LM2735升压&SEPIC直流稳压器技术详解

LM2735

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1.6 MHz



Thermal

SHDN

LIMIT

NMOS

UVLO = 2.3V

SENSE-AMP

Internal

Compensation

Soft-Start

Corrective - Ramp

Oscillator

Control Logic

REF

= 1.255V

GND

LM2735

www.ti.com.cn

ZHCSHW2I –JUNE 2007–REVISED SEPTEMBER 2018

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Current Limit

The LM2735 uses cycle-by-cycle current limiting to protect the internal NMOS switch. It is important to note that

this current limit will not protect the output from excessive current during an output short circuit. The input supply

is connected to the output by the series connection of an inductor and a diode. If a short circuit is placed on the

output, excessive current can damage both the inductor and diode.

7.3.2 Thermal Shutdown

Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature

exceeds 160°C. After thermal shutdown occurs, the output switch does not turn on until the junction temperature

drops to approximately 150°C.

7.3.3 Soft Start

This function forces V

OUT

to increase at a controlled rate during start-up. During soft start, the reference voltage

of the error amplifier ramps to its nominal value of 1.255 V in approximately 4 ms. This forces the regulator

output to ramp up in a more linear and controlled fashion, which helps reduce inrush current.

7.3.4 Compensation

The LM2735 uses constant-frequency peak current mode control. This mode of control allows for a simple

external compensation scheme that can be optimized for each application. A complicated mathematical analysis

can be completed to fully explain the internal and external compensation of the LM2735, but for simplicity, a

graphical approach with simple equations will be used. Below is a Gain and Phase plot of a LM2735 that

produces a 12-V output from a 5-V input voltage. The Bode plot shows the total loop Gain and Phase without

external compensation.

( )

Load

Lx2S

RHP

ZERO

CFPOLE

2S((R

) x C

)

( )

CFZERO-

LOAD

Load

OUT

)2S

RCP-

LM2735

www.ti.com.cn

ZHCSHW2I –JUNE 2007–REVISED SEPTEMBER 2018

Feature Description (continued)

Lower output voltages will have the zero set closer to 10 kHz, and higher output voltages will usually have the

zero set closer to 5 kHz. TI recommends obtaining a Gain and Phase plot for your actual application. See

Application and Implementation to obtain examples of working applications and the associated component

values.

Pole at origin due to internal GM amplifier:

P-ORIGIN

(3)

Pole due to output load and capacitor:

(4)

This equation only determines the frequency of the pole for perfect current mode control (CMC). That is, it

doesn’t take into account the additional internal artificial ramp that is added to the current signal for stability

reasons. By adding artificial ramp, you begin to move away from CMC to voltage mode control (VMC). The

artifact is that the pole due to the output load and output capacitor will actually be slightly higher in frequency

than calculated. In this example, it is calculated at 650 Hz, but in reality, it is around 1 kHz.

The zero created with capacitor C3 & resistor R2:

Figure 17. Setting External Pole-Zero

(5)

There is an associated pole with the zero that was created in the above equation.

(6)

It is always higher in frequency than the zero.

A right-half plane zero (RHPZ) is inherent to all boost converters. One must remember that the gain associated

with a right-half plane zero increases at 20 dB per decade, but the phase decreases by 45° per decade. For

most applications there is little concern with the RHPZ due to the fact that the frequency at which it shows up is

well beyond crossover, and has little to no effect on loop stability. One must be concerned with this condition for

large inductor values and high output currents.

(7)

There are miscellaneous poles and zeros associated with parasitics internal to the LM2735, external

components, and the PCB. They are located well over the crossover frequency, and for simplicity are not

discussed.

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