UC1846-SP_15 Datasheet, PDF (9/29 Pages) Texas Instruments – Current-Mode PWM Controller

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7 Detailed Description
UC1846-SP
SLUS871C – JANUARY 2009 – REVISED OCTOBER 2015
7.1 Overview
The UC1846-SP control devices provide all of the necessary features to implement fixed frequency, current
mode control schemes while maintaining a minimum external parts count. The superior performance of this
technique can be measured in improved line regulation, enhanced load response characteristics, and a simpler,
easier-to-design control loop. Topological advantages include inherent pulse-by-pulse current limiting capability,
automatic symmetry correction for push-pull converters, and the ability to parallel “power modules" while
maintaining equal current sharing.
7.2 Functional Block Diagram
VIN 15
SYNC 10
RT 9
CT 8
C/S- 3
C/S+ 4
E/A+ 5
E/A– 6
OSC
COMP
X3
0.5 V
+
E/A
5.1 V
REFERENCE
REGULATOR
UVLO
LOCKOUT
SR
Q
S
0.5 mA
COMP 7
F/F
Q
T
Q
2 VREF
13 VC
11 A OUT
Output Stage
14 B OUT
12 GND
350 mV
1 C/L SS
16 SHUTDOWN
6k
NOTE: Pin numbers shown are for the J package.
7.3 Feature Description
UC1846-SP is a current mode controller, used to support various topologies such as forward, flyback, half-
bridge, full bridge, push-pull configurations.
Current mode control is a two-loop system. The switching power supply inductor is hidden within the inner
current control loop. This simplifies the design of the outer voltage control loop and improves power supply
performance in many ways, including better dynamics. The objective of this inner loop is to control the state-
space averaged inductor current, but in practice the instantaneous peak inductor current is the basis for control
(switch current - equal to inductor current during the on time - is often sensed). If the inductor ripple current is
small, peak inductor current control is nearly equivalent to average inductor current control.
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