LTC1627 Просмотр технического описания (PDF) - Linear Technology

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LTC1627

Monolithic Synchronous Step-Down Switching Regulator

Linear Technology 

U

OPERATIO

load slightly allows constant frequency PWM operation

to resume.

Frequency synchronization is inhibited when the feedback

voltage VFB is below 0.6V. This prevents the external clock

from interfering with the frequency foldback for short-

circuit protection.

Dropout Operation

When the input supply voltage decreases toward the out-

put voltage, the duty cycle increases toward the maximum

on-time. Further reduction of the supply voltage forces the

main switch to remain on for more than one cycle until it

reaches 100% duty cycle. The output voltage will then be

determined by the input voltage minus the voltage drop

across the P-channel MOSFET and the inductor.

In Burst Mode operation or pulse skipping mode operation

(externally synchronized) with the output lightly loaded,

the LTC1627 transitions through continuous mode as it

enters dropout.

Undervoltage Lockout

A precision undervoltage lockout shuts down the LTC1627

when VIN drops below 2.5V, making it ideal for single

lithium-ion battery applications. In lockout, the LTC1627

draws only several microamperes, which is low enough to

prevent deep discharge and possible damage to the lithium-

ion battery nearing its end of charge. A 150mV hysteresis

ensures reliable operation with noisy supplies.

Low Supply Operation

The LTC1627 is designed to operate down to 2.65V supply

voltage. At this voltage the converter is most likely to be

running at high duty cycles or in dropout where the main

switch is on continuously. Hence, the I2R loss is due

mainly to the RDS(ON) of the P-channel MOSFET. See

Efficiency Considerations in the Applications Information

section.

When VIN is low (< 4.5V) the RDS(ON) of the P-channel

MOSFET can be lowered by driving its gate below ground.

The top P-channel MOSFET driver makes use of a floating

return pin, VDR, to allow biasing below GND. A simple

charge pump bootstrapped to the SW pin realizes a

negative voltage at the VDR pin as shown in Figure 2. Using

LTC1627

VDR

LTC1627 VIN

SW

C1

D1 L1

0.1µF

VIN < 4.5V

C2

0.1µF

+

VOUT

COUT

100µF

D2

1627 F02

Figure 2. Using a Charge Pump to Bias VDR

the charge pump at VIN ≥ 4.5V is not recommended to

ensure that (VIN – VDR) does not exceed its absolute

maximum voltage.

When VIN decreases to a voltage close to VOUT, the loop

may enter dropout and attempt to turn on the P-channel

MOSFET continuously. When the VDR charge pump is

enabled, a dropout detector counts the number of oscilla-

tor cycles that the P-channel MOSFET remains on, and

periodically forces a brief off period to allow C1 to

recharge. 100% duty cycle is allowed when VDR is grounded.

Slope Compensation and Inductor Peak Current

Slope compensation provides stability by preventing

subharmonic oscillations. It works by internally adding a

ramp to the inductor current signal at duty cycles in excess

of 40%. As a result, the maximum inductor peak current

is lower for VOUT/ VIN > 0.4 than when VOUT/VIN < 0.4. See

the inductor peak current as a function of duty cycle graph

in Figure 3. The worst-case peak current reduction occurs

with the oscillator synchronized at its minimum frequency,

i.e., to a clock just above the oscillator free-running

950

900

WITHOUT

EXTERNAL

850

CLOCK SYNC

800

750

WORST CASE

EXTERNAL

700

CLOCK SYNC

650

600

550

VIN = 5V

500

0 10 20 30 40 50 60 70 80 90 100

DUTY CYCLE (%)

1627 F03

Figure 3. Maximum Inductor Peak Current vs Duty Cycle

7

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