Datasheet

V
IN
L
OUT
C
OUT
R
FB1
R
FB2
C
C2
C
C1
R
C1
V
OUT
C
C3
R
C2
FB
+
-
R
DCR
COMP
DRIVER
EA
+
-
PWM
Error Amplifier and Compensation
R
O
PWM Modulator
SW
R
ESR
Power Train
0.6V
582.4
R
ILIM
(k:) =
I
HSMAX
- 14.2
'I
LMAX
I
HSMAX
=
2
+ I
DCMAX
LM21215
www.ti.com SNVS625E –FEBRUARY 2011–REVISED MARCH 2013
where
• V
INMAX
, V
INMIN
, V
OUTMAX
, V
OUTMIN
, L
MIN
and F
SWMIN
are the respective maximum and minimum conditions of the
system as defined by the component tolerance and device variation (9)
From this, the maximum allowable current through the high-side FET (I
HSMAX
) of device can be described as:
where
• I
OUTMAX
is the maximum defined DC output current, up to 15 A
• (10)
Once the I
HSMAX
value has been determined, a nominal value of the R
ILIM
resistor can be calculated as follows:
where
• R
ILIM
value is the nominal resistance necessary for the given I
HSMAX
value (11)
A conservative design should also take into account the device variation over V
IN
and temperature, as seen in
the Electrical Characteristics table for the I
CLR
parameter and the typical performance characteristics. These
variations can cause the I
HSMAX
value to increase, depending on the range of the input voltage and junction
temperature.
CONTROL LOOP COMPENSATION
The LM21215 incorporates a high bandwidth amplifier between the FB and COMP pins to allow the user to
design a compensation network that matches the application. This section will walk through the various steps in
obtaining the open loop transfer function.
There are three main blocks of a voltage mode buck switcher that the power supply designer must consider
when designing the control system; the power train, modulator, and the compensated error amplifier. A closed
loop diagram is shown in Figure 32.
Figure 32. Loop Diagram
The power train consists of the output inductor (L) with DCR (DC resistance R
DCR
), output capacitor (C
0
) with
ESR (effective series resistance R
ESR
), and load resistance (R
o
). The error amplifier (EA) constantly forces FB to
0.6V. The passive compensation components around the error amplifier help maintain system stability. The
modulator creates the duty cycle by comparing the error amplifier signal with an internally generated ramp set at
the switching frequency.
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