LM2642MTC/NOPB National Semiconductor, LM2642MTC/NOPB Datasheet - Page 19

• Manufacturer Part Number: LM2642MTC/NOPB
• Description: IC CTRLR SW SYNC STPDN 28TSSOP
• Manufacturer: National Semiconductor

Specifications of LM2642MTC/NOPB

• Applications: Embedded systems, Console/Set-Top boxes
• Current - Supply: 1mA
• Voltage - Supply: 4.5 V ~ 30 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP
• Dc To Dc Converter Type: Synchronous Step Down Controller
• Number Of Outputs: 2
• Pin Count: 28
• Input Voltage: 4.5 to 30V
• Output Voltage: 1.3 to 30V
• Output Current: 20A
• Package Type: TSSOP
• Mounting: Surface Mount
• Operating Temperature Classification: Automotive
• Operating Temperature (min): -40C
• Operating Temperature (max): 125C
• Primary Input Voltage: 30V
• No. Of Outputs: 1
• No. Of Pins: 28
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 3 - 168 Hours
• Control Mode: Current
• Rohs Compliant: Yes
• For Use With: LM2642REVD EVAL - BOARD EVALUATION LM2642
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM2642MTC; *LM2642MTC/NOPB; LM2642MTC

Loop Compensation
As shown in Figure 10, the control-output transfer function
consists of one pole (fp), one zero (fz), and a double pole at
fn (half the switching frequency). The following can be done
to create a -20dB /decade roll-off of the loop gain: Place the
first pole at 0Hz, the first zero at fp, the second pole at fz,
and the second zero at fn. The resulting output-control trans-
fer function is shown in Figure 11.
The control-output corner frequencies, and thus the desired
compensation corner frequencies, can be determined ap-
proximately by the following equations:
Since fp is determined by the output network, it will shift with
loading (Ro) and duty cycle. First determine the range of
frequencies (fpmin/max) of the pole across the expected
load range, then place the first compensation zero within that
range.
Example: R
50Ω, R
FIGURE 10. Control-Output Transfer Function
FIGURE 11. Output-Control Transfer Function
omin
= 5V/3A = 1.7Ω:
e
= 20mΩ, C
o
= 100µF, R
(Continued)
omax
= 5V/100mA =
20046214
20046212
19
Once the fp range is determined, R
using:
Where B is the desired gain in V/V at fp (fz1), gm is the
transconductance of the error amplifier, and R1 and R2 are
the feedback resistors. A gain value around 10dB (3.3v/v) is
generally a good starting point.
Example: B = 3.3 v/v, gm=650 m, R1 = 20 KΩ, R2 = 60.4 KΩ:
Bandwidth will vary proportional to the value of Rc1. Next,
Cc1 can be determined with the following equation:
Example: fpmin = 363 Hz, Rc1=20 KΩ:
The value of C
Fpmin/max. A higher value will generally provide a more
stable loop, but too high a value will slow the transient
response time.
The compensation network (Figure 12) will also introduce a
low frequency pole which will be close to 0Hz.
A second pole should also be placed at fz. This pole can be
created with a single capacitor Cc2 and a shorted Rc2 (see
Figure 12). The minimum value for this capacitor can be
calculated by:
Cc2 may not be necessary, however it does create a more
stable control loop. This is especially important with high
load currents and in current sharing mode.
Example: fz = 80 kHz, Rc1 = 20 KΩ:
c1
should be within the range determined by
c1
should be calculated
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