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LT3663HMS8E#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LT3663
7
3663fc
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operaTion
The LT3663 is a constant frequency, current mode step
down regulator. A switch cycle is initiated when the os
-
cillator enables the RS flip flop, turning on the internal
power switch, Q1. The sense amplifier (A1) monitors the
switch current via the voltage dropped across the current
sense resistor R
SENSE1
. The comparator compares the
amplified current signal with the output (V
C
) of the error
amplifier (G1). The switch is turned off when this current
exceeds a value determined by the V
C
voltage. The error
amplifier monitors the V
OUT
voltage through an internal
resistor divider and, when not driven externally, servos
the V
C
voltage to regulate V
OUT
. If the V
OUT
voltage drops,
the V
C
voltage will be driven higher increasing the output
current and V
OUT
voltage. An active clamp (not shown)
on the V
C
node provides current limit. The LT3663 is
internally compensated with a pole-zero combination on
the V
C
node.
An external capacitor and internal diode, D2, generate a
voltage at the BOOST pin that is higher than the input supply.
(Refer to Block Diagram)
This allows the driver to fully saturate the internal bipolar
NPN power switch for efficient operation. The switch driver
operates from either V
IN
or BOOST to ensure startup.
An internal regulator provides power to the control circuitry.
This regulator includes input undervoltage and overvoltage
protection which disables switching action when V
IN
is
out of range. When switching is disabled, the LT3663
safely sustains input voltages up to 60V. Note that while
switching is disabled the output will discharge.
Output current limiting is provided via the servo action of
amplifier G2. The voltage across sense resistor, R
SENSE2
,
is compared to a voltage programmed by external resistor
R
ILIM
on the I
LIM
pin. A capacitor averages the inductor
ripple current. If the average inductor current exceeds the
programmed value then the V
C
voltage is pulled low, reduc-
ing the current in the regulator. The output current limit
circuit allows for a lower current rated inductor and diode
and provides better control of system power dissipation.
LT3663
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3663fc
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2.5µs/DIV
3663 F02
V
SW
20V/DIV
I
L
1A/DIV
V
OUT
200mV/DIV
AC-COUPLED
Figure 2. Pulse-Skip Mode
V
SW
20V/DIV
I
L
1A/DIV
V
OUT
200mV/DIV
AC-COUPLED
2.5µs/DIV
3663 F01
applicaTions inForMaTion
FB Resistor Network
The output voltage of the LT3663 is programmed with a
resistor divider. Choose the 1% resistors according to:
R1 = R2 (V
OUT
/0.8 – 1)
Reference designators refer to the Block Diagram.
The resistor divider is internal on the fixed output voltage
versions, LT3663-X.
Minimum Duty Cycle
As the input voltage increases the duty cycle decreases. At
some point, the minimum duty cycle of the IC is reached
and pulse skipping begins. The output voltage continues to
regulate but the output voltage ripple increases. The input
voltage at which this occurs is calculated as follows:
V
VV
DC
VV
IN
OUT F
MIN
FS
W
=
+
()
+–
Where V
F
is the forward voltage drop of the catch diode,
D1, and V
SW
is the voltage drop on the internal switch,
Q1. LT3663 DC
MIN
is typically 0.12.
For V
F
= 0.4V, V
SW
= 0.3V, and V
OUT
= 3.3V pulse skipping
occurs at about 24.5V.
Figure 1 illustrates switching waveforms in normal mode
for a 3.3V output application with V
IN
= 20V.
Figure 2. illustrates switching waveforms in pulse skip
mode a 3.3V output application with V
IN
= 30V.
Figure 1. Normal Operation
Inductor Selection
A good first choice for the inductor value is:
L = V
OUT
+ V
D
(µH)
where V
D
is the voltage drop of the catch diode (~0.4V)
and L is in µH. With this value there is no sub-harmonic
oscillation for applications with 50% or greater duty
cycle. The inductor’s RMS current rating must be greater
than the maximum load current. Size the inductor so the
saturation current rating is about 30% higher than the
maximum load current. The output current limit circuit
tightly controls the maximum average inductor current,
therefore the inductor RMS current rating does not have
to be overrated to handle short circuit or overload condi
-
tions. For high efficiency, keep the series resistance (DCR)
less than 0.1Ω. A higher value inductor is larger in size
and slows transient response but reduces output voltage
ripple. A lower value inductor has higher ripple currents
but is physically smaller or, for the same size, it has lower
DCR typically resulting in higher efficiency.
LT3663
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3663fc
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applicaTions inForMaTion
Catch Diode
The catch diode conducts current only during switch-off
time. Average forward current in normal operation is
calculated from:
II
VV
V
D AVG OUT
IN OUT
IN
()
–
=
where I
OUT
is the maximum output load current pro-
grammed by the I
LIM
resistor. Peak reverse voltage is equal
to the regulator input voltage. Use a Schottky diode with a
reverse voltage rating greater than the input voltage. The
overvoltage protection feature in the LT3663 keeps the
switch off when V
IN
> 39V (typical) allowing the use of a
45V rated Schottky even when V
IN
ranges up to 60V.
Input Capacitor
Bypass the input of the LT3663 circuit with a 1µF or higher
value ceramic capacitor of X7R or X5R type. Y5V types
have poor performance over temperature and applied volt
-
age and are not recommended. If the input power source
has high impedance, or there is significant inductance
due to long wires or cables, additional bulk capacitance
may be necessary. This can be provided with a low per
-
formance electrolytic capacitor. Step-down regulators
draw current from the input supply in pulses with very
fast rise and fall times. The input capacitor is required
to reduce the resulting voltage ripple at the LT3663 and
to force this very high frequency switching current into
a tight local loop, minimizing EMI. Place the capacitor in
close proximity to the LT3663 and the catch diode; (see
the PCB Layout section).
Output Capacitor
A 10µF or greater ceramic capacitor is recommended to
provide low output ripple and good transient response.
Ceramic capacitors have very low equivalent series re
-
sistance (ESR) and provide the best ripple performance.
Use X5R or X7R types and keep in mind that a ceramic
capacitor biased with V
OUT
has less than its specified
nominal capacitance.
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose
one that is intended for use in switching regulators. Keep
the ESR less than 0.1Ω.
I
LIM
Resistor
The LT3663 output current limit controls the maximum
current delivered from the LT3663 regulator. This allows
tighter control of the system power dissipation and also
protects the inductor and diode from overheating during
an overload or short circuit condition. A resistor con
-
nected from I
LIM
pin to GND programs the output current.
Table 1 details the I
LIM
resistor values for specific output
current limits.
Table 1. Output Current Limit vs R
LIM
Value
OUTPUT CURRENT LIMIT (A) R
ILIM
VALUE (kΩ)
0.4 140
0.6 75
0.8 48.7
1.0 36.5
1.2 28.7
There may be a reason in some applications to move the
output current limit. The following simple circuit demon
-
strates how the output current limit can be adjusted:
Figure 3. Circuit for Multiple Values of Output Current Limit
LT3663
ON, R
ILIM
= RA
OFF, R
ILIM
= RA + RB
I
LIM
3663 F03
RA
RB
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LT3663HMS8E#PBF

Mfr. #:
Buy LT3663HMS8E#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 36V (60V Transient), 1.2A (Iout), 1.5MHz Input Regulator with Output Current Limit in MSOP-8E
Lifecycle:
New from this manufacturer.
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