LTC1700_15 datasheet(9/16 Pages) LINER

Part #	LTC1700

Description	No RSENSE Synchronous Step-Up DC/DC Controller

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Manufacturer	LINER [Linear Technology]

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LTC1700

9

1700fa

load current. When the LTC1700 is operating in continu-

ous mode, the duty cycles for the MOSFETs are:

Main MOSFET Duty Cycle = 1 – VIN/VOUT

Synchronous MOSFET Duty Cycle = VIN/VOUT

The MOSFET power dissipations at maximum output

current are:

PMAIN = (1 – VIN/VOUT)(IO(MAX)2)(ρT(MAIN))(RDS(ON))

+ (k)(V0UT2)(IO(MAX))CRSS(f)

PSYNC = (VIN/VOUT)(IO(MAX)2)(ρT(BOT))(RDS(ON))

Both MOSFETs have I2R losses and the PMAIN equation

includes an additional term for transition losses, which are

largest at high output voltages. The constant k = 2.5 can be

used to estimate the amount of transition loss. The syn-

chronous MOSFET losses are greatest at high input volt-

age and low output voltage.

Start-Up Load Current

In start-up mode, the current limit is set at 60mA and the

oscillator runs at 210kHz with 50% duty cycle at

VIN = 1.8V. Since the current limit is low, the amount of

energy that is stored in the inductor during the on time is

small. Therefore the LTC1700 is incapable of supplying

the full load current. Figure 4 shows the amount of load

current the LTC1700 can provide while successfully exit-

ing out of the start-up mode. If the load current exceeds

the amount shown in Figure 4 during start-up, the output

voltage will not increase but will “hang” at a value below

the regulated voltage. However, if the load current is lower,

then there is a net positive amount of energy stored in the

output capacitor for every cycle. The output voltage then

rises and once it exceeds 2.3V, the LTC1700 will success-

fully exit out of its start-up mode.

Operating Frequency and Synchronization

The choice of operating frequency and inductor value is a

trade-off between efficiency and component size. Low

frequency operation improves efficiency by reducing

MOSFET switching losses, both gate charge loss and

transition loss. However, lower frequency operation re-

quires more inductance for a given amount of ripple

current.

The internal oscillator runs at a nominal 530kHz frequency

when the SYNC/MODE pin is either connected to GND or

VIN. When a CMOS compatible clock is applied to the

SYNC/MODE pin, the internal oscillator will lock on to the

external clock. The LTC1700 uses a novel technique to

phase lock to the external clock without the requirement of

an external PLL filter, hence minimizing components. The

capture range is between 400kHz to 750kHz. Do not

synchronize below or above the capture range as this will

cause abnormal operation. During synchronization, Burst

Mode operation is inhibited.

The LTC1700 will lock on at the leading edge of the external

clock and the minimum pulse width required is

200ns.

Remember just because you can operate at a high switch-

ing frequency doesn’t always mean you should. At higher

frequencies the switching loss increases, so the CRSS of

the N-channel MOSFET becomes very critical to keep

efficiencies high.

Slope Compensation and Peak Inductor Current

Current mode switching regulators that operate with a

duty cycle greater than 50% with continuous inductor

current can exhibit duty cycle instability. While the regu-

lator will not be damaged and may even continue to

function acceptably, a look at its frequency spectrum

will indicate harmonics. These harmonics may interfere

with other sensitive devices and will cause non-optimal

performance.

Figure 4. Start-Up Load Current

APPLICATIONS INFORMATION

VIN (V)

1.0

2.2

1700 • G04

1.4

1.8

40

35

30

25

20

15

10

5

0

1.2

1.6

2.0

2.4

A

B

C

D

E

A = 15µH

B = 10µH

C = 6.2µH

D = 4.2µH

E = 2.2µH

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