, from Equation 2 and Equation 3 : C_f = ∼4.1 pF and fp = 39
MHz.
For this example, optimum C_f was empirically determined to be around 5 pF. This time domain response is
shown in Figure 58 showing about 9 ns rise/fall times, corresponding to about 39 MHz for fp. The overall supply
current from the +5-V supply is around 5 mA with no load.
9.2.3 Application Curves
Input
Output _V+
V_IN (1 V/DIV)
V_S = ±2.5V
V^-
A_V = +1
1V/DIV
200 ns/DIV
Figure 56. Input and Output Shown with CMVR Exceeded
V_S=±5V, V_IN=5V_PP
A_V=+5, R_F=R_L=2k
V_OUT (2 V/DIV)
2 V/DIV
100 ns/DIV
Figure 57. Overload Recovery Waveform
200 mV/DIV
20 ns/DIV
Figure 58. Converter Step Response (1 V_PP, 20 ns/DIV)
10 Power Supply Recommendations
The LMH6644-MIL device can operate off a single supply or with dual supplies. The input CM capability of the
parts (CMVR) extends all the way down to the V– rail to simplify single supply applications. Supplies should be
decoupled with low inductance, often ceramic, capacitors to ground less than 0.5 inches from the device pins.
The use of ground plane is recommended, and as in most high speed devices, it is advisable to remove ground
plane close to device sensitive pins such as the inputs.
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