CS5171, CS5172, CS5173, CS5174
http://onsemi.com
16
This circuit, shown in Figure 40, requires a minimum
number of components and allows the Soft−Start circuitry to
activate any time the SS pin is used to restart the converter.
Figure 40. Soft Start
V
C
R1
C2
C1
D2
D1
V
CC
C3
IN
SS
SS
Resistor R1 and capacitors C1 and C2 form the
compensation network. At turn on, the voltage at the V
C
pin
starts to come up, charging capacitor C3 through Schottky
diode D2, clamping the voltage at the V
C
pin such that
switching begins when V
C
reaches the V
C
threshold,
typically 1.05 V (refer to graphs for detail over temperature).
V
C
+ V
F(D2)
)V
C3
Therefore, C3 slows the startup of the circuit by limiting
the voltage on the V
C
pin. The Soft−Start time increases with
the size of C3.
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 V
IN
.
Calculating Junction Temperature
To ensure safe operation of the CS5171/2/3/4, the
designer must calculate the on−chip power dissipation and
determine its expected junction temperature. Internal
thermal protection circuitry will turn the part off once the
junction temperature exceeds 180°C ± 30°. However,
repeated operation at such high temperatures will ensure a
reduced operating life.
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 CS517x:
• biasing of internal control circuitry, P
BIAS
• switch driver, P
DRIVER
• switch saturation, P
SAT
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, I
Q
,
due to this circuitry is 5.5 mA. Additional guidance can be
found in the graph of operating current vs. temperature. This
graph shows that IQ is strongly dependent on input voltage,
V
IN
, and temperature. Then
P
BIAS
+ V
IN
I
Q
Since the onboard switch is an NPN transistor, the base
drive current must be factored in as well. This current is
drawn from the V
IN
pin, in addition to the control circuitry
current. The base drive current is listed in the specifications
as DI
CC
/DI
SW
, or switch transconductance. As before, the
designer will find additional guidance in the graphs. With
that information, the designer can calculate
P
DRIVER
+ V
IN
I
SW
I
CC
DI
SW
D
where:
I
SW
= the current through the switch;
D = the duty cycle or percentage of switch on−time.
I
SW
and D are dependent on the type of converter. In a
boost converter,
I
SW(AVG)
^ I
L
(
AVG
)
D
1
Efficiency
D ^
V
OUT
* V
IN
V
OUT
In a flyback converter,
I
SW(AVG)
^
V
OUT
I
LOAD
V
IN
1
Efficiency
1
D
D ^
V
OUT
V
OUT
)
N
S
N
P
V
IN
The switch saturation voltage, V
(CE)SAT
, is the last major
source of on−chip power loss. V
(CE)SAT
is the
collector−emitter voltage of the internal NPN transistor
when it is driven into saturation by its base drive current. The
value for V
(CE)SAT
can be obtained from the specifications
or from the graphs, as “Switch Saturation Voltage.” Thus,
P
SAT
^ V
(CE)SAT
I
SW
D
Finally, the total on−chip power losses are
P
D
+ P
BIAS
)P
DRIVER
)P
SAT
Power dissipation in a semiconductor device results in the
generation of heat in the junctions at the surface of the chip.
This heat is transferred to the surface of the IC package, but
a thermal gradient exists due to the resistive properties of the
package molding compound. The magnitude of the thermal
gradient is expressed in manufacturers’ data sheets as q
JA
,
or junction−to−ambient thermal resistance. The on−chip
junction temperature can be calculated if q
JA
, the air
temperature near the surface of the IC, and the on−chip
power dissipation are known.
