TC28C46EOE TELCOM [TelCom Semiconductor, Inc], TC28C46EOE Datasheet - Page 5

• Manufacturer Part Number: TC28C46EOE
• Description: CMOS CURRENT MODE PWM CONTROLLER
• Manufacturer: TELCOM [TelCom Semiconductor, Inc]

where:
when the peak current limit is exceeded. When the sensed
current goes below I
Output Shutdown
SHUTDOWN input (pin 16). A signal greater than 360 mV
at pin 16 forces the shutdown comparator output HIGH.
The PWM latch is held set, disabling the outputs.
positive feedback through the lock-up amplifier and Q1
keeps the inverting PWM comparator inverting input below
0.65V. Q3 remains ON even after the shutdown input
signal is removed. This is because the lock-up amplifier is
in latched mode driving Q3 ON. This state can be cleared
only through a power-up cycle. Outputs will be disabled
whenever the potential at pin 1 is below 0.65V.
able the PWM controller output transistors. System protec-
tion and remote shutdown applications are possible.
Peak Current Limit Setup
1) set the peak current limit (Figure 1). The potential at pin
1 is easily calculated:
ture is not latched for (V
for currents greater than 140 A.
going above V1 through the limit buffer amplifier. Peak
current is sensed by RS and amplified by the current
amplifier which has a fixed gain of 3.
the PWM comparator noninverting input to exceed V1, the
potential at the inverting input. Once the comparator trip
point is exceeded, both outputs are disabled.
CMOS CURRENT MODE
PWM CONTROLLERS
V1 = V
V
3 = Gain of current-sense amplifier
0.65V = Current limit offset
Both driver OUTPUTs (pins 11 and 14) are OFF (LOW)
The outputs can be turned OFF quickly through the
Q2 is also turned ON. If V
The shutdown terminal gives a fast, direct way to dis-
Resistors R1 and R2 at the CURRENT LIMIT input (pin
V1 = V
R1 should be selected first. The shutdown circuit fea-
The error amplifier output voltage is clamped from
I
I
I
PCL
PCL
PCL
TELCOM SEMICONDUCTOR, INC.
REF
, the peak current limit, is the current that causes
is easily calculated:
=
= Internal voltage reference = 5.1V
REF
REF
V1 – 0.65V
3 (RS)
R1 + R2
R2
R1 + R2
PCL
R2
REF
, the circuit operates normally.
– 0.35)/R1 <65 A and is latched
REF
/R1 is greater than 140 A,
Soft-Start Power-Up
start cycle. As the input voltage (pin 15) exceeds the
under-voltage lockout potential (7V), Q4 is turned OFF,
ending undervoltage lockout. Whenever the PWM com-
parator inverting input is below 0.65V, both outputs are
disabled.
ceeded, the capacitor begins to charge. The PWM duty
cycle increases until the operating output voltage is reached.
Soft-start operation forces the PWM output drivers to initially
operate with minimum duty cycle and low peak current.
Current-Sense Amplifier
3. Maximum differential input voltage (V
Common-mode input voltage range is 0V to V
Figure 2(A), a simple RC filter limits transient voltage spikes
at pin 4, caused by external output transistor-collector
capacitance. Transformer coupling (Figure 3) offers isola-
tion and better power efficiency, but cost and complexity
increase.
to the current amplifier to the output terminals, the current
ramp should be in the order of 1 sec in width (min). Typical
time delay values are in the 225nsec region at 25 C. The
delay time increases with device temperature so that at
50 C, the delay times may be increased by as much as
100nsec.
wide and have an amplitude of at least 1V in order to get the
minimum propagation delay from input to output. If these
parameters are met, the delay should be less than 400nsec
at 25 C; however, the delay time will increase as the device
temperature rises.
Soft Restart From Shutdown
down operation is used.
potential toward V1. When the voltage at pin 1 reaches
0.75V, the PWM latch set input is removed and the circuit
establishes a regulated output voltage. The soft-start opera-
tion forces the PWM output drivers to initially operate with
minimum duty cycle and low peak currents.
insert a capacitor between pin 1 and ground if the current I
is greater than 140 A. This capacitor will prevent "noise
triggering" of the latch, yet minimize the soft-start effect.
During power-up, a capacitor at R1, R2 initiates a soft-
When the undervoltage lockout start threshold is ex-
The current-sense amplifier operates at a fixed gain of
Resistive-sensing methods are shown in Figure 2. In
In order to minimize the propagation delay from the input
The input pulse to pin 16 should be at least 100nsec
A soft restart can be programmed if nonlatched shut-
A capacitor at pin 1 will cause a gradual increase in
Even if a soft start is not required, it is necessary to
PIN4
–V
TC18C46
TC28C46
TC38C46
PIN3
IN
– 3V.
) is 1.1V.
4-105
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