LT1308A/LT1308B
APPLICATIONS INFORMATION
OPERATION
The LT1308A combines a current mode, fixed frequency
PWM architecture with Burst Mode micropower operation
to maintain high efficiency at light loads. Operation can be
best understood by referring to the block diagram in Figure
2. Q1 and Q2 form a bandgap reference core whose loop
is closed around the output of the converter. When V
IN
is
1V, the feedback voltage of 1.22V, along with an 80mV
drop across R5 and R6, forward biases Q1 and Q2’s base
collector junctions to 300mV. Because this is not enough
to saturate either transistor, FB can be at a higher voltage
than V
IN
. When there is no load, FB rises slightly above
1.22V, causing V
C
(the error amplifier’s output) to
decrease. When V
C
reaches the bias voltage on hysteretic
comparator A1, A1’s output goes low, turning off all
circuitry except the input stage, error amplifier and low-
battery detector. Total current consumption in this state is
140μA. As output loading causes the FB voltage to
decrease, A1’s output goes high, enabling the rest of the
IC. Switch current is limited to approximately 400mA
initially after A1’s output goes high. If the load is light, the
output voltage (and FB voltage) will increase until A1’s
output goes low, turning off the rest of the LT1308A. Low
frequency ripple voltage appears at the output. The ripple
frequency is dependent on load current and output capaci-
tance. This Burst Mode operation keeps the output regu-
lated and reduces average current into the IC, resulting in
high efficiency even at load currents of 1mA or less.
If the output load increases sufficiently, A1’s output
remains high, resulting in continuous operation. When the
LT1308A is running continuously, peak switch current is
controlled by V
C
to regulate the output voltage. The switch
is turned on at the beginning of each switch cycle. When
the summation of a signal representing switch current and
a ramp generator (introduced to avoid subharmonic oscil-
lations at duty factors greater than 50%) exceeds the V
C
signal, comparator A2 changes state, resetting the flip-flop
and turning off the switch. Output voltage increases as
switch current is increased. The output, attenuated by a
resistor divider, appears at the FB pin, closing the overall
loop. Frequency compensation is provided by an external
series RC network connected between the V
C
pin and
ground.
Low-battery detector A4’s open-collector output (LBO)
pulls low when the LBI pin voltage drops below 200mV.
There is no hysteresis in A4, allowing it to be used as an
amplifier in some applications. The entire device is dis-
abled when the SHDN pin is brought low. To enable the
converter, SHDN must be at 1V or greater. It need not be
tied to V
IN
as on the LT1308.
The LT1308B differs from the LT1308A in that there is no
hysteresis in comparator A1. Also, the bias point on A1 is
set lower than on the LT1308B so that switching can occur
at inductor current less than 100mA. Because A1 has no
hysteresis, there is no Burst Mode operation at light loads
and the device continues switching at constant frequency.
This results in the absence of low frequency output voltage
ripple at the expense of efficiency.
The difference between the two devices is clearly illus-
trated in Figure 3. The top two traces in Figure 3 shows an
LT1308A/LT1308B circuit, using the components indi-
cated in Figure 1, set to a 5V output. Input voltage is 3V.
Load current is stepped from 50mA to 800mA for both
circuits. Low frequency Burst Mode operation voltage
ripple is observed on Trace A, while none is observed on
Trace B.
At light loads, the LT1308B will begin to skip alternate
cycles. The load point at which this occurs can be de-
creased by increasing the inductor value. However, output
ripple will continue to be significantly less than the LT1308A
output ripple. Further, the LT1308B can be forced into
micropower mode, where I
Q
falls from 3mA to 200μA by
sinking 40μA or more out of the V
C
pin. This stops
switching by causing A1’s output to go low.
8
U
W
U
U
TRACE A: LT1308A
V
OUT
, 100mV/DIV
AC COUPLED
TRACE B: LT1308B
V
OUT
, 100mV/DIV
AC COUPLED
800mA
I
LOAD
50mA
V
IN
= 3V
200μs/DIV
(CIRCUIT OF FIGURE 1)
1308 F03
Figure 3. LT1308A Exhibits Burst Mode Operation Output
Voltage Ripple at 50mA Load, LT1308B Does Not
1308abfa
LINER [ LINEAR TECHNOLOGY ]
