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LT3460ES5#TRPBF

P1-P3P4-P6P7-P9P10-P12
LT3460/LT3460-1
4
3460fb
(ThinSOT/SC70/DFN Packages)
SW (Pin 1/Pin 1/Pin 3): Switch Pin. Connect inductor/diode
here. Minimize trace at this pin to reduce EMI.
GND (Pin 2/Pins 2 and 5/Exposed Pad Pin 7): Ground
Pin. Tie directly to local ground plane.
FB (Pin 3/Pin 3/Pin 1): Feedback Pin. Reference
voltage is 1.255V. Connect resistor divider tap here.
Minimize trace area at FB. Set V
OUT
according to
V
OUT
= 1.255V (1 + R1/R2).
SHDN (Pin 4/Pin 4/Pin 6): Shutdown Pin. Tie to 1.5V or
higher to enable device; 0.4V or less to disable device.
Also functions as soft-start. Use RC fi lter (47k, 47nF typ)
as shown in Figure 1.
V
IN
(Pin 5/Pin 6/Pin 4): Input Supply Pin. Must be locally
bypassed.
NC (NA/NA/Pins 2, 5): No-Connects. These pins are not
connected to internal circuitry. They should be tied to
ground to improve thermal and electrical performance.
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Bias Current Feedback Voltage
TEMPERATURE (°C)
–50
15
20
25
25 75
3460 G04
10
5
–25 0
50 100
0
FEEDBACK BIAS CURRENT (nA)
30
TEMPERATURE (°C)
–50
1.255
25 75
3460 G05
1.250
1.245
–25 0
50 100
1.240
V
FB
(V)
1.260
Switch Saturation Voltage
(V
CESAT
) Current Limit vs Duty Cycle
TEMPERATURE (°C)
–50
250
300
350
25 75
3460 G06
200
150
100
–25 0
50 100
50
0
V
CESAT
(mV)
400
(LT3460)
(LT3460)
(LT3460)
(LT3460-1)
I
C
= 100mA
I
C
= 200mA
I
C
= 250mA
I
C
= 50mA
DUTY CYCLE
0
250
300
350
0.6
3460 G07
200
150
100
0.2 0.4
0.8 1.0
50
0
CURRENT LIMIT (mA)
450
400
LT3460
LT3460-1
PIN FUNCTIONS
LT3460/LT3460-1
5
3460fb
BLOCK DIAGRAM
Figure 1. Block Diagram, LT3460
–
+
–
+
–
+
V
OUT
R1 (EXTERNAL)
R2 (EXTERNAL)
R
S
(EXTERNAL)
C
S
(EXTERNAL)
FB
SHUTDOWN
SHDN
RAMP
GENERATOR
1.255V
REFERENCE
1.3MHz
OSCILLATOR
R
S
Q
A1
A2
COMPARATOR
DRIVER
R
C
C
C
SW
Q1
0.1Ω
GND
V
IN
FB
3460 BD
R
S
, C
S
OPTIONAL SOFT-START COMPONENTS
OPERATION
The LT3460/LT3460-1 uses a constant frequency, current
mode control scheme to provide excellent line and load
regulation. Operation can be best understood by referring to
the block diagram in Figure 1. At the start of each oscillator
cycle, the SR latch is set, which turns on the power switch
Q1. A voltage proportional to the switch current is added
to a stabilizing ramp and the resulting sum is fed into the
positive terminal of the PWM comparator A2. When this
voltage exceeds the level at the negative input of A2, the
SR latch is reset turning off the power switch. The level at
the negative input of A2 is set by the error amplifi er A1, and
is simply an amplifi ed version of the difference between
the feedback voltage and the reference voltage of 1.255V.
In this manner, the error amplifi er sets the correct peak
current level to keep the output in regulation. If the error
amplifi er’s output increases, more current is delivered to
the output; if it decreases, less current is delivered.
Feedback Loop Compensation
The LT3460/LT3460-1 has an internal feedback compensa-
tion network as shown in Figure 1 (R
C
and C
C
). However,
because the small signal characteristics of a boost converter
change with operation conditions, the internal compensa-
tion network cannot satisfy all applications. A properly
designed external feed forward capacitor from V
OUT
to
FB (C
F
in Figure 2) will correct the loop compensation for
most applications.
