LTC1701B 查看數據表（PDF） - Linear Technology

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LTC1701B

1MHz Step-Down DC/DC Converters in SOT-23

Linear Technology 

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OPERATIO

The LTC1701 uses a contant off-time, current mode archi-

tecture. The operating frequency is then determined by the

off-time and the difference between VIN and VOUT.

The output voltage is set by an external divider returned to

the VFB pin. An error amplfier compares the divided output

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 VFB voltage is below the reference

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 ITH/RUN pin, which is the output of the error

amplifier.This amplifier compares the VFB pin to the 1.25V

reference. When the load current increases, the FB voltage

decreases slightly below the reference. This decrease

causes the error amplifier to increase the ITH/RUN voltage

until the average inductor current matches the new load

current.

The main control loop is shut down by pulling the ITH/RUN

pin to ground. When the pin is released an external resistor

is used to charge the compensation capacitor. When the

voltage at the ITH/RUN pin reaches 0.8V, the main control

LTC1701/LTC1701B

loop is enabled and the error amplifier drives the ITH/RUN

pin. Soft-start can be implemented by ramping the voltage

on the ITH/RUN pin (see Applications Information sec-

tion).

Low Current Operation

To optimize efficiency when the load is relatively light, the

LTC1701 automatically switches to Burst Mode operation

in which the internal PMOS switch operates intermittently

based on load demand. The main control loop is inter-

rupted when the output voltage reaches the desired regu-

lated value. The hysteretic voltage comparator trips when

ITH/RUN is below 1.5V, shutting off the switch and reduc-

ing the power consumed. The output capacitor and the

inductor supply the power to the load until the output

voltage drops slightly and the ITH/RUN pin exceeds 1.5V,

turning on the switch and the main control loop which

starts another cycle.

For reduced output ripple, the LTC1701B doesn't use

Burst Mode operation and operates continuously down to

very low currents where the part starts skipping cycles.

Dropout Operation

In dropout, the internal PMOS switch is turned on continu-

ously (100% duty cycle) providing low dropout operation

with VOUT at VIN. Since the LTC1701 does not incorporate

an under voltage lockout, care should be taken to shut

down the LTC1701 for VIN < 2.5V.

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 CIN and COUT.

L Selection and Operating Frequency

The operating frequency is fixed by VIN, VOUT and the

constant off-time of about 500ns. The complete expres-

sion for operating frequency is given by:

fO

=





VIN − VOUT

VIN + VD









1

TOFF





Although the inductor does not influence the operating

frequency, the inductor value has a direct effect on ripple

current. The inductor ripple current ∆IL decreases with

higher inductance and increases with higher VIN or VOUT:

∆IL

=





VIN

− VOUT  

fL  

VOUT + VD 

VIN + VD 

where VD is the output Schottky diode forward drop.

5

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