OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

NCP1406SNT1

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P19
NCP1406, NCV1406
http://onsemi.com
10
SIMPLIFIED OPERATING DESCRIPTION
Operation
The NCP1406 is a monolithic DC−DC switching
converter optimized for single Lithium, two− or three−cell
AA/AAA size battery−powered portable products.
The NCP1406 operates in a Pulse Frequency Modulation
(PFM) scheme with constant peak current control. This
scheme maintains high efficiencies over the entire load
current range.
The device is designed to operate in Discontinuous
Conduction Mode (DCM). When the inductor releases its
energy to the output and its current reaches zero before a
new cycle starts, the converter is said to operate in DCM.
If a new cycle starts before the inductor current reaches
zero, the converter is said to operate in Continuous
Conduction Mode (CCM).
The operation of the NCP1406 is not limited to the
discontinuous conduction mode. The device can also be
operated in continuous conduction mode, but its stability is
not guaranteed.
Figure 31 depicts the three phases of operation in DCM
in a simplified way. During the first interval, the switch is
turned on and the diode is reverse biased. The inductor
stores energy through the battery while the load is supplied
by the output capacitor to maintain regulation. During the
second interval, the switch is turned off and the diode is
forward biased, this allows the energy stored in the inductor
to be supplied to both the load and the capacitor. During the
third interval, the switch is kept off and the diode is reverse
biased, the capacitor supplies the current to the load.
In DCM, the voltage ratio can be expressed as:
V
OUT
V
IN
+
1 ) 1 )
4 D
2
K
Ǹ
2
(eq. 1)
Where D +
T
ON
T
SW
,KD+
2 L I
OUT
V
OUT
T
SW
Unlike in CCM, the voltage ratio of a boost converter in
DCM is load dependent.
Figure 31. Simplified Boost Converted Operation− Discontinuous Mode
D
C
V
OUT
R
V
SW
V
L
V
IN
L
V
D
C
V
OUT
R
V
L
V
IN
L
D
C
V
OUT
R
V
L
V
IN
L
V
D
C
V
OUT
R
V
L
V
IN
L
T
ON
T
OFF
T
IDLE
I
L
I
C
= I
O
I
L
= I
D
= I
C
+ I
O
I
C
= I
O
T
ON
T
IDLE
T
OFF
Time
V
IN
V
SW
I
AVG
I
AVG
I
L
Time
T
ON
T
IDLE
T
OFF
Time
I
O
I
D
I
SW
Time
12 3
12 3
+
−
+
−
+
−
+
−
NCP1406, NCV1406
http://onsemi.com
11
The detailed operation of NCP1406 can be best
understood by referring to the block diagram and typical
application circuits in Figures 1, 3, 4, and 5. The PFM
comparator monitors the output voltage via the external
feedback resistor divider by comparing the feedback
voltage with the reference voltage. When the feedback
voltage is lower than the reference voltage, the PFM
controller and driver circuit turn the internal switch on and
the current ramps up in the inductor. The switch will remain
on for the maximum on−time, 0.90 ms, or until the current
limit is reached (0.8 A), whichever occurs first. The
internal switch is then turned off and the inductor current
ramps down. The energy stored in the inductor will be
discharged to the output capacitor and load through the
Schottky diode. The internal switch will be turned off for
at least the minimum off−time, 0.13 ms, and will remain off
until the feedback voltage becomes lower than the
reference voltage. If the inductor current reaches zero
before then, the Schottky diode will be reverse biased and
the output capacitor will sustain the regulation by
providing current to the load, while the switch pin will be
left floating. The switch will turn back on when the
feedback voltage becomes again lower than reference
voltage. This switching cycle is then repeated to attain
voltage regulation. The device operating current is
typically 15 mA (not switching), and can be further reduced
to about 0.3 mA when the chip is disabled (VCE < 0.3 V).
Soft Start
There is a soft start circuit in NCP1406. When power is
applied to the device, the soft−start circuit limits the device
to switch at a small duty cycle initially. The duty cycle is
then increased gradually until the output voltage is in
regulation. With the soft−start circuit, the output voltage
over−shoot is minimized and the startup capability with
heavy loads is also improved.
ON/OFF Timing Control
The timing control of the converter is application
dependent. The maximum on−time (inductor current
ramping up) is set at a typical 0.9 ms if the inductor current
does not reach current limit 0.8 A. The minimum off−time
(inductor current ramping down) is set at a typical 0.13 ms
to ensure the complete energy transfer to the output. The
switching frequency can be as high as 1.0 MHz.
Voltage Reference and Output Voltage
The internal bandgap voltage reference is trimmed to
1.19 V with an accuracy of "1.0% at 25°C. The voltage
reference is connected to the non inverting input of the
PFM comparator and the inverting input of the PFM
comparator is connected to the FB pin. The output voltage
can be set by connecting an external resistor divider to the
output and using the FB pin. The output voltage
programmable range is from VIN to 25 V.
Current Limit
The current limit circuit limits the maximum current
flowing through the LX pin to a typical of 0.8 A during the
internal switch turn−on period. When the current limit is
exceeded, the switch will be turned off. Since the peak
inductor current is limited to the current limit, saturation of
inductor is prevented and output voltage over−shoot during
startup is also minimized.
Internal Switch
The NCP1406 integrates a 26 V open drain internal
switch which allows high output voltage up to 25 V to be
generated from simple step−up topology.
FB Pin Short−Circuit/Open−Circuit Protection
The FB protection circuit is realized by sensing the
drain−to−source leakage current of the N−Ch MOSFET.
When the FB pin connection is shorted or opened, the
converter switches at maximum duty cycle, the peak of
V
LX
and the V
OUT
will build up, and the leakage current
will increase. When the leakage current increases to a
certain level, the protection circuitry will trigger and the
converter will stop switching. Therefore, the peak of V
LX
will immediately stop increasing at a certain level before
the N−Ch MOSFET is damaged. However, the sensing of
the leakage current is not very accurate and cannot be too
close to the normal 26 V maximum operating condition.
Therefore, the VLX is around 30 V to 40 V during a FB pin
protection fault. This is not destructive to the chip though.
Input Undervoltage Lockout
An undervoltage lockout circuit continuously monitors
the voltage at the VDD pin. The device will be disabled if
the VDD pin voltage drops below the UVLO threshold
voltage. In the same manner, the device will be enabled if
the VDD pin voltage goes above the UVLO threshold.
Thermal Shutdown
When the chip junction temperature exceeds 140°C, the
entire IC is shutdown. The IC will resume operation when
the junction temperature drops below 130°C.
Enable/Disable Operation
An external pin, CE, allows the user to enable or disable
the converter. This feature proves useful when the system
is in a standby mode by increasing battery life through
significantly decreased current consumption. A 150 nA
pull−up current source ties the CE pin to the VDD pin
internally. Therefore, leaving the CE pin floating will
enable the NCP1406.
With no other connections to the CE pin, it can be
independently controlled by an external signal. When the
voltage at the CE pin is equal to or greater than 0.9 V, the
chip will be enabled, which means the device is in normal
operation. When the voltage at the CE pin is less than 0.3 V,
NCP1406, NCV1406
http://onsemi.com
12
the chip is disabled and is shutdown. During shutdown, the
IC supply current reduces to 0.3 mA and the LX pin enters
high impedance state. However, the input remains
connected to the output through the inductor and the
Schottky diode, keeping the output voltage one diode
forward voltage drop below the input voltage.
When the NCP1406 is used to drive white LEDs, the EN
pin can be pulse width modulated to control LED
brightness.
External Component Selection
Inductor
Because it uses a PFM peak current control scheme in
DCM, the NCP1406 is inherently stable. The inductor
value does not affect the stability of the device. The
NCP1406 is designed to work well with a range of
inductance values; the actual inductance value depends on
the specific application, output current, efficiency, and
output ripple voltage. For step−up conversion, the device
works well with inductance ranging from 1 mH to 47 m H.
The selection of the inductor value along with the load
current, input and output voltages determines the switching
frequency at which the converter will operate.
In general, an inductor with small DCR is used to
minimize loss and increase efficiency. It is necessary to
choose an inductor with saturation current greater than the
peak switching current in the application.
A lower inductor value increases the switching
frequency, hence increases the losses which yields a lower
overall efficiency.
As stated before, the NCP1406 is designed to operate in
DCM. Stable operation in CCM is not guaranteed.
For all the mathematical equations given below, V
IN
is
the input voltage, T
ON_MAX
is the maximum on−time
which is typically 0.9 ms, I
LIM
is the current limit which is
typically 0.8 A, L is the selected inductance, V
OUT
is the
desired output voltage, V
D
is the Schottky diode forward
voltage, and h is the conversion efficiency which can be
assumed typically 80% for better margin for estimation.
Mode determination
T
ON
T
IDLE
T
OFF
I
PK
I
L
Time
I
O
D = T
ON
T
SW
I
PK
= V
IN
T
ON
L
D
2
= T
OFF
(1 − D) T
SW
The on−time (inductor ramp up) can be expressed as
following:
T
ON
+
L
V
IN
I
PK
(eq. 2)
The output current and the switching frequency can be
expressed as following:
I
OUT
+
V
IN
T
ON
2 L
1
ǒ
1
D
2
* 1
Ǔ
)
V
OUT
)V
D
h V
IN
(eq. 3)
F
SW
+
ǒ
1 * h
V
IN
V
OUT
)V
D
Ǔ
T
ON
ǒ
1 ) h
V
IN
V
OUT
)V
D
ǒ
1
D
2
* 1
ǓǓ
In the above equations, D
2
gives us the information about
the mode of operation (DCM or CCM). The value of D
2
will increase as load increases until it reaches 1, which
corresponds to the state of critical conduction when the
inductor current starts ramping up immediately after it
reaches zero (starting a new cycle).
The value of the output current and the switching
frequency at the critical mode transition (D
2
= 1) can be
expressed as following:
I
OUT
+
V
IN
2
T
ON
2 L
(
V
OUT
) V
D
)
h
(eq. 4)
F
SW
(
load
)
+
2 L I
OUT
V
IN
T
ON
2
ǒ
V
OUT
) V
D
h V
IN
* 1
Ǔ
One can determine the mode of operation using the factor
I
ROOM
defined as:
I
ROOM
+
V
IN
2
T
ON
2 L
(
V
OUT
) V
D
)
h * I
OUT
(eq. 5)
If I
ROOM
< 0,
the converter operates in continuous conduction mode.
If I
ROOM
= 0,
the converter operates in critical conduction mode.
If I
ROOM
> 0,
the converter operates in discontinuous conduction mode.
The Discontinuous Conduction Mode
For each switching cycle, if the internal MOSFET is
switched on, it will be switched off only when either the
maximum on−time, T
ON
, of typical 0.9 ms is reached or the
inductor current limit of 0.8 A is met, whichever is earlier.
Therefore, the designer can choose to use either the
maximum on−time or the current limit to turn off the
internal switch.
Minimizing the output ripple voltage
If the aim is to minimize output ripple voltage, the
maximum on−time of 0.9 ms should be used to turn off the
MOSFET; however, the maximum output current will be
reduced. It is critical to ensure that the maximum on−time
has been reached before the current limit is met.
T
ON_MAX
t
L
V
IN
I
LIM
(eq. 6)
To ensure this condition is met, the inductance L should
be selected according to the following equation:
L u
V
IN
I
LIM
T
ON_MAX
(eq. 7)
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P19

NCP1406SNT1

Mfr. #:
Buy NCP1406SNT1
Manufacturer:
ON Semiconductor
Description:
Switching Voltage Regulators 25V 25mA Boost PFM
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet