LM2642MTC/NOPB

Manufacturer Part NumberLM2642MTC/NOPB
DescriptionIC CTRLR SW SYNC STPDN 28TSSOP
ManufacturerNational Semiconductor
LM2642MTC/NOPB datasheet
 


Specifications of LM2642MTC/NOPB

ApplicationsEmbedded systems, Console/Set-Top boxesCurrent - Supply1mA
Voltage - Supply4.5 V ~ 30 VOperating Temperature-40°C ~ 125°C
Mounting TypeSurface MountPackage / Case28-TSSOP
Dc To Dc Converter TypeSynchronous Step Down ControllerNumber Of Outputs2
Pin Count28Input Voltage4.5 to 30V
Output Voltage1.3 to 30VOutput Current20A
Package TypeTSSOPMountingSurface Mount
Operating Temperature ClassificationAutomotiveOperating Temperature (min)-40C
Operating Temperature (max)125CPrimary Input Voltage30V
No. Of Outputs1No. Of Pins28
Operating Temperature Range-40°C To +125°CMslMSL 3 - 168 Hours
Control ModeCurrentRohs CompliantYes
For Use WithLM2642REVD EVAL - BOARD EVALUATION LM2642Lead Free Status / RoHS StatusLead free / RoHS Compliant
Other names*LM2642MTC
*LM2642MTC/NOPB
LM2642MTC
  
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Loop Compensation
(Continued)
FIGURE 10. Control-Output Transfer Function
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.
FIGURE 11. Output-Control Transfer Function
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
= 20mΩ, C
= 100µF, R
e
o
50Ω, R
= 5V/3A = 1.7Ω:
omin
Once the fp range is determined, R
using:
20046214
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:
20046212
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
= 5V/100mA =
stable control loop. This is especially important with high
omax
load currents and in current sharing mode.
Example: fz = 80 kHz, Rc1 = 20 KΩ:
19
should be calculated
c1
should be within the range determined by
c1
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