lt3685emse-trpbf Linear Technology Corporation, lt3685emse-trpbf Datasheet - Page 15

• Manufacturer Part Number: lt3685emse-trpbf
• Description: 38v, 2a, 2.4mhz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
300mV above V
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3685, requiring a higher
input voltage to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT3685,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC fi lter to
create a voltage ramp at this pin. Figure 6 shows the start-
up and shut-down waveforms with the soft-start circuit.
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
6.0
5.5 TO START
5.0
4.5
4.0
3.5
3.0
2.5
2.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1
1
TO RUN
TO RUN
(WORST CASE)
V
T
L = 8.2μH
f = 700kHz
TO START
(WORST CASE)
V
T
L = 8.2μH
f = 700kHz
OUT
OUT
A
OUT
A
= 25°C
= 25°C
= 3.3V
= 5V
. At higher load currents, the inductor
10
10
LOAD CURRENT (A)
LOAD CURRENT (A)
100
100
1000
1000
3685 F05
10000
10000
By choosing a large RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply 20μA when the RUN/SS
pin reaches 2.5V.
Synchronization
Synchronizing the LT3685 oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.3V
and peaks that are above 0.8V (up to 6V).
The LT3685 may be synchronized over a 250kHz to 2MHz
range. The R
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
250kHz and higher, the R
To assure reliable and safe operation the LT3685 will only
synchronize when the output voltage is near regulation
as indicated by the PG fl ag. It is therefore necessary to
choose a large enough inductor value to supply the required
output current at the frequency set by the R
Inductor Selection section. It is also important to note that
slope compensation is set by the R
frequency is much higher than the one set by R
compensation will be signifi cantly reduced which may
require a larger inductor value to prevent subharmonic
oscillation.
RUN
15k
Figure 6. To Soft-Start the LT3685, Add a Resisitor
and Capacitor to the RUN/SS Pin
RUN/SS
GND
T
resistor should be chosen to set the LT3685
T
should be chosen for 200kHz.
2ms/DIV
T
value: When the sync
T
LT3685
resistor. See
T
, the slope
3685 F06
15
I
1A/DIV
V
2V/DIV
V
2V/DIV
L
RUN/SS
OUT
3685f