Datasheet

LT1933
7
1933fe
OPERATION
(Refer to Block Diagram)
The LT1933 is a constant frequency, current mode step
down regulator. A 500kHz oscillator enables an RS fl ip-
fl op, turning on the internal 750mA power switch Q1. An
amplifi er and comparator monitor the current fl owing
between the V
IN
and SW pins, turning the switch off when
this current reaches a level determined by the voltage at
V
C
. An error amplifi er measures the output voltage through
an external resistor divider tied to the FB pin and servos
the V
C
node. If the error amplifi er’s output increases, more
current is delivered to the output; if it decreases, less cur-
rent is delivered. An active clamp (not shown) on the V
C
node provides current limit. The V
C
node is also clamped
to the voltage on the SHDN pin; soft-start is implemented
by generating a voltage ramp at the SHDN pin using an
external resistor and capacitor.
An internal regulator provides power to the control cir-
cuitry. This regulator includes an undervoltage lockout
to prevent switching when V
IN
is less than ~3.35V. The
SHDN pin is used to place the LT1933 in shutdown, dis-
connecting the output and reducing the input current to
less than 2µA.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate
the internal bipolar NPN power switch for effi cient opera-
tion.
The oscillator reduces the LT1933’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during startup
and overload.
APPLICATIONS INFORMATION
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
R1 = R2(V
OUT
/1.245 – 1)
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Input Voltage Range
The input voltage range for LT1933 applications depends
on the output voltage and on the absolute maximum rat-
ings of the V
IN
and BOOST pins.
The minimum input voltage is determined by either the
LT1933’s minimum operating voltage of ~3.35V, or by its
maximum duty cycle. The duty cycle is the fraction of
time that the internal switch is on and is determined by
the input and output voltages:
DC = (V
OUT
+ V
D
)/(V
IN
– V
SW
+ V
D
)
where V
D
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
(~0.4V at maximum load). This leads to a minimum input
voltage of:
V
IN(MIN)
= (V
OUT
+ V
D
)/DC
MAX
– V
D
+ V
SW
with DC
MAX
= 0.88
The maximum input voltage is determined by the absolute
maximum ratings of the V
IN
and BOOST pins and by the
minimum duty cycle DC
MIN
= 0.08 (corresponding to a
minimum on time of 130ns):
V
IN(MAX)
= (V
OUT
+ V
D
)/DC
MIN
– V
D
+ V
SW
Note that this is a restriction on the operating input voltage;
the circuit will tolerate transient inputs up to the absolute
maximum ratings of the V
IN
and BOOST pins.
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
L = 5 (V
OUT
+ V
D
)
where V
D
is the voltage drop of the catch diode (~0.4V)
and L is in µH. With this value the maximum load current
will be above 500mA. The inductor’s RMS current rating
must be greater than your maximum load current and its
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