LT3509_15 Datasheet, PDF (9/24 Pages) Linear Technology – Dual 36V, 700mA Step-Down Regulator

LT3509
APPLICATIONS INFORMATION
Shutdown and Soft Start
When the RUN/SS pins are pulled to ground, the part will
shut down to its lowest current state of approximately
9μA. If driving a large capacitive load it may be desirable
to use the current limiting soft-start feature. Connecting
capacitors to ground from the RUN/SS pins will control
the delay until full current is available. The pull-up current
is 1μA and the full current threshold is 2V so the start-up
time is given by:
T = 2 • C • 10⁶ s
For example a 0.005μF capacitor will give a time to full
current of 10ms. If both outputs can come up together
then the two inputs can be paralleled and tied to one ca-
pacitor. In this case use twice the capacitor value to obtain
the same start-up time. During the soft-start time both
the peak current threshold and the DA current threshold
will track so the part will skip pulses as required to limit
the maximum inductor current. Starting up into a large
capacitor is not much different to starting into a short-
circuit in this respect.
V_SW
10V/DIV
I_L
0.2A/DIV
V_OUT
5V/DIV
TIME 1ms/DIV
3509 F02
Figure 2. Soft-Start
Setting The Output Voltage
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistors
according to:
R1=R2 • ^VOUT – 1
0.8
The designators correspond to Figure 1. R2 should be 20k
or less to avoid bias current errors.
Frequency Setting
The timing resistor, R_T, for any desired frequency in the
range 264kHz to 2.2MHz can be calculated from the
following formula:
R_T
=




1.215
f_SW
–

0.215

•
40.2
where f_SW is in MHz and R_T is in kΩ.
Table 1. Standard E96 Resistors for Common Frequencies
FREQUENCY
264 kHz
TIMING RESISTOR R_T (kΩ)
178
300 kHz
154
400kHz
113
500kHz
88.7
1MHz
40.2
2MHz
15.8
2.2MHz
13.7
Note: The device is specified for operation down to 300kHz. The 264kHz
value is to allow external synchronization at 300kHz
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