LT3435EFE#PBF Linear Technology, LT3435EFE#PBF Datasheet - Page 12

• Manufacturer Part Number: LT3435EFE#PBF
• Description: IC REG SW HV 3A 500KHZ 16-TSSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3435EFE#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.25 ~ 54 V
• Current - Output: 3A
• Frequency - Switching: 500kHz
• Voltage - Input: 3.3 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Primary Input Voltage: 60V
• No. Of Outputs: 1
• Output Voltage: 68V
• Output Current: 2.4A
• No. Of Pins: 16
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LT3435
APPLICATIO S I FOR ATIO
12
For high frequency switchers the ripple current slew rate
is also relevant and can be calculated from:
Peak-to-peak output ripple voltage is the sum of a triwave
created by peak-to-peak ripple current times ESR and a
square wave created by parasitic inductance (ESL) and
ripple current slew rate. Capacitive reactance is assumed
to be small compared to ESR or ESL.
Example: with V
0.08Ω, ESL = 10nH:
MAXIMUM OUTPUT LOAD CURRENT
Maximum load current for a buck converter is limited by
the maximum switch current rating (I
rating for the LT3435 is 3A. Unlike most current mode
converters, the LT3435 maximum switch current limit
V
I
AC-COUPLED
C
AC-COUPLED
C
V
dt
dt
di
P-P
di
ESR 100mΩ
OUT
OUT
TANTALUM
RIPPLE
RIPPLE
20mV/DIV
20mV/DIV
CERAMIC
=
=
= 100µF
= 100µF
5V/DIV
=
V
V
V
V
15
OUT
OUT
SW
L
( )
Figure 3. LT3435 Ripple Voltage Waveform
IN
12 15
12
= (0.319A)(0.08) + (10e – 9)(0.8e6)
= 0.026 + 0.008 = 34mV
e
=
V
V
I
L = 15µH
LOAD
IN
OUT
–
( )(
( )( )
3 3 12 3 3
= 12V
(
I
6
P-P
= 3.3V
= 1A
.
IN
=
e
= 12V, V
0 8 6
−
U
ESR
.
6 500 3
– .
)(
e
+
U
500ns/DIV
)
( )
OUT
e
ESL
)
= 3.3V, L = 15µH, ESR =
=
dt
di
0 319
W
P-P
.
PK
A
). The current
3435 F03
U
does not fall off at high duty cycles. Most current mode
converters suffer a drop off of peak switch current for duty
cycles above 50%. This is due to the effects of slope
compensation required to prevent subharmonic oscilla-
tions in current mode converters. (For detailed analysis,
see Application Note 19.)
The LT3435 is able to maintain peak switch current limit
over the full duty cycle range by using patented circuitry to
cancel the effects of slope compensation on peak switch
current without affecting the frequency compensation it
provides.
Maximum load current would be equal to maximum
switch current for an infinitely large inductor, but with
finite inductor size, maximum load current is reduced by
one-half peak-to-peak inductor current. The following
formula assumes continuous mode operation, implying
that the term on the right (I
Discontinuous operation occurs when:
For V
Note that there is less load current available at the higher
input voltage because inductor ripple current increases. At
V
conditions:
IN
I
I
I
I
OUT MAX
OUT DIS
OUT MAX
OUT MAX
= 15V, duty cycle is 33% and for the same set of
OUT
(
(
(
(
= 5V, V
)
)
)
)
≤
=
=
=
=
=
V
I
3 0 125 2 875
3 0 22 2 88
3
3
OUT
PK
– .
– .
IN
–
–
2
( )( )(
–
2 15
2 15
L f V
(
= 8V and L = 15µH:
V
(
(
( )(
IN
V
OUT
e
e
=
–
2
( )( )
( )(
=
–
–
( )( )( )
5 8 5
5 15 5
V
IN
L f V
P-P
OUT
6 500 3 8
6 500 3 15
.
)
.
V
)(
)(
IN
/2) is less than I
–
A
)
–
–
A
IN
V
e
e
)
OUT
)( )
)( )
)
=
I
PK
OUT
–
I
P
.
2
-P
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