Datasheet

f
P
=
1
2SR
O
C
O
+
1 - D - .5
2SfLC
O
LM5642, LM5642X
www.ti.com SNVS219K –JUNE 2003–REVISED APRIL 2013
The control-output corner frequencies, and thus the desired compensation corner frequencies, can be
determined approximately by the following equations:
(29)
(30)
Since fp is determined by the output network, it will shift with loading (Ro). It is best to use a minimum Iout value
of approximately 100mA when determining the maximum Ro value.
Example: Re = 20 mΩ, Co = 100 uF, Romax = 5V/100 mA = 50Ω:
(31)
(32)
First determine the minimum frequency (fpmin) of the pole across the expected load range, then place the first
compensation zero at or below that value. Once fpmin is determined, Rc1 should be calculated using:
where
• B is the desired gain in V/V at fp (fz1)
• gm is the transconductance of the error amplifier
• R1 and R2 are the feedback resistors (33)
A gain value around 10dB (3.3v/v) is generally a good starting point.
Example: B = 3.3v/v, gm = 650m, R1 = 20 kKΩ, R2 = 60.4 kΩ:
(34)
Bandwidth will vary proportional to the value of Rc1. Next, Cc1 can be determined with the following equation:
(35)
Example: fpmin = 995 Hz, Rc1 = 20 kΩ:
(36)
The compensation network (Figure 35) will also introduce a low frequency pole which will be close to 0 Hz.
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 35). The minimum value for this capacitor can be calculated by:
(37)
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:
(38)
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