Datasheet

12
LT3433
3433f
Design Example
4V-60V to 5V DC/DC converter (the application on the
front page of this data sheet), load capability for T
A
= 85°C.
Application Specific
Constants: LT3433 W/C Constants:
V
IN
= 4V I
MAX
= 0.55A
V
OUT
= 5V R
SWH
= 1.2Ω
L = 100µHR
SWL
= 1Ω
R
L
= 0.28Ω f
O
= 190kHz
V
F1
= 0.45V ∆
BST
= 0.05
V
F2
= 0.4V ∆
OUT
= 0.05
R
CESR
= 0.01Ω I
VIN
= 600µA
I
BIAS
= 800µA
The LT3433 operates in bridged mode with V
IN
= 4V, so the
relations used are:
DC = [V
OUT
+ V
F1
+ V
F2
– I
SW
• (R
L
+ R
ESR
)]/[V
IN
–
I
SW
• (R
SWH
+ R
SWL
+ 2R
L
+ R
ESR
) + V
OUT
+ V
F1
+
V
F2
]
∆I = (V
OUT
+ V
F1
+ V
F2
- I
SW
• R
L
) • (1 – DC)/(L • f
O
)
I
OUT(MAX)
= I
SW
• [1 – DC • (1 + ∆
BST
+ ∆
OUT
)] – I
BIAS
Iteration procedure for DC:
(1) Set initial seed value for ∆I (this example will set
∆I = 0).
(2) Using seed value for ∆I, determine I
SW
(I
SW
= 0.55 –
0 = 0.55).
(3) Use calculated I
SW
and above design constants to
solve the DC relation (DC = 0.683).
(4) Use calculated DC to solve the ∆I relation (yields ∆I =
0.0949).
(5) If calculated ∆I is equal to the seed value, stop.
Otherwise, use calculated ∆I as new seed value and
repeat (2) through (4).
CALCULATED VALUES
ITERATION # SEED ∆II
SW
DC ∆I
1 0 0.55 0.683 0.095
2 0.095 0.503 0.674 0.098
3 0.098 0.501 0.674 0.098
After iteration, DC = 0.674 and ∆I = 0.098.
Use iteration result for DC and above design constants to
solve the I
OUT(MAX)
relation:
I
OUT(MAX)
= 0.501 • [1 – 0.674 • (1 + 0.05 + 0.05)] –
800µA
I
OUT(MAX)
= 129mA
Increased Output Voltages
The LT3433 can be used in converter applications with
output voltages from 3.3V through 20V, but as converter
output voltages increase, output current and duty cycle
limitations prevent operation with V
IN
at the extreme low
end of the LT3433 operational range. When a converter
operates as a buck/boost, the output current becomes
discontinuous, which reduces output current capability by
roughly a factor of 1 – DC, where DC = duty cycle. As such,
the output current requirement dictates a minimum input
voltage where output regulation can be maintained.
APPLICATIO S I FOR ATIO
WUUU
V
OUT
(V)
4
V
IN(MIN)
(V)
12
16
200mA
125mA
175mA
20
3433 AI03
8
4
8
12
16
24
20
150mA
Typical Minimum Input Voltage as a Function of
Output Voltage and Required Load Current