TC170 Datasheet, PDF (6/16 Pages) TelCom Semiconductor, Inc – CMOS CURRENT MODE PWM CONTROLLER

TC170
3.0 DETAILED DESCRIPTION
3.1 Peak Current Limit Setup
Resistors R1 and R2 at the current limit input (pin 1) set
the TC170 peak current limit (Figure 3-1). The potential
at pin 1 is easily calculated:
V1 = V_REF
R2
R1 + R2
R1 should be selected first. The shutdown circuit
feature is not latched for (V_REF – 0.35)/R1 < 50µA and
is latched for currents greater than 125µA.
The error amplifier output voltage is clamped from
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 3.15.
I_PCL, the peak current limit, is the current that causes
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.
I_PCL is easily calculated:
I_PCL =
V1 – 0.75V
3.15 (RS)
where:
V1 = V_REF
R2
R1 + R2
V_REF = Internal voltage reference = 5.1V
3.15 = Gain of current-sense amplifier
0.75V = Current limit offset
Both driver outputs (pins 11 and 14) are OFF (LOW)
when the peak current limit is exceeded. When the
sensed current goes below I_PCL, the circuit operates
normally.
3.2 Output Shutdown
The TC170 outputs can be turned OFF quickly through
the shutdown input (pin 16). A signal greater than
350mV at pin 16 forces the shutdown comparator
output HIGH. The PWM latch is held set, disabling the
outputs.
Q2 is also turned ON. If V_REF/R1 is greater than 125µA,
positive feedback through the lockup amplifier and Q1
keeps the inverting PWM comparator inverting input
below 0.75V. Q3 remains ON even after the shutdown
input signal is removed, because of the positive
feedback. The state can be cleared only through a
power-up cycle. Outputs will be disabled whenever the
potential at pin 1 is below 0.75V.
The shutdown terminal gives a fast, direct way to
disable the TC170 output transistors. System protec-
tion and remote shutdown applications are possible.
The input pulse to pin 16 should be at least 500nsec
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 600nsec at 25°C; however, the delay time
will increase as the device temperature rises.
3.3 Soft Restart From Shutdown
A soft restart can be programmed if nonlatched
shutdown operation is used.
A capacitor at pin 1 will cause a gradual increase in
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 operation forces the PWM output drivers to initially
operate with minimum duty cycle and low peak
currents.
Even if a soft start is not required, it is necessary to
insert a capacitor between pin 1 and ground if the
current I_L is greater than 125µA. This capacitor will
prevent "noise triggering" of the latch, yet minimize the
soft-start effect.
3.4 Soft-Start Power-Up
During power-up, a capacitor at R1, R2 initiates a soft-
start cycle. As the input voltage (pin 15) exceeds the
undervoltage lockout potential (7.7V), Q4 is turned
OFF, ending undervoltage lockout. Whenever the
PWM comparator inverting input is below 0.5V, both
outputs are disabled.
When the undervoltage lockout level is passed, 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.
3.5 Current-Sense Amplifier
The current-sense amplifier operates at a fixed gain
of 3.15. Maximum differential input voltage (V_PIN4 –
V_PIN3) is 1.1V. Common-mode input voltage range is
0V to V_IN – 3V.
Resistive-sensing methods are shown in Figure 3-2
and Figure 3-3. In Figure 3-2, a simple RC filter limits
transient voltage spikes at pin 4, caused by external
output transistor-collector capacitance. Transformer
coupling (Figure 3-4) offers isolation and better power
efficiency, but cost and complexity increase.
In order to minimize the propagation delay from the
input to the current amplifier to the output terminals, the
current ramp should be in the order of 1µsec in width
(min). Typical time delay values are in the 300 to
400nsec 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.
DS21395B-page 6
© 2002 Microchip Technology Inc.