7

5

4

2

1

3

8

7

6

5

V

CC

Gnd

R

V

FB

Comp

C/S

V

REF

5.0 V

50 mA

V

C

Output

Pwr

Ground

PWM

Latch

5-V

REF

S/R

UVLO

34 V

2.5 V

Osc

Error

Amplifier

2R

R

1 V

Current

Sense

Comparator

Internal

Bias

V

Good

Logic

REF

S

R

UC1842A-SP, UC1843A-SP, UC1844A-SP

www.ti.com

SLUS872D – JANUARY 2009 – REVISED MAY 2015

9 Detailed Description

9.1 Overview

The UC184xA-SP control IC is a pin-for-pin compatible improved version of the UC184x family. Providing the

necessary features to control current-mode switched-mode power supplies, this device has the following

improved features. Startup current is specified to be less than 0.5 mA. Oscillator discharge is trimmed to 8.3 mA.

9.2 Functional Block Diagram

9.3 Feature Description

UC184xA is a current mode controller, used to support various topologies such as forward, flyback, Buck, Boost

and using an external interface circuit will also support half-bridge, full bridge, and push-pull configurations.

Figure 2 shows the two-loop current-mode control system in a typical buck regulator application. A clock signal

initiates power pulses at a fixed frequency. The termination of each pulse occurs when an analog of the inductor

current reaches a threshold established by the error signal. In this way, the error signal actually controls peak

inductor current. This contrasts with conventional schemes in which the error signal directly controls pulse width

without regard to inductor current.

Several performance advantages result from the use of current-mode control. First, an input voltage feed-forward

characteristic is achieved; that is, the control circuit instantaneously corrects for input voltage variations without

using up any of the error amplifier’s dynamic range. Therefore, line regulation is excellent and the error amplifier

can be dedicated to correcting for load variations exclusively.

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Product Folder Links: UC1842A-SP UC1843A-SP UC1844A-SP