The LT3460/LT3460-1 uses peak current mode control.
The current feedback makes the inductor very similar
to a current source in the medium frequency range. The
power stage transfer function in the medium frequency
range can be approximated as:
G
P(s)
=
K1
s•C2
,
where C2 is the output capacitance, and K1 is a constant
based on the operating point of the converter. In continuous
current mode, K1 increases as the duty cycle decreases.
The internal compensation network R
C
, C
C
can be ap-
proximated as follows in medium frequency range:
G
C(s)
=K2 •
s•R
C
•C
C
+ 1
s•C
C
The zero
f
Z
=
1
2• •R
C
•C
C
is about 70kHz.
LT3460/LT3460-1
6
3460fb
OPERATION
Figure 3
FREQUENCY (kHz)
PHASE
GAIN
1
GAIN (dB)
60
50
40
30
20
PHASE (DEG)
10
0
–10
–20
–30
–40
90
45
0
–45
–90
–135
–180
–225
–270
–315
–360
10 100 1000
3460 F03
Figure 4
60
50
40
30
20
10
0
–10
–20
–30
–40
PHASE (DEG)
90
45
0
–45
–90
–135
–180
–225
–270
–315
–360
FREQUENCY (kHz)
PHASE
GAIN
1
GAIN (dB)
10 100 1000
3460 F04
The feedback loop gain T(s) = K3 • G
P
(s) • G
C
(s). If it
crosses over 0dB far before f
Z
, the phase margin will be
small. Figure 3 is the Bode plot of the feedback loop gain
measured from the converter shown in Figure 2 without
the feedforward capacitor C
F
. The result agrees with
the previous discussion: Phase margin of about 20° is
insuffi cient.
In order to improve the phase margin, a feed-forward
capacitor C
F
in Figure 2 can be used.
Without the feed-forward capacitor, the transfer function
from V
OUT
to FB is:
FB
V
OUT
=
R1
R1+ R2
With the feed-forward capacitor C
F
, the transfer function
becomes:
FB
V
OUT
=
R1
R1+ R2
•
s•R2•C
F
+ 1
s•
R1• R2
R1+ R
2
•C
F
+ 1
The feed-forward capacitor C
F
generates a zero and a pole.
The zero always appears before the pole. The frequency
distance between the zero and the pole is determined
only by the ratio between V
OUT
and FB. To give maximum
phase margin, C
F
should be chosen so that the midpoint
frequency between the zero and the pole is at the cross
over frequency.
With C
F
= 20pF, the feedback loop Bode plot is reshaped
as shown in Figure 4. The phase margin is about 60°.
Figure 2. 5V to 12V Step-Up Converter
OFF ON
V
IN
5V
L1
22μH
D1
R2
130k
R1
15k
C2
1μF
C
F
22pF
C1
4.7μF
V
OUT
12V
70mA
51
3
2
4
V
IN
SW
FB
SHDN
GND
LT3460
C1: TAIYO YUDEN X5R JMK212BJ475KG
C2: TAIYO YUDEN X5R EMK316BJ105
D1: CENTRAL SEMICONDUCTOR CMDSH2-3
L1: MURATA LQH32CN-220 OR EQUIVALENT
3460 F02
The feed-forward capacitor increases the gain at high
frequency. The feedback loop therefore needs to have
enough attenuation at the switching frequency to reject
the switching noise. Additional internal compensation
components have taken this into consideration.
For most of the applications of LT3460/LT3460-1, the
output capacitor ESR zero is at very high frequency and
can be ignored. If a low frequency ESR zero exists, for
example, when a high-ESR Tantalum capacitor is used at
the output, the phase margin may be enough even without
a feed-forward capacitor. In these cases, the feed-forward
capacitor should not be added because it may cause
the feedback loop to not have enough attenuation at the
switching frequency.
P1-P3P4-P6P7-P9P10-P12

LT3460ES5#TRPBF

Mfr. #:
Buy LT3460ES5#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 300mA, 1.3MHz Boost Converter
Lifecycle:
New from this manufacturer.
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