Application Information
power switch is turned off by the output of the PWM
Theory of Operation
Current Mode Control
Comparator.
A TTL-compatible sync input at the SS pin is capable of
syncing up to 1.8 times the base oscillator frequency. As
shown in Figure 2, in order to sync to a higher frequency, a
positive transition turns on the power switch before the
output of the oscillator goes high thereby resetting the
oscillator. The sync operation allows multiple power sup-
plies to operate at the same frequency.
VCC
Oscillator
S
R
Q
L
VC
-
+
D1
Power Switch
In
PWM LATCH
VSW
PWM Comparator
A sustained logic low at the SS pin will shut down the IC
and reduce the supply current.
Out
Driver
C0
RLOAD
X5
An additional feature includes frequency shift to 20% of
the nominal frequency when the FB pin triggers the thresh-
old. During power up, overload, or short circuit condi-
tions, the minimum switch on time is limited by the PWM
comparator blanking period. Extra switch off time reduces
the minimum duty cycle to protect external components
and the IC itself.
63mΩ
Slope Compensation
Figure 1. Current Mode Control Scheme
The CS5171 incorporates a current mode control scheme, in
which the PWM ramp signal is derived from power switch
current. This ramp signal is compared to the output of the
error amplifier to control the on time of the power switch.
The oscillator is used as a fixed frequency clock to ensure a
constant operational frequency. The resulting control
scheme features several advantages over the conventional
voltage mode controller. First, derived directly from the
inductor, the ramp signal responds immediately to line
voltage changes. This eliminates the delay caused by the
output filter and error amplifier, which is commonly suf-
fered in voltage mode controllers. The second benefit
comes from inherent pulse-by-pulse current limiting by
merely clamping the peak switching current. Finally, since
current mode commands an output current rather than
voltage, the filter offers only a single pole to the feedback
loop. This allows both simpler compensation and a higher
gain bandwidth over a comparable voltage mode circuit.
As previously mentioned, this block also produces a ramp
for the slope compensation to improve regulator stability.
Error Amplifier
+
-
1.276V
V
C
C1
0.01µF
FB
1MΩ
Voltage
Clamp
120pF
positive error-amp
CS5171
R1
5kΩ
Figure 3. Error amplifier equivalent circuit.
Without discrediting its apparent merits, current mode
control comes with its own peculiar problems, mainly, sub-
harmonic oscillation at duty cycle over 50%. The CS5171
solves this problem by adopting a slope compensation
scheme in which a fixed ramp generated by the oscillator is
added to the current ramp. A proper slope rate is provided
to improve circuit stability without sacrificing the advan-
tages of current mode control.
The FB pin is directly connected to the inverting input of
the positive error amplifier, whose non–inverting input is
fed by the 1.276V reference. Both amplifiers are transcon-
ductance amplifiers with a high output impedance of
approximately 1MΩ as shown in Figure 3. The VC pin is
connected to the output of the error amplifiers and is inter-
nally clamped between 0.5V and 1.7V. A typical connec-
tion at the VC pin includes a capacitor in series with a resis-
tor to ground, forming a pole/zero for loop compensation.
Oscillator and Shutdown
An external shunt can be connected between the VC pin
and ground to reduce its clamp voltage. Consequently, the
current limit of the internal power transistor current is
reduced from its nominal value.
Sync
Current Ramp
V
SW
Switch Driver and Power Switch
The switch driver receives a control signal from the logic
section to drive the output power switch. The switch is
grounded through emitter resistors (63mΩ total) to the
PGnd pin. PGnd is not connected to the IC substrate so
that switching noise can be isolated from the analog
ground. The peak switching current is clamped by an
Figure 2. Timing Diagram of Sync and Shut Down.
The oscillator is trimmed to guarantee an 13% frequency
accuracy. As shown in Figure 1, the output of the oscillator
turns on the power switch at a frequency of 270kHz. The
7
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