LT3505
APPLICATIONS INFORMATION
Component Selection for Input Voltage Frequency
Foldback Circuit
and VSW is the voltage drop of the internal power switch
(~0.4V at maximum load), VIN(MAX) is the maximum input
voltage for the application (must be less than 36V), and
tON(MIN) is the worst-case minimum on-time for the in-
tended application. The worst-case minimum on-time can
bedeterminedfromthegraphsinthe“TypicalPerformance
Characteristics” section of the datasheet. Next look up the
resistance that corresponds to fSW(MIN) in Table 1. This
resistance is RT(MAX), the effective resistance from the RT
pin to ground at VIN(MAX) that programs the oscillator to
To determine the values of R4, R5, and D3 for a specific
application follow the procedure outlined in this section.
First select the value of R4 from Table 1.
Table 1. RT Pin Resistance
SWITCHING FREQUENCY (MHz)
RT PIN RESISTANCE (kΩ)
357
237
165
124
100
0.20
0.30
0.40
0.50
0.60
0.69
0.80
0.91
1.00
1.11
1.21
1.31
1.39
1.50
1.60
1.70
1.80
1.90
2.02
2.10
2.22
2.31
2.39
2.48
2.62
2.71
2.81
2.90
3.01
a switching frequency equal to fSW(MIN)
.
Finally determine R5 from the following equation:
R5 = 2 • (VIN(MAX) – VZENER)/(1/R4 – 1/RT(MAX)
84.5
71.5
61.9
54.9
48.7
44.2
40.2
37.4
34.0
31.6
29.4
27.4
25.5
23.7
22.6
21.0
20.0
19.1
18.2
16.9
16.2
15.4
14.7
13.7
)
where V
IN(MAX)
applied to the V pin. V
is the zener diode breakdown voltage,
ZENER
and V
is the maximum input voltage that will be
must not exceed 36V, the
IN
IN(MAX)
maximum operating input voltage of the LT3505. The
equation to determine R5 assumes that R5 will com-
pensate a percentage of the current flowing through R4
equal to R4/R
. Be careful not to select a value of
T(MAX)
R5muchlessthanthatdeterminedbytheequationabove
because it may become possible for R5 to compensate
100% of the current flowing through R4 reducing the
frequency to 30kHz. In this state the part is not able to
start into large output current loads.
Whenever the voltage at the FB pin is below 600mV, the
LT3505 folds back the switching frequency by reducing
the bias voltage at the RT pin. If the input voltage is higher
than the zener voltage, the reduced voltage at the RT pin
results in a larger voltage drop across R5, and a reduced
voltage drop across R4. The current carried by R5 may
be large enough to completely compensate the current
flowing through R4, reducing the frequency to 30kHz. In
thissituationtheinputvoltagewillhavetobereduceduntil
the input voltage is less than the zener voltage.
Second, determine the value of VIN(PS) from the equation
in the “Input Voltage Range” section of the data sheet.
Select the zener diode, D3, to have a breakdown voltage
(VZENER)belowVIN(PS).Nextdeterminethedesiredfoldback
frequency from the following equation:
Note that when VIN is above VZENER and the frequency is
reduced, the inductor ripple current will be higher and the
maximum load that the LT3505 can regulate will be lower.
See the Inductor Selection and Maximum Output Current
section of this data sheet for more information.
fSW(MIN) =(VOUT +VD)/[tON(MIN) •(VIN(MAX) +VD –VSW)]
where VD is the forward drop of the catch diode (~0.4V),
3505fc
12
Linear [ Linear ]
