LT3430 Linear Technology, LT3430 Datasheet - Page 21

• Manufacturer Part Number: LT3430
• Description: High Voltage/ 3A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Input Voltage vs Operating Frequency Considerations
The absolute maximum input supply voltage for the LT3430
is specified at 60V. This is based solely on internal semi-
conductor junction breakdown effects. Due to internal
power dissipation, the actual maximum V
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 switch-
ing loss is also proportional to the square of input voltage.
For example, while the combination of V
5V at 2A and f
simultaneously raising V
not possible. Nevertheless, input voltage transients up to
60V can usually be accommodated, assuming the result-
ing increase in internal dissipation is of insufficient time
duration to raise die temperature significantly.
A second consideration is controllability. A potential limi-
tation occurs with a high step-down ratio of V
as this requires a correspondingly narrow minimum switch
on time. An approximate expression for this (assuming
continuous mode operation) is given as follows:
where:
A potential controllability problem arises if the LT3430 is
called upon to produce an on time shorter than it is able to
produce. Feedback loop action will lower then reduce the
V
skipping or odd/even cycle behavior is exhibited.
In summary:
1. Be aware that the simultaneous requirements of high
C
V
V
V
f
V
practice due to internal dissipation. The Thermal Con-
siderations section offers a basis to estimate internal
control voltage to the point where some sort of cycle-
OSC
Min t
IN
OUT
F
IN
= Schottky diode forward drop
, high I
= input voltage
= switching frequency
= output voltage
ON
OUT
V
V f
OSC
OUT
IN OSC
and high f
U
= 200kHz may be easily achievable,
V
F
IN
U
to 60V and f
OSC
may not be achievable in
W
OSC
IN
IN
= 40V, V
achievable in
to 700kHz is
IN
U
to V
OUT
OUT
=
,
2. The simultaneous requirements of high V
FREQUENCY COMPENSATION
Before starting on the theoretical analysis of frequency
response, the following should be remembered—the worse
the board layout, the more difficult the circuit will be to
stabilize. This is true of almost all high frequency analog
circuits, read the Layout Considerations section first.
Common layout errors that appear as stability problems
are distant placement of input decoupling capacitor and/
or catch diode, and connecting the V
ground track carrying significant switch current. In addi-
tion, the theoretical analysis considers only first order
non-ideal component behavior. For these reasons, it is
important that a final stability check is made with produc-
tion layout and components.
The LT3430 uses current mode control. This alleviates
many of the phase shift problems associated with the
inductor. The basic regulator loop is shown in Figure 10.
The LT3430 can be considered as two g
amplifier and the power stage.
Figure 11 shows the overall loop response. At the V
the frequency compensation components used are:
R
capacitor used is a 100 F, 10V tantalum capacitor with
typical ESR of 100m .
The ESR of the tantalum output capacitor provides a useful
zero in the loop frequency response for maintaining stabil-
ity. This ESR, however, contributes significantly to the
ripple voltage at the output (see Output Ripple Voltage in
the Applications Information section). It is possible to
reduce capacitor size and output ripple voltage by replac-
ing the tantalum output capacitor with a ceramic output
capacitor because of its very low ESR. The zero provided
by the tantalum output capacitor must now be reinserted
back into the loop. Alternatively, there may be cases
C
power. In questionable cases a prototype supply should
be built and exercised to verify acceptable operation.
and high f
minimum switch on time. Cycle skipping and/or odd/
even cycle behavior will result although correct output
voltage is usually maintained.
= 3.3k, C
C
OSC
= 0.022 F and C
can result in an unacceptably short
F
= 220pF. The output
C
compensation to a
m
blocks, the error
LT3430
IN
, low V
sn3430 3430is
21
C
pin,
OUT