LT1766 Linear Technology, LT1766 Datasheet - Page 20

• Manufacturer Part Number: LT1766
• Description: 5.5V to 60V 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1766/LT1766-5
voltage required for full saturation of the internal power
switch. For output voltages of 5V, V
During switch turn on, V
C2 is dicharged by the boost pin. In the previous Boost Pin
section, the value of C2 was designed for a 0.7V droop in
V
would still allow the minimum 3.3V for the boost function
using the C2 capacitor calculated. If a target output voltage
of 12V is required, however, an excess of 8V is placed
across the boost capacitor which is not required for the
boost function but still dissipates additional power.
What is required is a voltage drop in the path of D2 to
achieve minimal power dissipation while still maintaining
minimum boost voltage across C2. A zener, D4, placed in
series with D2 (see Figure 9), drops voltage to C2.
Example : the BOOST pin power dissipation for a 20V input
to 12V output conversion at 1A is given by:
If a 7V zener D4 is placed in series with D2, then power
dissipation becomes :
20
V
C2
IN
P
P
BOOST
BOOST
= V
DROOP
C3
Figure 9. Boost Pin, Diode Selection
12 1 36 5
12 1 36 12
V
SHDN
SYNC
. Hence, an output voltage as low as 4V
GND
IN
• ( /
C
• ( /
LT1766
BOOST
C
R
C
U
20
20
BIAS
V
SW
)•
C
)•
C2
FB
U
C
will fall as the boost capacitor
F
0 084
.
0 2
D1
C2
.
C2
W
W
is approximately 5V.
W
D2
D2
L1
R1
R2
1766 F09
D4
U
+
C1
V
OUT
For an FE package with thermal resistance of 45 C/W,
ambient temperature savings would be, T(ambient) sav-
ings = 0.116W • 45 C/W = 5c. For a GN Package with
thermal resistance of 85 C/W, ambient temperature sav-
ings would be T/(ambient) savings = 0.116 • 85 C/W =
10c. The 7V zener should be sized for excess of 0.116W
operation. The tolerances of the zener should be consid-
ered to ensure minimum V
Input Voltage vs Operating Frequency Considerations
The absolute maximum input supply voltage for the LT1766
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 1A 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:
V
V
V
f
A potential controllability problem arises if the LT1766 is
called upon to produce an on time shorter than it is able to
produce. Feedback loop action will lower then reduce the
OSC
IN
OUT
F
= Schottky diode forward drop
Min t
= input voltage
= switching frequency
= output voltage
ON
V
V
OSC
OUT
IN OSC
(
f
= 200kHz may be easily achievable,
V
F
)
IN
C2
to 60V and f
exceeds 3.3V + V
OSC
IN
IN
= 40V, V
achievable in
to 700kHz is
IN
DROOP
to V
OUT
OUT
1766fa
.
=
,