Datasheet

LTC3526L/LTC3526LB
8
3526lfc
exceeds the input by 0.24V, the IC powers itself from
V
OUT
instead of V
IN
. At this point the internal circuitry has
no dependency on the V
IN
input voltage, eliminating the
requirement for a large input capacitor. The input voltage
can drop as low as 0.5V. The limiting factor for the ap
-
plication becomes
the availability of the power source to
supply sufficient energy to the output at low voltages, and
maximum duty cycle, which is clamped at 90% typical.
Note that at low input voltages, small voltage drops due
to series resistance become critical, and greatly limit the
power delivery capability of the converter.
LOW NOISE FIXED FREQUENCY OPERATION
Soft-Start
The L
TC3526L/LTC3526LB contain internal circuitry to
provide soft-start operation. The soft-start circuitry slowly
ramps the peak inductor current from zero to its peak value
of 750mA (typical) in approximately 0.5ms, allowing start-
up into heavy loads. The soft-start circuitry is reset in the
event of a shutdown command or a thermal shutdown.
Oscillator
An internal
oscillator sets the switching frequency to
1MHz.
Shutdown
Shutdown is accomplished by pulling the SHDN pin
below
0.3V and enabled by pulling the SHDN pin above
0.8V. Although SHDN can be
driven above V
IN
or V
OUT
(up to the absolute maximum rating) without damage,
the LTC3526L/LTC3526LB have a proprietary test mode
that may be engaged if
SHDN is held in the range of 0.5V
to 1V higher than the greater of V
IN
or V
OUT
. If the test
mode is engaged, normal PWM switching action is inter-
rupted, which
can cause undesirable operation in some
applications. Therefore, in applications where
SHDN may
be
driven above V
IN
, a resistor divider or other means
must be employed to keep the SHDN voltage below
(V
IN
+ 0.4V) to prevent the possibility of the test mode being
engaged. Please refer to Figure 1 for two possible imple
-
mentations.
Error Amplifier
The
positive
input of the transconductance error amplifier
is internally connected to the 1.195V reference and the
negative input is connected to FB. Clamps limit the mini
-
mum and maximum error amp output voltage for improved
large-signal transient response. Power converter control
loop compensation is provided internally. An external
resistive voltage divider from V
OUT
to ground programs
the output voltage via FB from 1.5V to 5.25V.
V V
R
R
OUT
= +






1 195 1
2
1
. •
Current Sensing
Lossless current sensing converts the peak current signal of
the N-channel MOSFET switch into a voltage that is summed
with the internal slope compensation. The summed signal
is compared to the error amplifier output to provide a peak
current control command for the PWM.
Current Limit
The current limit comparator shuts off the N-channel
MOSFET switch once its threshold is reached. The cur
-
rent limit
comparator delay to output is typically 60ns.
Peak switch current is limited to approximately 750mA,
independent of input or output voltage, unless V
OUT
falls
below 0.7V, in which case the current limit is cut in half.
Zero Current Comparator
The zero
current comparator monitors the inductor cur-
rent to
the output and shuts off the synchronous rectifier
(Refer to Block Diagram)
operAtion
ZETEX ZC2811E
3526l F01
LTC3526L/LTC3526LB
SHDN
V
IN
V
CNTRL
4M
±30%
LTC3526L/LTC3526LB
SHDN
4M
±30%
1M
R
V
CNTRL
1M
R > (V
CNTRL
/(V
IN
+ 0.4) – 1)MΩ
Figure 1. Recommended Shutdown Circuits when Driving
SHDN above V
IN