10
LT1776
Input Voltage vs Operating Frequency Considerations
The absolute maximum input supply voltage for the LT1776
is specified at 60V. This is based solely on internal semi-
conductor 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 switch-
ing loss is also proportional to the square of input voltage.
For example, while the combination of V
IN
= 40V, V
OUT
=
5V at 500mA and f
OSC
= 200kHz may be easily achievable,
simultaneously raising V
IN
to 60V and f
OSC
to 400kHz is
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
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:
M
VV
Vf
ON
OUT F
IN OSC
in t =
+
()
where:
V
IN
= input voltage
V
OUT
= output voltage
V
F
= Schottky diode forward drop
f
OSC
= switching frequency
It is important to understand the nature of minimum
switch ON time as given in the data sheet. This test is
intended to mimic behavior under short-circuit condi-
tions. It is performed with the V
C
control voltage at its
clamp level (V
CL
) and uses a fixed resistive load from V
SW
to ground for simplicity. The resulting ON time behavior is
overconservative as a general operating design value for
two reasons. First, actual power supply application cir-
cuits present an inductive load to the V
SW
node. The
APPLICATIONS INFORMATION
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resulting ramping current behavior helps overdrive the
current comparator (current mode switching) and reduce
its propagation delay, hastening output switch turnoff.
Second, and more importantly, actual power supply op-
eration involves a feedback amplifier that adjusts the V
C
node control voltage to maintain proper output voltage. As
progressively shorter ON times are required, the feedback
loop acts to reduce V
C
, and the resulting overdrive further
reduces the propagation delay in the current comparator.
A suggested worst-case limit for minimum switch ON time
in actual operation is 350ns.
A potential controllability problem arises if the LT1776 is
called upon to produce an ON time shorter than its ability.
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.
Minimum Load Considerations
As discussed previously, a lightly loaded LT1776 with V
C
pin control voltage below the boost threshold will operate
in low dV/dt mode. This affords greater controllability at
light loads, as minimum t
ON
requirements are relaxed.
However, some users may be indifferent to pulse skipping
behavior, but instead may be concerned with maintaining
maximum possible efficiency at light loads. This require-
ment can be satisfied by forcing the part into Burst Mode
TM
operation. The use of an external comparator whose
Burst Mode is a trademark of Linear Technology Corporation.
