lt3844 Linear Technology Corporation, lt3844 Datasheet - Page 15

• Manufacturer Part Number: lt3844
• Description: High Voltage, Current Mode Switching Regulator Controller With Programmable Operating Frequency
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
V
lockout is disabled and the supply turns on.
Example: Select R
on a 15V minimum input voltage)
If low supply current in standby mode is required, select
a higher value of R
The supply turn off voltage is 9% below turn on. In the
example the V
If additional hysteresis is desired for the enable function,
an external positive feedback resistor can be used from the
LT3844 regulator output.
The shutdown function can be disabled by connecting the
SHDN pin to the V
This pin contains a low impedance clamp at 6V, so the
SHDN pin will sink current from the pull-up resistor(R
Because this arrangement will clamp the SHDN pin to the
6V, it will violate the 5V absolute maximum voltage rating
of the pin. This is permitted, however, as long as the
absolute maximum input current rating of 1mA is not
exceeded. Input SHDN pin currents of <100μA are recom-
mended: a 1MΩ or greater pull-up resistor is typically
used for this configuration.
Soft-Start
The desired soft-start time (t
C
The amount of time in which the power supply can
withstand a V
(t
is approximated by the following formula:
SUPPLY(ON)
SS
C
FAULT
R
I
SS
SHDN
capacitor as follows:
A
=
=
) before the C
= 486.1kΩ (499kΩ resistor is selected)
49 9
2
=
1 231
μA t
.
.
V
IN
is the input voltage at which the undervoltage
k
R
•
Ω
SUPPLY(OFF)
– 6
PU
IN
SS
V
, V
•
B
IN
⎛
⎝ ⎜
V
B
= 49.9kΩ, V
U
14 5
1 35
through a large value pull-up resistor.
CC
.
SS
.
.
pin voltage enters its active region
or V
V
V
U
would be 13.2V.
–
1
SHDN
⎞
⎠ ⎟
SS
SUPPLY(ON)
) is programmed via the
UVLO fault condition
W
= 14.5V (based
U
PU
):
Oscillator SYNC
The oscillator can be synchronized to an external clock.
Set the R
frequency.
It is recommended that the SYNC pin be driven with a
square wave that has amplitude greater than 2V, pulse
width greater than 1μs and rise time less than 500ns. The
rising edge of the sync wave form triggers the discharge
of the internal oscillator capacitor.
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%.
Express percent efficiency as:
where L1, L2, etc. are individual loss terms as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main contributors usually account for most of
the losses in LT3844 circuits:
1. LT3844 V
2. I
3. MOSFET transition loss
4. Schottky diode conduction loss
1. The V
currents of the LT3844 and the MOSFET drive currents.
The quiescent currents are in the LT3844 Electrical Char-
acteristics table. The MOSFET drive current is a result of
charging the gate capacitance of the power MOSFET each
cycle with a packet of charge, Q
MOSFET data sheet. The average charging current is
calculated as Q
currents can be reduced by backdriving V
voltage than V
t
FAULT
% Efficiency = 100% - (L1 + L2 + L3 + ...)
2
R conduction losses
IN
SET
=
and V
C
IN
resistor at least 10% below the desired sync
SS
IN
and V
50
G
• . 0 65
CC
such as V
• f
μA
SW
currents are the sum of the quiescent
CC
. The power loss term due to these
V
current loss
OUT
.
G
. Q
G
is found in the
CC
LT3844
with a lower
15
3844fa