LTC1701 LINER [Linear Technology], LTC1701 Datasheet - Page 4

• Manufacturer Part Number: LTC1701
• Description: 1MHz Step-Down DC/DC Converter in SOT-23
• Manufacturer: LINER [Linear Technology]

OPERATIO
LTC1701
The LTC1701 uses a contant off-time, current mode
architecture. The operating frequency is then determined
by the off-time and the difference between V
To optimize efficiency, the LTC1701 automatically switches
between continuous and Burst Mode operation.
The output voltage is set by an external divider returned to
the V
voltage with a reference voltage of 1.25V and adjusts the
peak inductor current accordingly.
Main Control Loop
During normal operation, the internal PMOS switch is
turned on when the V
voltage. The current into the inductor and the load in-
creases until the current limit is reached. The switch turns
off and energy stored in the inductor flows through the
external Schottky diode into the load. After the constant
off-time interval, the switch turns on and the cycle repeats.
The peak inductor current is controlled by the voltage on
the I
amplifier.This amplifier compares the V
reference. When the load current increases, the FB voltage
decreases slightly below the reference. This decrease
causes the error amplifier to increase the I
until the average inductor current matches the new load
current.
APPLICATIO S I FOR ATIO
The basic LTC1701 application circuit is shown in
Figure 1. External component selection is driven by the
load requirement and begins with the selection of L1. Once
L1 is chosen, the Schottky diode D1 can be selected
followed by C
L Selection and Operating Frequency
The operating frequency is fixed by V
constant off-time of about 500ns. The complete expres-
sion for operating frequency is given by:
4
FB
TH
pin. An error amplfier compares the divided output
/RUN pin, which is the output of the error
IN
and C
U
U
OUT
FB
.
voltage is below the reference
U
W
FB
IN
pin to the 1.25V
TH
, V
/RUN voltage
IN
OUT
and V
U
and the
OUT
.
The main control loop is shut down by pulling the I
pin to ground. When the pin is released an external resistor
is used to charge the compensation capacitor. When the
voltage at the I
loop is enabled and the error amplifier drives the I
pin. Soft-start can be implemented by ramping the voltage
on the I
tion).
Low Current Operation
When the load is relatively light, the LTC1701 automati-
cally switches to Burst Mode operation in which the
internal PMOS switch operates intermittently based on
load demand. The main control loop is interrupted when
the output voltage reaches the desired regulated value.
The hysteretic voltage comparator trips when I
below 1.5V, shutting off the switch and reducing the
power consumed. The output capacitor and the inductor
supply the power to the load until the output voltage drops
slightly and the I
switch and the main control loop which starts another
cycle.
Dropout Operation
In dropout, the internal PMOS switch is turned on continu-
ously (100% duty cycle) providing low dropout operation
with V
an under voltage lockout, care should be taken to shut
down the LTC1701 for V
Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current I
higher inductance and increases with higher V
f
O
I
L
OUT
TH
V
at V
IN
/RUN pin (see Applications Information sec-
V
V
IN
IN
IN
TH
V
fL
. Since the LTC1701 does not incorporate
OUT
V
TH
/RUN pin reaches 0.8V, the main control
V
D
OUT
/RUN pin exceeds 1.5V, turning on the
T
IN
OFF
V
1
V
OUT
< 2.5V.
IN
V
V
D
D
L
decreases with
IN
TH
or V
TH
TH
/RUN is
/RUN
/RUN
OUT
: