NCP3163 データシートの表示（PDF） - ON Semiconductor
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NCP3163 Datasheet PDF : 20 Pages
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NCP3163, NCV3163
Feedback and Low Voltage Indicator Comparators
Output voltage control is established by the Feedback
comparator. The inverting input is internally biased at 1.25 V
and is not pinned out. The converter output voltage is
typically divided down with two external resistors and
monitored by the high impedance noninverting input at Pin 2.
The maximum input bias current is ± 0.4 mA, which can cause
an output voltage error that is equal to the product of the input
bias current and the upper divider resistance value. For
applications that require 5.0 V, the converter output can be
directly connected to the noninverting input at Pin 3. The high
impedance input, Pin 2, must be grounded to prevent noise
pickup. The internal resistor divider is set for a nominal
voltage of 5.05 V. The additional 50 mV compensates for a
1.0% voltage drop in the cable and connector from the
converter output to the load. The Feedback comparator’s
output state is controlled by the highest voltage applied to
either of the two noninverting inputs.
The Low Voltage Indicator (LVI) comparator is designed
for use as a reset controller in microprocessor−based
systems. The inverting input is internally biased at 1.125 V,
which sets the noninverting input thresholds to 90% of
nominal. The LVI comparator has 15 mV of hysteresis to
prevent erratic reset operation. The Open Collector output is
capable of sinking in excess of 6.0 mA (see Figure 13). An
external resistor (RLVI) and capacitor (CDLY) can be used to
program a reset delay time (tDLY) by the formula shown
below, where Vth(MPU) is the microprocessor reset input
threshold. Refer to Figure 20.
ǒ Ǔ tDLY = RLVI ⋅ CDLY ⋅ In
1
1−
Vth(MPU)
Vout
3
14
Low Voltage
Indicator Output
2
RLVI
1
CDLY
+
+
-
LVI
+ Feedback
+
-
Comparator
1.25 V
1.125 V
(Bottom View)
15
16
L
CO
Vout
Figure 20. Partial Application Schematic Showing
Implementation of LVI Delay with RLVI and CDLY
Current Limit Comparator, Latch and Thermal
Shutdown
With a voltage mode ripple converter operating under
normal conditions, output switch conduction is initiated by
the oscillator and terminated by the Voltage Feedback
comparator. Abnormal operating conditions occur when the
converter output is overloaded or when feedback voltage
sensing is lost. Under these conditions, the Current Limit
comparator will protect the Output Switch.
The switch current is converted to a voltage by inserting
a fractional ohm resistor, RSC, in series with VCC and output
switch transistor Q2. The voltage drop across RSC is
monitored by the Current Sense comparator. If the voltage
drop exceeds 250 mV with respect to VCC, the comparator
will set the latch and terminate output switch conduction on
a cycle−by−cycle basis. This Comparator/Latch
configuration ensures that the Output Switch has only a
single on−time during a given oscillator cycle. The
calculation for a value of RSC is:
RSC
+
0.25 V
Ipk (Switch)
Figures 14 and 15 show that the Current Sense comparator
threshold is tightly controlled over temperature and has a
typical input bias current of 1.0 mA. The propagation delay
from the comparator input to the Output Switch is typically
200 ns. The parasitic inductance associated with RSC and the
circuit layout should be minimized. This will prevent
unwanted voltage spikes that may falsely trip the Current
Limit comparator.
Internal thermal shutdown circuitry is provided to protect
the IC in the event that the maximum junction temperature
is exceeded. When activated, typically at 170°C, the Latch
is forced into the “Set” state, disabling the Output Switch.
This feature is provided to prevent catastrophic failures from
accidental device overheating. It is not intended to be used
as a replacement for proper heatsinking.
Driver and Output Switch
To aid in system design flexibility and conversion
efficiency, the driver current source and collector, and
output switch collector and emitter are pinned out
separately. This allows the designer the option of driving the
output switch into saturation with a selected force gain or
driving it near saturation when connected as a Darlington.
The output switch has a typical current gain of 70 at 2.5 A
and is designed to switch a maximum of 40 V collector to
emitter, with up to 3.4 A peak collector current. The
minimum value for RSC is:
RSC(min)
+
0.25 V
3.4 A
+
0.0735
W
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