1.5A, 280kHz, Boost Regulator
LM5171
Subharmonic Oscillation
Subharmonic oscillation (SHM) is a problem found in current-mode control systems, where instability results
when duty cycle exceeds 50%. SHM only occurs in switching regulators with a continuous inductor current.
This instability is not harmful to the converter and usually does not affect the output voltage regulation. SHM will
increase the radiated EM noise from the converter and can cause, under certain circumstances, the inductor to
emit high-frequency audible noise.
SHM is an easily remedied problem. The rising slope of the inductor current is supplemented with internal
“slope compensation” to prevent any duty cycle instability from carrying through to the next switching cycle. In
the LM5171, slope compensation is added during the entire switch on-time, typically in the amount of 180mA/μs.
In some cases, SHM can rear its ugly head despite the presence of the onboard slope compensation. The
simple cure to this problem is more slope compensation to avoid the unwanted oscillation. In that case, an
external circuit, shown in Figure 30, can be added to increase the amount of slope compensation used. This
circuit requires only a few components and is “tacked on” to the compensation network.
VSW
VSW
VC
R1
C2
R2
C1
R3
C3
Figure 30. Technique for Increasing Slope Compensation
The dashed box contains the normal compensation circuitry to limit the bandwidth of the error amplifier.
Resistors R2 and R3 form a voltage divider off of the VSW pin. In normal operation, VSW looks similar to a
square wave, and is dependent on the converter topology. Formulas for calculating VSW in the boost and flyback
topologies are given in the section “VSW Voltage Limit.” The voltage on VSW charges capacitor C3 when the
switch is off, causing the voltage at the VC pin to shift upwards. When the switch turns on, C3 discharges
through R3, producing a negative slope at the VC pin. This negative slope provides the slope compensation.
The amount of slope compensation added by this circuit is
-(1-D)
R3
fSW
ΔI
3
3 SW
= VSW
(
)(1- e R C f )(
)
ΔT
R2 + R3
(1- D)RE AV
where:
ΔI/ΔT = the amount of slope compensation added (A/s);
SW = the voltage at the switch node when the transistor is turned off (V);
SW = the switching frequency, typically 280 kHz;
V
f
D = the duty cycle;
RE = 63mꢀ, the value of the internal emitter resistor;
AV = 5V/V, the gain of the current sense amplifier.
Dec. 2010 - Rev. 1.2.1
- 16 -
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
