LT1766EFE#TRPBF Linear Technology, LT1766EFE#TRPBF Datasheet - Page 22

• Manufacturer Part Number: LT1766EFE#TRPBF
• Description: IC REG SW HV 1.5A 200KHZ 16TSSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1766EFE#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.2 ~ 54 V
• Current - Output: 1.5A
• Frequency - Switching: 200kHz
• Voltage - Input: 5.5 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LT1766/LT1766-5
APPLICATIONS INFORMATION
For output voltages of 5V, V
switch turn on, V
dicharged by the BOOST pin. In the previous BOOST Pin
section, the value of C2 was designed for a 0.7V droop in
V
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 :
22
C2
V
IN
P
P
= V
BOOST
BOOST
DROOP
C3
=
=
Figure 9. Boost Pin, Diode Selection
. Hence, an output voltage as low as 4V would
12 1 36 5
12 1 36 12
V
SHDN
SYNC
GND
IN
• ( /
• ( /
C2
C
LT1766
BOOST
C
R
20
C
20
will fall as the boost capacitor C2 is
BIAS
)•
V
)•
SW
C
FB
C2
C
F
=
is approximately 5V. During
=
0 084
.
0 2
D1
C2
.
W
W
D2
D2
L1
R1
R2
1766 F09
D4
+
C1
V
OUT
For an FE package with thermal resistance of 45°C/W,
ambient temperature savings would be, T(ambient) savings
= 0.116W • 45°C/W = 5c. For a GN Package with thermal
resistance of 85°C/W, ambient temperature savings 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 considered to ensure
minimum V
Input Voltage vs Operating Frequency Considerations
The absolute maximum input supply voltage for the
LT1766 is specifi ed at 60V. This is based solely on internal
semiconductor 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 switching
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 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
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 LT1766 is
called upon to produce an on time shorter than it is able
to produce. Feedback loop action will lower then reduce
V
V
V
f
OSC
Min t
IN
OUT
F
= Schottky diode forward drop
= Input voltage
= Switching frequency
= Output voltage
ON
C2
=
V
exceeds 3.3V + V
V
OUT
IN OSC
OSC
(
f
+
= 200kHz may be easily achievable,
V
F
)
IN
to 60V and f
DROOP
.
OSC
IN
IN
achievable in
IN
to 700kHz is
= 40V, V
to V
OUT
1766fc
OUT
, as