Application Information: continued
ducing a negative slope at the VC pin. This negative slope
V
provides the slope compensation.
IN
The amount of slope compensation added by this circuit is
V
V
CC
−(1 − D)
∆I
∆T
R3
R2 + R3
fSW
(1 − D)REAV
SS
SS
= VSW
1 − eR C f
3
3 SW
C
(
)(
)(
)
where:
∆I/∆T = the amount of slope compensation added (A/s),
VSW = the voltage at the switch node when the transistor
is turned off (V),
D2
D1
R1
fSW = the switching frequency, typically 270kHz (Hz),
D = the duty cycle,
RE = .063Ω, the value of the internal emitter resistor,
AV = 5V/V, the gain of the current sense amplifier.
C1
C2
C3
In selecting appropriate values for the slope compensation
network, the designer is advised to choose a convenient
capacitor, then select values for R2 and R3 such that the
amount of slope compensation added is 100mA/µs. Then
R2 may be increased or decreased as necessary. Of course,
the series combination of R2 and R3 should be large
Figure 12: Soft Start.
Resistor R1 and capacitors C1 and C2 form the compensa-
tion network. At turn on, the voltage at the VC pin starts to
come up, charging capacitor C3 through Schottky diode
D2, clamping the voltage at the VC pin such that
enough to avoid drawing excessive current from VSW
.
Additionally, to ensure that the control loop stability is
improved, the time constant formed by the additional com-
ponents should be chosen such that
VC = VF(D2) + VC3
1 − D
R3C3 <
fSW
Therefore, C3 slows the startup of the circuit by limiting
the voltage on the VC pin. The soft-start time increases with
the size of C3.
Finally, it is worth mentioning that the added slope com-
pensation is a trade-off between duty cycle stability and
transient response. The more slope compensation a
designer adds, the slower the transient response will be,
due to the external circuitry interfering with the proper
operation of the error amplifier.
Diode D1 discharges C3 when SS is low. If the shutdown
function is not used with this part, the cathode of D1
should be connected to VIN.
Calculating Junction Temperature
Soft Start
To ensure safe operation of the CS5171 part, the designer
must calculate the on-chip power dissipation and deter-
mine its expected junction temperature. Internal thermal
protection circuitry will turn the part off once the junction
temperature exceeds 180°C 30°. However, repeated oper-
ation at such high temperatures will ensure a reduced
operating life.
Through the addition of an external circuit, a soft-start
function can be added to the CS5171 family of components.
Soft-start circuitry prevents the VC pin from slamming
high during startup, thereby inhibiting the inductor cur-
rent from rising at a high slope.
This circuit, shown in Figure 12, requires a minimum num-
ber of components and allows the soft-start circuitry to
activate any time the SS pin is used to restart the converter.
Calculation of the junction temperature is an imprecise but
simple task. First, the power losses must be quantified.
There are three major sources of power loss on the CS5171:
• biasing of internal control circuitry, PBIAS
• switch driver, PDRIVER
• switch saturation, PSAT
The internal control circuitry, including the oscillator and
linear regulator, requires a small amount of power even
when the switch is turned off. The specifications section of
this datasheet reveals that the typical operating current, IQ,
due to this circuitry is 5.5mA. Additional guidance can be
found in the graph of operating current vs. temperature.
This graph shows that IQ is strongly dependent on input
voltage, VIN, and the ambient temperature, TA. Then
PBIAS = VINIQ
12
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
