lt3663 Linear Technology Corporation, lt3663 Datasheet - Page 6

• Manufacturer Part Number: lt3663
• Description: 1.2a, 1.5mhz Step-down Switching Regulator With Output Current Limit
• Manufacturer: Linear Technology Corporation

OPERATION
LT3663
FB Resistor Network
The output voltage is programmed with a resistor divider.
Choose the 1% resistors according to:
Reference designators refer to the Block Diagram.
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:
Where V
D1, and V
Q1. DC
The LT3663 is a constant frequency, current mode step
down regulator. A switch cycle is initiated when the os-
cillator enables the RS fl ip fl op, turning on the internal
power switch, Q1. The sense amplifi er (A1) monitors the
switch current via the voltage dropped across the current
sense resistor R
amplifi ed current signal with the output (V
amplifi er (G1). The switch is turned off when this current
exceeds a value determined by the V
amplifi er monitors the V
resistor divider and, when not driven externally, servos
the V
the V
current and V
on the V
internally compensated with a pole-zero combination on
the V
An external capacitor and internal diode, D2, generate a
voltage at the BOOST pin that is higher than the input supply.
APPLICATIONS INFORMATION
6
R1 = R2 (V
V
IN
C
C
C
voltage to regulate V
=
voltage will be driven higher increasing the output
node.
MIN
F
(
C
V
is the forward voltage drop of the catch diode,
SW
is typically 0.12.
OUT
DC
node provides current limit. The LT3663 is
OUT
is the voltage drop on the internal switch,
OUT
MIN
+
SENSE1
/0.8 – 1)
V
voltage. An active clamp (not shown)
F
)
–
(Refer to Block Diagram)
V
. The comparator compares the
F
OUT
+
OUT
V
voltage through an internal
SW
. If the V
C
OUT
voltage. The error
voltage drops,
C
) of the error
This allows the driver to fully saturate the internal bipolar
NPN power switch for effi cient operation. The switch driver
operates from either V
An internal regulator provides power to the control circuitry.
This regulator includes input undervoltage and overvoltage
protection which disables switching action when V
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
amplifi er G2. The voltage across sense resistor, R
is compared to a voltage programmed by external resistor
R
ripple current. If the average inductor current exceeds the
programmed value then the V
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.
For V
occurs at about 24.5V.
Figure 1 illustrates switching waveforms in normal mode
for a 3.3V output application with V
Figure 2. illustrates switching waveforms in pulse skip
mode a 3.3V output application with V
ILIM
AC-COUPLED
F
200mV/DIV
on the I
= 0.4V, V
20V/DIV
1A/DIV
V
V
OUT
SW
I
L
LIM
SW
Figure 1. Normal Operation
pin. A capacitor averages the inductor
= 0.3V, and V
IN
or BOOST to ensure startup.
2.5μs/DIV
C
voltage is pulled low, reduc-
OUT
= 3.3V pulse skipping
IN
= 20V.
IN
= 30V.
3663 F01
IN
SENSE2
is out
3663fa
,