NCP5173MNR2 ON Semiconductor, NCP5173MNR2 Datasheet - Page 14

• Manufacturer Part Number: NCP5173MNR2
• Description: IC PWM BUCK BST FLYBCK INV 8DIP
• Manufacturer: ON Semiconductor
• Type: Step-Up (Boost), Inverting, Flyback, Forward Converter, Sepic, PWM - Current Moder

Specifications of NCP5173MNR2

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adjustable
• Current - Output: 1.5A
• Frequency - Switching: 560kHz
• Voltage - Input: 2.7 ~ 30 V
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: *
• Package / Case: 8-TDFN Exposed Pad
• Voltage - Supply: 2.7 V ~ 30 V
• Frequency-max: 1MHz
• Duty Cycle: 90%
• Pwm Type: Current Mode
• Buck: Yes
• Boost: Yes
• Flyback: Yes
• Inverting: Yes
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: NCP5173MNR2OS

circuitry to limit the bandwidth of the error amplifier.
Resistors R2 and R3 form a voltage divider off of the V
pin. In normal operation, V
wave, and is dependent on the converter topology. Formulas
for calculating V
given in the section “V
V
the voltage at the V
turns on, C3 discharges through R3, producing a negative
slope at the V
compensation.
where:
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 100 mA/ms. Then
R2 may be increased or decreased as necessary. Of course,
the series combination of R2 and R3 should be large enough
to avoid drawing excessive current from V
to ensure that the control loop stability is improved, the time
constant formed by the additional components should be
chosen such that:
compensation 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.
Soft−Start
function can be added to the NCP5173. Soft−start circuitry
prevents the V
thereby inhibiting the inductor current from rising at a high
slope.
number of components and allows the soft−start circuitry to
activate any time the SS pin is used to restart the converter.
R 3 C 3 t 1 * D
DT
DI
SW
The dashed box contains the normal compensation
The amount of slope compensation added by this circuit is:
DI/DT = the amount of slope compensation added (A/s);
V
f
D = the duty cycle;
R
A
In selecting appropriate values for the slope compensation
Finally, it is worth mentioning that the added slope
Through the addition of an external circuit, a soft−start
This circuit, shown in Figure 33, requires a minimum
SW
V
E
+ V SW
SW
charges capacitor C3 when the switch is off, causing
= 0.063 W, the value of the internal emitter resistor;
= 5.0 V/V, the gain of the current sense amplifier.
= the switching frequency, typically 560 kHz;
= the voltage at the switch node when the transistor
is turned off (V);
f SW
C
R 2 )R 3
C
pin. This negative slope provides the slope
R 3
SW
pin from slamming high during startup,
C
in the boost and flyback topologies are
pin to shift upwards. When the switch
SW
1 * e
Voltage Limit.” The voltage on
SW
R 3 C 3 f SW
*(1*D)
looks similar to a square
(1 * D)R E A V
SW
f SW
. Additionally,
http://onsemi.com
SW
14
V
compensation network. At turn on, the voltage at the V
starts to come up, charging capacitor C3 through Schottky
diode D2, clamping the voltage at the V
switching begins when V
typically 1.05 V (refer to graphs for detail over
temperature).
the voltage on the V
the size of C3.
function is not used with this part, the cathode of D1 should
be connected to V
Calculating Junction Temperature
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 reduce operating life.
but simple task. First, the power losses must be quantified.
There are three major sources of power loss on the
NCP5173:
• biasing of internal control circuitry, P
• switch driver, P
• switch saturation, P
V C + V F(D2) )V C3
SS
IN
Resistor R1 and capacitors C1 and C2 form the
Therefore, C3 slows the startup of the circuit by limiting
Diode D1 discharges C3 when SS is low. If the shutdown
To ensure safe operation, the designer must calculate the
Calculation of the junction temperature is an imprecise
D1
IN
DRIVER
C
.
Figure 33. Soft−Start
pin. The soft−start time increases with
SAT
SS
C3
D2
C
V
V
reaches the V
CC
C
C1
BIAS
C
R1
C2
pin such that
C
threshold,
C
pin