LT1173CN8 Linear Technology, LT1173CN8 Datasheet - Page 5

• Manufacturer Part Number: LT1173CN8
• Description: IC DC/DC CONV ADJUSTBL OUT 8-DIP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost), Invertingr

Specifications of LT1173CN8

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.25 ~ 50 V
• Current - Output: 400mA
• Frequency - Switching: 23kHz
• Voltage - Input: 2 ~ 30 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• Power - Output: 500mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

A
LT1173 OPER
The LT1173 is a gated oscillator switcher. This type archi-
tecture has very low supply current because the switch is
cycled only when the feedback pin voltage drops below the
reference voltage. Circuit operation can best be under-
stood by referring to the LT1173 block diagram. Compara-
tor A1 compares the feedback pin voltage with the 1.245V
reference voltage. When feedback drops below 1.245V, A1
switches on the 24kHz oscillator. The driver amplifier
boosts the signal level to drive the output NPN power
switch. An adaptive base drive circuit senses switch
current and provides just enough base drive to ensure
switch saturation without overdriving the switch, resulting
in higher efficiency. The switch cycling action raises the
output voltage and feedback pin voltage. When the feed-
back voltage is sufficient to trip A1, the oscillator is gated
off. A small amount of hysteresis built into A1 ensures loop
stability without external frequency compensation. When
the comparator is low the oscillator and all high current
circuitry is turned off, lowering device quiescent current
to just 110 A, for the reference, A1 and A2.
The oscillator is set internally for 23 s ON time and 19 s
OFF time, optimizing the device for circuits where V
and V
3V to 5V step-up converter or a 9V to 5V step-down
converter.
Measuring Input Current at Zero or Light Load
Obtaining meaningful numbers for quiescent current and
efficiency at low output current involves understanding
how the LT1173 operates. At very low or zero load current,
the device is idling for seconds at a time. When the output
voltage falls enough to trip the comparator, the power
switch comes on for a few cycles until the output voltage
rises sufficiently to overcome the comparator hysteresis.
When the power switch is on, inductor current builds up
to hundreds of milliamperes. Ordinary digital multimeters
are not capable of measuring average current because of
bandwidth and dynamic range limitations. A different
PPLICATI
IN
differ by roughly a factor of 2. Examples include a
O
U
ATI
S
I FOR ATIO
U
O
U
W
U
OUT
A2 is a versatile gain block that can serve as a low battery
detector, a linear post regulator, or drive an under voltage
lockout circuit. The negative input of A2 is internally
connected to the 1.245V reference. A resistor divider from
V
provides the trip voltage in a low battery detector applica-
tion. The gain block output (AO) can sink 100 A (use a 47k
resistor pull-up to + 5V). This line can signal a microcon-
troller that the battery voltage has dropped below the
preset level.
A resistor connected between the I
maximum switch current. When the switch current ex-
ceeds the set value, the switch cycle is prematurely
terminated. If current limit is not used, I
directly to V
circuitry is approximately 2 s.
In step-up mode the switch emitter (SW2) is connected to
ground and the switch collector (SW1) drives the induc-
tor; in step-down mode the collector is connected to V
and the emitter drives the inductor.
The LT1173-5 and LT1173-12 are functionally identical to
the LT1173. The -5 and -12 versions have on-chip voltage
setting resistors for fixed 5V or 12V outputs. Pin 8 on the
fixed versions should be connected to the output. No
external resistors are needed.
approach is required to measure the 100 A off-state and
500mA on-state currents of the circuit.
Quiescent current can be accurately measured using the
circuit in Figure 1. V
LT1173. The circuit must be “booted” by shorting V2 to
V
nect the short. Input voltage is V2, and average input
current can be calculated by this formula:
IN
SET
I
IN
to GND, with the mid-point connected to the SET pin
. After the LT1173 output voltage has settled, discon-
V
100
2
IN
. Propagation delay through the current limit
V
1
SET
is set to the input voltage of the
LIM
LIM
pin and V
should be tied
LT1173
IN
sets
5
01
IN