LT3430/LT3430-1
21
34301fa
ings = 0.233W • 45°C/W = 11°C. The 7V zener should be
sized for excess of 0.233W operaton. The tolerances of
the zener should be considered to ensure minimum V
C2
exceeds 3.3V + V
DROOP
.
Input Voltage vs Operating Frequency Considerations
The absolute maximum input supply voltage for the LT3430/
LT3430-1 is specifi ed at 60V. This is based solely on internal
semiconductor junction breakdown effects. Due to internal
power dissipation, the actual maximum V
IN
achievable in
a particular application may be less than this.
A detailed theoretical basis for estimating internal power
loss is given in the section, Thermal Considerations. Note
that AC switching loss is proportional to both operating
frequency and output current. The majority of AC switching
loss is also proportional to the square of input voltage.
For example, while the combination of V
IN
= 40V, V
OUT
= 5V at 2A and f
OSC
= 200kHz may be easily achievable,
simultaneously raising V
IN
to 60V and f
OSC
to 700kHz is
not possible. Nevertheless, input voltage transients up to
60V can usually be accommodated, assuming the result-
ing increase in internal dissipation is of insuffi cient time
duration to raise die temperature signifi cantly.
A second consideration is controllability. A potential limita-
tion occurs with a high step-down ratio of V
IN
to V
OUT
, as
this requires a correspondingly narrow minimum switch
on time. An approximate expression for this (assuming
continuous mode operation) is given as follows:
min t
VV
Vf
ON
OUT F
IN OSC
=
+
()
where:
V
IN
= input voltage
V
OUT
= output voltage
V
F
= Schottky diode forward drop
f
OSC
= switching frequency
A potential controllability problem arises if the LT3430/
LT3430-1 are called upon to produce an on time shorter
than it is able to produce. Feedback loop action will lower
then reduce the V
C
control voltage to the point where
some sort of cycle-skipping or odd/even cycle behavior
is exhibited.
In summary:
1. Be aware that the simultaneous requirements of high
V
IN
, high I
OUT
and high f
OSC
may not be achievable in
practice due to internal dissipation. The Thermal Con-
siderations section offers a basis to estimate internal
power. In questionable cases a prototype supply should
be built and exercised to verify acceptable operation.
2. The simultaneous requirements of high V
IN
, low V
OUT
and
high f
OSC
can result in an unacceptably short minimum
switch on time. Cycle skipping and/or odd/even cycle
behavior will result although correct output voltage is
usually maintained. The LT3430-1 100kHz switching
frequency will allow higher V
IN
/V
OUT
ratios without
pulse skipping.
FREQUENCY COMPENSATION
Before starting on the theoretical analysis of frequency
response, the following should be remembered—the
worse the board layout, the more diffi cult the circuit will
be to stabilize. This is true of almost all high frequency
analog circuits, read the Layout Considerations section
fi rst. Common layout errors that appear as stability prob-
lems are distant placement of input decoupling capacitor
and/or catch diode, and connecting the V
C
compensation
to a ground track carrying signifi cant switch current. In
addition, the theoretical analysis considers only fi rst
order non-ideal component behavior. For these reasons,
it is important that a fi nal stability check is made with
production layout and components.
The LT3430/LT3430-1 use current mode control. This al-
leviates many of the phase shift problems associated with
the inductor. The basic regulator loop is shown in Figure
10. The LT3430/LT3430-1 can be considered as two g
m
blocks, the error amplifi er and the power stage.
Figure 11 shows the overall loop response. At the V
C
pin, the frequency compensation components used are:
R
C
= 3.3k, C
C
= 0.022µF and C
F
= 220pF. The output
capacitor used is a 100µF, 10V tantalum capacitor with
typical ESR of 100mΩ. LT3430-1 uses two of these
capacitors in parallel.
The ESR of the tantalum output capacitor provides a use-
ful zero in the loop frequency response for maintaining
APPLICATIONS INFORMATION
