LT3435EFE#PBF Linear Technology, LT3435EFE#PBF Datasheet - Page 22

• 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
catch diode and connecting the V
ground track carrying significant switch current. In addi-
tion the theoretical analysis considers only first order non-
ideal component behavior. For these reasons, it is important
that a final stability check is made with production layout
and components.
The LT3435 uses current mode control. This alleviates
many of the phase shift problems associated with the
inductor. The basic regulator loop is shown in Figure 12.
The LT3435 can be considered as two g
amplifier and the power stage.
Figure 13 shows the overall loop response with a 330pF V
capacitor and a typical 100µF tantalum output capacitor.
The response is set by the following terms:
The pole set by C
Unity gain frequency is set by C
Powerstage: DC gain is set by g
Pole set by C
Unity gain set by C
Tantalum output capacitor zero is set by C
ESR
The zero produced by the ESR of the tantalum output
capacitor is very useful in maintaining stability. If better
transient response is required, a zero can be added to the
loop using a resistor (R
22
APPLICATIO S I FOR ATIO
Error amplifier: DC gain is set by g
EA Gain = 650µ • 1.5M = 975
EA Pole = 1/(2π • 1.5M • 470pF) = 220Hz
EA Unity Gain Frequency = 650µF/(2π • 470pF)
PS DC Gain = 6 • 10 = 60
PS Pole = 1/(2π • 100µF • 10) = 159Hz
PS Unity Gain Freq = 6/(2π • 100µF) = 94kHz.
Output Capacitor Zero = 1/(2π • 100µF • 0.1) = 15.9kHz
OUT
F
and R
and R
OUT
Ω
U
C
L
and g
) in series with the compensation
:
L
:
U
= 220kHz
m
:
m
F
and g
and R
C
W
compensation to a
m
m
m
and R
L
blocks, the error
:
(assume 10Ω):
OUT
O
U
:
and C
OUT
C
C
capacitor. As the value of R
sponse will generally improve but two effects limit its
value. First, the combination of output capacitor ESR and
a large R
Second, if the loop gain is not rolled off sufficiently at the
switching frequency output ripple will perturb the V
enough to cause unstable duty cycle switching similar to
subharmonic oscillation. This may not be apparent at the
output. Small-signal analysis will not show this since a
continuous time system is assumed.
When checking loop stability the circuit should be oper-
ated over the application’s full voltage, current and tem-
perature range. Any transient loads should be applied and
the output voltage monitored for a well-damped behavior.
R
C
C
C
F
11
C
LT3435
100
–40
–80
V
80
40
CURRENT MODE
Figure 13. Model for Loop Response
may stop loop gain rolling off altogether.
0
POWER STAGE
C
10
Figure 14. Overall Loop Response
g
m
= 6
Ω
100
g
1.5M
m
= 650µ
FREQUENCY (Hz)
1k
ERROR
Ω
AMP
C
–
+
V
C
C
R
C
I
10k
LOAD
is increased, transient re-
OUT
OUT
F
C
C
= 470pF
= 10k
= 4700pF
= 3.3V
= 100µF, 0.1Ω
= 1A
1.25V
100k
SW
FB
12
2
3435 F14
R1
R2
1M
180
160
120
80
40
0
3435 F13
C
FB
ESR
C
OUTPUT
C
OUT
3435fa
pin