MC34167, MC33167
http://onsemi.com
7
INTRODUCTION
The MC34167, MC33167 series are monolithic power
switching regulators that are optimized for dc−to−dc
converter applications. These devices operate as fixed
frequency, voltage mode regulators containing all the active
functions required to directly implement step−down and
voltage−inverting converters with a minimum number of
external components. They can also be used cost effectively
in step−up converter applications. Potential markets include
automotive, computer, industrial, and cost sensitive
consumer products. A description of each section of the
device is given below with the representative block diagram
shown in Figure 14.
Oscillator
The oscillator frequency is internally programmed to
72 kHz by capacitor C
T
and a trimmed current source. The
charge to discharge ratio is controlled to yield a 95%
maximum duty cycle at the Switch Output. During the
discharge of C
T
, the oscillator generates an internal blanking
pulse that holds the inverting input of the AND gate high,
disabling the output switch transistor. The nominal
oscillator peak and valley thresholds are 4.1 V and 2.3 V
respectively.
Pulse Width Modulator
The Pulse Width Modulator consists of a comparator with
the oscillator ramp voltage applied to the noninverting input,
while the error amplifier output is applied into the inverting
input. Output switch conduction is initiated when C
T
is
discharged to the oscillator valley voltage. As C
T
charges to
a voltage that exceeds the error amplifier output, the latch
resets, terminating output transistor conduction for the
duration of the oscillator ramp−up period. This PWM/Latch
combination prevents multiple output pulses during a given
oscillator clock cycle. Figures 7 and 15 illustrate the switch
output duty cycle versus the compensation voltage.
Current Sense
The MC34167 series utilizes cycle−by−cycle current
limiting as a means of protecting the output switch transistor
from overstress. Each on cycle is treated as a separate
situation. Current limiting is implemented by monitoring the
output switch transistor current buildup during conduction,
and upon sensing an overcurrent condition, immediately
turning off the switch for the duration of the oscillator
ramp−up period.
The collector current is converted to a voltage by an
internal trimmed resistor and compared against a reference
by the Current Sense comparator. When the current limit
threshold is reached, the comparator resets the PWM latch.
The current limit threshold is typically set at 6.5 A.
Figure 10 illustrates switch output current limit threshold
versus temperature.
Error Amplifier and Reference
A high gain Error Amplifier is provided with access to the
inverting input and output. This amplifier features a typical
dc voltage gain of 80 dB, and a unity gain bandwidth of
600 kHz with 70 degrees of phase margin (Figure 4). The
noninverting input is biased to the internal 5.05 V reference
and is not pinned out. The reference has an accuracy of
± 2.0% at room temperature. To provide 5.0 V at the load,
the reference is programmed 50 mV above 5.0 V to
compensate for a 1.0% voltage drop in the cable and
connector from the converter output. If the converter design
requires an output voltage greater than 5.05 V, resistor R
1
must be added to form a divider network at the feedback
input as shown in Figures 14 and 19. The equation for
determining the output voltage with the divider network is:
V
out
+ 5.05
ǒ
R
2
R
1
) 1
Ǔ
External loop compensation is required for converter
stability. A simple low−pass filter is formed by connecting
a resistor (R
2
) from the regulated output to the inverting
input, and a series resistor−capacitor (R
F
, C
F
) between Pins
1 and 5. The compensation network component values
shown in each of the applications circuits were selected to
provide stability over the tested operating conditions. The
step−down converter (Figure 19) is the easiest to
compensate for stability. The step−up (Figure 21) and
voltage−inverting (Figure 23) configurations operate as
continuous conduction flyback converters, and are more
difficult to compensate. The simplest way to optimize the
compensation network is to observe the response of the
output voltage to a step load change, while adjusting R
F
and
C
F
for critical damping. The final circuit should be verified
for stability under four boundary conditions. These
conditions are minimum and maximum input voltages, with
minimum and maximum loads.
By clamping the voltage on the error amplifier output
(Pin 5) to less than 150 mV, the internal circuitry will be
placed into a low power standby mode, reducing the
power supply current to 36 mA with a 12 V supply voltage.
Figure 11 illustrates the standby supply current versus
supply voltage.
The Error Amplifier output has a 100 mA current source
pull−up that can be used to implement soft−start. Figure 18
shows the current source charging capacitor C
SS
through a
series diode. The diode disconnects C
SS
from the feedback
loop when the 1.0 M resistor charges it above the operating
range of Pin 5.
