lt3505ems8e-trpbf Linear Technology Corporation, lt3505ems8e-trpbf Datasheet - Page 12

• Manufacturer Part Number: lt3505ems8e-trpbf
• Description: 1.2a, Step-down Switching Regulator In 3mm X 3mm Dfn
• Manufacturer: Linear Technology Corporation

LT3505
APPLICATIONS INFORMATION
Component Selection for Input Voltage Frequency
Foldback Circuit
To determine the values of R4, R5, and D3 for a specifi c
application follow the procedure outlined in this section.
First select the value of R4 from Table 1.
Table 1. R
Second, determine the value of V
in the “Input Voltage Range” section of the data sheet.
Select the zener diode, D3, to have a breakdown voltage
(V
frequency from the following equation:
where V
12
f
ZENER
SW(MIN)
R
T
PIN RESISTANCE (kΩ)
) below V
D
T
is the forward drop of the catch diode (~0.4V),
Pin Resistance
= (V
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
357
237
165
124
100
OUT
IN(PS)
+ V
D
. Next determine the desired foldback
)/[t
ON(MIN)
SWITCHING FREQUENCY (MHz)
IN(PS)
• (V
IN(MAX)
from the equation
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
+ V
D
– V
SW
)]
and V
(~0.4V at maximum load), V
voltage for the application (must be less than 36V), and
t
tended application. The worst-case minimum on-time can
be determined from the graphs in the “Typical Performance
Characteristics” section of the datasheet. Next look up the
resistance that corresponds to f
resistance is R
pin to ground at V
a switching frequency equal to f
Finally determine R5 from the following equation:
where V
and V
applied to the V
maximum operating input voltage of the LT3505. The
equation to determine R5 assumes that R5 will com-
pensate a percentage of the current fl owing through R4
equal to R4/R
R5 much less than that determined by the equation above
because it may become possible for R5 to compensate
100% of the current fl owing 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 R
than the zener voltage, the reduced voltage at the R
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
fl owing through R4, reducing the frequency to 30kHz. In
this situation the input voltage will have to be reduced until
the input voltage is less than the zener voltage.
Note that when V
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.
ON(MIN)
R5 = 2 • (V
SW
IN(MAX)
is the worst-case minimum on-time for the in-
ZENER
is the voltage drop of the internal power switch
IN(MAX)
is the maximum input voltage that will be
T(MAX)
T(MAX)
is the zener diode breakdown voltage,
IN
IN
pin. V
IN(MAX)
is above V
. Be careful not to select a value of
, the effective resistance from the R
– V
T
IN(MAX)
ZENER
pin. If the input voltage is higher
that programs the oscillator to
IN(MAX)
ZENER
)/(1/R4 – 1/R
must not exceed 36V, the
SW(MIN)
SW(MIN)
is the maximum input
and the frequency is
.
in Table 1. This
T(MAX)
)
T
3505fc
pin
T