LT1933ES6#PBF Linear Technology, LT1933ES6#PBF Datasheet - Page 7

• Manufacturer Part Number: LT1933ES6#PBF
• Description: IC, STEP-DOWN REGULATOR, 6-TSOT-23
• Manufacturer: Linear Technology

Specifications of LT1933ES6#PBF

• Primary Input Voltage: 36V
• No. Of Outputs: 1
• Output Current: 600mA
• No. Of Pins: 6
• Operating Temperature Range: -40°C To +85°C
• Supply Voltage Range: 3.6V To 36V
• Switching Frequency Max: 600kHz
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

OPERATION
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
this current reaches a level determined by the voltage at
V
an external resistor divider tied to the FB pin and servos
the V
current is delivered to the output; if it decreases, less cur-
rent is delivered. An active clamp (not shown) on the V
node provides current limit. The V
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.
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:
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
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:
where V
(~0.4V) and V
C
. An error amplifi er measures the output voltage through
R1 = R2(V
DC = (V
C
node. If the error amplifi er’s output increases, more
D
is the forward voltage drop of the catch diode
OUT
IN
OUT
SW
IN
+ V
and BOOST pins.
and SW pins, turning the switch off when
/1.245 – 1)
is the voltage drop of the internal switch
D
)/(V
IN
(Refer to Block Diagram)
– V
SW
+ V
C
node is also clamped
D
)
C
An internal regulator provides power to the control cir-
cuitry. This regulator includes an undervoltage lockout
to prevent switching when V
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.
(~0.4V at maximum load). This leads to a minimum input
voltage of:
with DC
The maximum input voltage is determined by the absolute
maximum ratings of the V
minimum duty cycle DC
minimum on time of 130ns):
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
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
where V
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
V
V
L = 5 (V
IN(MIN)
IN(MAX)
MAX
D
is the voltage drop of the catch diode (~0.4V)
OUT
= (V
= (V
= 0.88
+ V
OUT
OUT
D
)
+ V
+ V
D
D
)/DC
MIN
)/DC
IN
IN
and BOOST pins.
and BOOST pins and by the
MAX
= 0.08 (corresponding to a
MIN
IN
is less than ~3.35V. The
– V
– V
D
D
+ V
+ V
SW
SW
LT1933
1933fe
7