NCV890130
http://onsemi.com
13
OUTPUT PRECHARGE DETECTION
Prior to Soft−start, the FB pin is monitored to ensure the
SW voltage is low enough to have charged the external
bootstrap capacitor (C
BST
). If the FB pin is higher than
V
SSEN
, restart is delayed until the output has discharged.
Figure 28 shows the IC starts to switch after the voltage on
FB pin reaches VSSEN, even the EN pin is high. After the
IC is switching, the FB pin follows the soft starts reference
to reach the final set point.
Figure 28. Output Voltage Detection
EN
FB
SW
Time
Time
Time
V
SSEN
THERMAL SHUTDOWN
A thermal shutdown circuit inhibits switching, resets the
Soft−start circuit, and removes DRV voltage if internal
temperature exceeds a safe level. Switching is automatically
restored when temperature returns to a safe level.
MINIMUM DROPOUT VOLTAGE
When operating at low input voltages, two parameters
play a major role in imposing a minimum voltage drop
across the regulator: the minimum off time (that sets the
maximum duty cycle), and the on state resistance.
When operating in continuous conduction mode (CCM),
the output voltage is equal to the input voltage multiplied by
the duty ratio. Because the NCV890130 needs a sufficient
bootstrap voltage to operate, its duty cycle cannot be 100%:
it needs a minimum off time (t
OFFmin
) to periodically re−fuel
the bootstrap capacitor C
BST
. This imposes a maximum duty
ratio D
MAX
= 1 − t
OFFmin
.F
SW(min)
, with the switching
frequency being folded back down to F
SW(min)
= 500 kHz to
keep regulating at the lowest input voltage possible.
The drop due to the on−state resistance is simply the
voltage drop across the Switch resistance R
DSON
at the
given output current: V
SWdrop
= I
OUT
.R
DSon
.
Which leads to the maximum output voltage in low Vin
condition: V
OUT
= D
MAX
.V
IN(min)
− V
SWdrop
Figure 29. Minimum Input Voltage vs. Output Curren
I
OUT
(A)
0.90.80.70.60.30.20.10
0
1
2
3
4
5
6
IN
0.4 0.5 1.0 1.1 1
V
OUT
= 5 V
EXPOSED PAD
The exposed pad (EPAD) on the back of the package must
be electrically connected to the electrical ground (GND pin)
for proper, noise−free operation.
DESIGN METHODOLOGY
The NCV890130 being a fixed−frequency regulator with
the switching element integrated, is optimized for one value
of inductor. This value is set to 4.7 mH, and the slope
compensation is adjusted for this inductor. The only
components left to be designed are the input and output
capacitor and the freewheeling diode. Please refer to the
design spreadsheet www.onsemi.com NCV890130 page
that helps with the calculation.
Output capacitor:
The minimum output capacitor value can be calculated
based on the specification for output voltage ripple:
C
OUTmin
+
DI
L
8 @ DV
OUT
@ F
SW
(eq. 1)
With
− DI
L
the inductor ripple current:
DI
L
+
V
OUT
@
ǒ
1 *
V
OUT
V
IN
Ǔ
L @ F
SW
(eq. 2)
− DV
OUT
the desired voltage ripple.
However, the ESR of the output capacitor also contributes
to the output voltage ripple, so to comply with the
requirement, the ESR cannot exceed R
ESRmax
:
R
ESRmax
+
DV
OUT
@ L @ F
SW
V
OUT
ǒ
1 *
V
OUT
V
IN
Ǔ
(eq. 3)
Finally, the output capacitor must be able to sustain the ac
current (or RMS ripple current):
I
OUTac
+
DI
L
23
Ǹ
(eq. 4)
Typically, with the recommended 4.7 mH inductor, two ceramic
capacitors of 10 mF each in parallel give very good results.
