LTC1700EMS 查看數據表（PDF） - Linear Technology

零件编号

产品描述 (功能)

生产厂家

LTC1700EMS

No RSENSE Synchronous Step-Up DC/DC Controller

Linear Technology 

LTC1700

APPLICATIONS INFORMATION

The efficiency of a switching regulator is equal to the

output power divided by the input power (× 100%).

Percent efficiency can be expressed as:

% Efficiency = 100%–(L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percentage

of input power. It is often useful to analyze individual

losses to determine what is limiting the efficiency and

which change would produce the most improvement.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1700 circuits:

1. LTC1700 supply current. This DC supply current, given

in the electrical characteristics, excludes MOSFET drivers

and control current. This supply current results in a small

loss which increases with VOUT.

2. MOSFETs gate charge current results from switching

the gate capacitance of the power MOSFETs. Each time a

MOSFET gate is switched on and then off, a packet of gate

charge Qg moves from VOUT to ground. The resulting

current out from VOUT is typically much larger than the

control circuit current. In continuous mode, IGATECHG =

f(Qg(TOP) + Qg(BOT)). At high switching frequencies, this

loss becomes increasingly important.

3. DC I2R Losses. Since there is no sense resistor needed,

DC I2R losses arise only from the resistances of the

MOSFETs and inductor. In continuous mode, the average

current flows through the inductor but is “chopped”

between the synchronous P-channel MOSFET and the

main N-channel MOSFET. If the two MOSFETs have ap-

proximately the same RDS(ON), then the resistance of one

MOSFET can simply be summed with the resistance of the

inductor to obtain the DC I2R loss. For example, if each

RDS(ON) = 0.05Ω and RL = 0.15Ω, then the total resistance

is 0.2Ω. This results in losses ranging from 2% to 8% as

the output current increases from 0.5A to 2A for a 5V

output. I2R losses cause the efficiency to drop at high

output currents.

4. Transition losses apply to the main external MOSFET

and increase at higher operating frequencies and output

voltages. Transition losses can be estimated from:

Transition Loss = 2.5(VOUT)2IO(MAX)CRSS(f)

Other losses including CIN and COUT ESR dissipative

losses, and inductor core losses, generally account for

less than 2% total loss.

Run/Soft-Start Function

The RUN/SS pin is a dual purpose pin that provides the

soft-start function and a means to shut down the LTC1700.

Soft-start reduces input surge current from VIN by gradu-

ally increasing the internal current limit. Power supply

sequencing can also be accomplished using this pin.

An internal 3.8µA current source charges up an external

capacitor CSS. When the voltage on the RUN/SS pin

reaches 0.7V, the LTC1700 begins operating. As the

voltage on RUN/SS continues to ramp from 0.7V to 1V, the

internal current limit is also ramped at a proportional linear

rate. The current limit begins near 0A (at VRUN/SS = 0.7V)

and ends at 0.078/RDS(ON) (VRUN/SS ≈ 2.2V). The output

current thus ramps up slowly, reducing the starting surge

current required from the input power supply. If the RUN/

SS has been pulled all the way to ground, there will be a

delay before the current limit starts increasing and is given

by:

tDELAY = 1.13CSS/ICHG

For input voltages less than 2.3V during the start-up

duration, the soft-start function has no effect on the

internal 60mA current limit. Therefore to fully take advan-

tage of this feature, the soft-start capacitor has to be sized

accordingly to account for the time it takes VOUT to reach

2.3V. An approximate mathematical representation for the

time it takes VOUT to reach 2.3V upon powering up is given

by:

tPOWER−UP

=

COUT (2.3 –

260(L)

2.3 – VIN

VIN – VD)

– IOUT

1700fa

12

Share Link: