Datasheet
LM21305
COMP
R
C
C
C1
Power Stage
Compensator
Feedback
-
+
V
REF
V
C
(s)
V
OUT
(s)
+
-
Loop T(s)
H(s)
F
p
(s) x F
h
(s)
F
comp
(s)
I
OUT-MAX
D0 50% 100%
5.0A
2.5A
LM21305
SNVS639F –DECEMBER 2009–REVISED MARCH 2013
www.ti.com
Figure 28. LM21305 Maximum Load Current Derating when D > 50%
CONTROL LOOP COMPENSATION
Figure 29. Control Block Diagram of a Peak Current-Mode Controlled Buck Converter
This section will not provide a rigorous analysis of current-mode control, but rather a simplified yet relatively
accurate method to determine the control loop compensation network. The LM21305 employs a peak current-
mode controller and therefore the control loop block diagram representation involves two feedback loops (see
Figure 29). The inner feedback loop derives its feedback from the sensed inductor current while the outer loop
monitors the output voltage. The LM21305 compensation components from COMP to AGND are shown in
Figure 30. The purpose of the compensator block is to stabilize the control loop and achieve high performance in
terms of the load transient response, audio susceptibility and output impedance. The LM21305 will typically
require only a single resistor R
c
and capacitor C
c1
for compensation. However, depending on the location of the
power stage ESR zero, a second (small) capacitor, C
c2
, may be required to create a high frequency pole.
Figure 30. LM21305 Compensation Network
The overall loop transfer function is a product of the power stage transfer function, internal amplifier gains and
the feedback network transfer function and can be expressed by:
T(s) = Gain
0
F
p
(s)F
h
(s)F
comp
(s)
where Gain
0
includes all the DC gains in the loop, F
p
(s) represents the power stage pole and zero (including the
inner current loop), F
h
(s) represents the sampling effect in such a current-mode converter and F
comp
(s) is the
transfer function of the external compensator. Figure 31 shows an asymptotic approximation plot of the loop gain.
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