LT1304-5.0V UTC, LT1304-5.0V Datasheet - Page 6

• Manufacturer Part Number: LT1304-5.0V
• Description: MICROPOWER DC/DC CONVERTERS
• Manufacturer: UTC

UTC LT1304/LT1304-3.3V/LT1304-5.0V
OPERATION
The LT1304 operation can best be understood by examining the block diagram in Figure 1.Comparator A1monitors
the output voltage via resistor divider string R3/R4 at the FB pin. When V
and the timers are turned off. Only the reference, A1 and A3 consume current, typically 120µA.As V
1.24V plus A1’s hysteresis (about 6mV),A1 enables the rest of the circuit. Power switch Q1 is then cycled on for 6µs,
or until current comparator A2 turns off the ON timer, Whichever comes first. Off-time is fixed at approximately 1.5µs.
Q1’s switching cause current to alternately build up in inductor L1 and discharge into output capacitor C2 via D1,
increasing the output voltage .As V
left to supply current to the load until V
If switch current reaches 1A,causing A2 to trip, switch ON time is reduced. This allows continuous mode operation
during bursts.A2 monitors the voltage across 7.2 Ω resistor R1,which is directly related to the switch current.Q2’s
collector current is set by the emitter-area ratio to 0.5% of Q1’s collector current. R1’s voltage drop exceeds
36mV,corresponding to 1A switch current,A2’s output goes high ,truncating the ON time part of the switch cycle. The
1A peak current can be reduced by tying a resistor between the I
appear across R2.The drop offsets some of the 36mV reference voltage, lowering peak current. A 22K resistor limits
current to approximately 550mA.A capacitor connected between I
accomplished by grounding the SHDN pin.
The low-battery detector A3 has its own 1.17V reference and is always on. The open collector output device can sink
up to 500µA.Approximately 35mV of hysteresis is built into A3 to reduce ”buzzing” as the battery voltage reaches the
trip level.
INDUCTOR SELECTION
Inductors used with the LT1304 must be capable of handling the worst-case peak switch current of 1.2A
without saturating. Open flux rod or drum core units may be biased into saturation by 20% with only a small
reduction in efficiency. For the majority of 2-cell or 3-cell input LT1304 applications, a 22
as the Sumida CD54-220 (drum) or Coiltronics CTX20-1 (toroid) will suffice. If switch current is reduced using the
ILIM pin, smaller inductors such as the Sumida CD43 series or Coilcraft DO1608 series can be used. Minimizing
UTC
Figure 2. LT1304-3.3/LT1304-5.0 Block Diagram
8
R4=355K
R4=195K
UNISONIC TECHNOLOGIES CO., LTD.
590K
R4
R3
(LT1304-3.3V)
(LT1304-5.0V)
LBI
1
FB
1.17V
1.24V
VREF
LINEAR INTEGRATED CIRCUIT
increases enough to overcome C1’s hysteresis, switching action ceases. C2 is
+
+
-
-
OUT
A1
2
decreases enough to force A1’s output high, and the entire cycle repeats.
A3
LBO
ENABLE
1.5μs OFF
6μs ON
TIMERS
UNDERVOLTA
GELOCKOUT
OFF
7
1.5V
SHDN
LIM
LIM
SHUTDOWN
BIAS
-1V
pin and ground, causing a voltage drop to
and ground provides soft start. Shutdown is
DRIVER
A2
FB
+
-
36mV
is higher than the 1.24V reference,A2
6
R2
Q3
1K
1K
ILIM
3
Q2
7.2 Ω
×1
R1
VIN
µ
H or 20
4
5
×200
Q1
GND
SW
µ
H inductor such
FB
drops below
QW-R103-018,A
6