lt3844 Linear Technology Corporation, lt3844 Datasheet - Page 11

• 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
Therefore, the peak inductor current is 100mV/R
The maximum output load current, I
inductor current minus half the peak-to-peak ripple cur-
rent, ΔI
Allowing adequate margin for ripple current and external
component tolerances, R
follows:
Typical values for R
to 0.05Ω.
Operating Frequency
The choice of operating frequency and inductor value is a
trade off between efficiency and component size. Low
frequency operation improves efficiency by reducing
MOSFET switching losses and gate charge losses. How-
ever, lower frequency operation requires more inductance
for a given amount of ripple current, resulting in a larger
inductor size and higher cost. If the ripple current is
allowed to increase, larger output capacitors may be
required to maintain the same output ripple. For convert-
ers with high step-down V
consideration is the minimum on-time of the LT3844 (see
the Minimum On-time Considerations section). A final
consideration for operating frequency is that in noise-
R
SENSE
L
.
=
Figure 1. Timing Resistor (R
200
180
160
140
120
100
60
80
40
20
I
OUT MAX
70
0
(
mV
U
100
SENSE
)
200
FREQUENCY (kHz)
U
SENSE
are in the range of 0.005Ω
IN
300
to V
can be calculated as
400
W
OUT(MAX)
SET
OUT
) Value
500
ratios, another
3844 G19
600
, is the peak
U
SENSE
.
Recommended 1% Standard Values
sensitive communications systems, it is often desirable to
keep the switching noise out of a sensitive frequency band.
The LT3844 uses a constant frequency architecture that
can be programmed over a 100kHz to 500kHz range with
a single resistor from the f
Figure 1. The nominal voltage on the f
current that flows from this pin is used to charge an
internal oscillator capacitor. The value of R
operating frequency can be chosen from Figure 4 or from
the following equation:
The following table lists typical resistor values for com-
mon operating frequencies:
Step-Down Converter: Inductor Selection
The critical parameters for selection of an inductor are
minimum inductance value, volt-second product, satura-
tion current and/or RMS current.
For a given ΔI, The minimum inductance value is calcu-
lated as follows:
f
SW
L V
≥
is the switch frequency.
R
SET
OUT
(
k
•
Ω = 8 4 10
80.6kΩ
63.4kΩ
49.9kΩ
40.2kΩ
33.2kΩ
27.4kΩ
23.2kΩ
191kΩ
118kΩ
R
f
SW
)
SET
V
IN MAX
•
(
. •
V
IN MAX
(
)
–
4
V
SET
)
OUT
•
•
f
SW
Δ
pin to ground, as shown in
I
L
(– . )
1 31
SET
100kHz
150kHz
200kHz
250kHz
300kHz
350kHz
400kHz
450kHz
500kHz
pin is 1V and the
f
SW
SET
LT3844
for a given
11
3844fa