NCP81038 D

NCP81038
Synchronous Buck Controller
with Auto Power Saving
Mode and Built-In LDO
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Wide Input Voltage Range: from 5.5 V to 28 V
Built−in 5 V / 60 mA LDO
Built−in selectable 3.3 V or 12 V / 10 mA LDO
Three Selectable Fixed Frequency 300 KHz, 400 KHz or 600 KHz
180 Interleaved Operation Between the Two Channels in
Continue−Conduction−Mode (CCM)
Selected Power−Saving Mode/Forced PWM Mode
Transient−Response−Enhancement (TRE) Control
Input Supply Voltage Feed Forward Control
Resistive or Lossless Inductor’s DCR Current Sensing
Over−Temperature Protection
Internal Fixed 8.5 ms Soft−Start
Fixed Output Voltages 5 V and 3.3 V
Power Good Outputs for Both Channels
Built−in Adaptive Gate Drivers
Output Discharge Operation
Built−in Over−Voltage, Under−Voltage Protection
Accurate Over−Current Protection
Thermal Shutdown
MARKING
DIAGRAM
81038
ALYWG
G
1
28 PIN QFN, 4x4
MN SUFFIX
CASE 485AR
81038
A
L
Y
W
G
= Specific Device Code
= Assembly Location
= Wafer Lot
= Year
= Work Week
= Pb−Free Package
(Note: Microdot may be in either location)
PIN CONNECTIONS
5V_LDOBYP
5V_LDOOUT
5V_LDOEN
SKIP
LDO2_EN
LDO2_OUT
VIN
Features
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GH2
BST2
SWN2
GL2
PGND2
CSP2
CSN2
28
1
GND
GH1
BST1
SWN1
GL1/FSET
PGND1
CSP1
CSN1
FB2
COMP2
EN2/SS2
PG
EN1/SS1
COMP1
FB1
NCP81038 is a dual synchronous buck controller that is optimized
for converting the battery voltage or adaptor voltage into multiple
power rails required in desktop and notebook system. NCP81038
consists of two buck switching controllers with fixed 5.0 V output on
channel 2, 3.3 V on channel 1 and two on−board LDOs with three
outputs: 5 V / 60 mA and 3.3 V or 12 V / 10 mA. NCP81038 supports
high efficiency, fast transient response and provides power good
signals. ON Semiconductor proprietary adaptive−ripple control
enables seamless transition from CCM to DCM, where converter runs
at reduced switching frequency with much higher efficiency at light
load. The part operates with supply voltage ranging from 5.5 V to
28 V. NCP81038 is available in a 28−pin QFN package.
(Top View)
Applications
• Desktop / Notebook Computers
• System Power Supplies
• I/O Power Supplies
ORDERING INFORMATION
Device
Package
Shipping†
NCP81038MNTWG
QFN−28
(Pb−Free)
4,000 /
Tape & Reel
†For information on tape and reel specifications,
including part orientation and tape sizes, please
refer to our Tape and Reel Packaging Specification
Brochure, BRD8011/D.
© Semiconductor Components Industries, LLC, 2013
October, 2013 − Rev. 10
1
Publication Order Number:
NCP81038/D
NCP81038
5V_LDOOUT
VIN
−
+
+
−
ENABLE
5V_LDOEN
5V_LDOBYP
LDO
VREF
LDO2_OUT
−
+
ENABLE
LDO2_EN
CSP1
CSN1
+
−
SKIP
CSA
BST1
COMP1
Control Logic
Ramp Generator
And
PWM Logic
Vbias
VREF
+ E/A
SS
−
FB1
OC & TRE Detection
EN1
PG
GND
PG1
PG2
Vbias
GH1
SW1
5V_LDOOUT
UVLO,
UVP, OVP,
Power Good
OCP, TSD and
Protection
GL1/
FSET
PGND1
VIN
Switcher 1 Shown
Figure 1. Block Diagram
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2
NCP81038
Table 1. PIN DESCRIPTIONS
Pin
No.
Symbol
1, 21
GH1, GH2
Description
Gate driver output of the top N−channel MOSFET.
2, 20
BST1, BST2
3, 19
SWN1, SWN2
Top gate driver input supply, a bootstrap capacitor connection between SWNx and this pin.
Switch node between the top MOSFET and bottom MOSFET.
4
GL2
Gate driver output of bottom N−channel MOSFET in channel2.
18
GL1/FSET
5, 17
PGND1, PGND2
6, 16
CSP1, CSP2
Inductor current differential sense non−inverting input.
7, 15
CSN1, CSN2
Inductor current differential sense inverting input.
8, 14
FB1, FB2
Gate driver output of bottom N−channel MOSFET in channel1. And it is also used to set up switching
frequency by connecting a resistor from this pin to ground.
Power ground for channel 1 & 2.
Output voltage feed back.
9, 13
COMP1, COMP2
10, 12
EN1, EN2
11
PG
Power good indicator for both output voltages. Open−drain output.
22
VIN
Battery or Adaptor input voltage
23
LDO2_OUT
24
LDO2_EN
25
SKIP
26
5V_LDOEN
27
5V_LDOOUT
The output for internal 5 V LDO. A capacitor of minimum 4.7 mF is recommended to connect between
this pin and ground.
28
5V_LDOBYP
5 V LDO bypass pin.
E−Pad
Output of the error amplifier.
Channel 1 and channel 2 enable pin. Short this pin to ground to disable the switcher channel. Pull this
pin high to enable the switcher channel.
Second internal LDO output. A capacitor of minimum 1.0 mF is recommended to connect between this
pin and ground.
Enable for second internal LDO
− Tie to VCC to setup LDO2 output at 12 V
− Tie to 1/2VCC to setup LDO2 output at 3.3 V
− Tie to ground to disable LDO
DCM programming pin:
− Ground this pin to setup automatic CCM−DCM transfer with 33 KHz minimum switching
frequency limitation;
− Connect this pin to VCC to force CCM operation;
− Leave this pin open to give automatic CCM−DCM transfer with 33 KHz minimum switching
frequency for channel 1 but forced CCM for channel 2.
Enable for internal 5 V LDO.
GND.
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3
NCP81038
LDO2_OUT
(3.3 V)
VIN
10 K
5V_LDO
10 K
28
27
VIN
LDO2_OUT
SKIP
22 mFx2
26 25 24 23 22
21
20
19
AGND
18
17
GH1
BST1
0.1 mF
SWN1
3.3 mH
Vout1
3.3 V / 6.0 A
GL1/FSET
PGND1
16 CSP1
CSN1
15
EN2/SS2
FB1
EN1/SS1
COMP1
10 11 12 13 14
PG
9
COMP2
8
FB2
5V_LDOEN
5V_LDOOUT
5V_LDOBYP
Vout2
5.0 V / 6.0 V
GH2
BST2 1
0.1 mF
2
SWN2
3
GL2
4
PGND2
5
CSP2
6
CSN2
7
LDO2_EN
EN5
SKIP
VOUT1
Figure 2. Application Circuit
Table 2. ABSOLUTE MAXIMUM RATINGS (Note 1)
VIN to GND, 5V_LDOEN to GND
−0.3 V (DC) to 28 V −1.0 V for T < 100 n
SWN1, SWN2 to GND
−0.6 V to 28 V, −10.0 V for T < 20 ns
BST1, BST2 to GND
−0.6 V to 34 V
GH1, GH2 to GND
−0.6 V to 34 V, −5.0 V for T < 100 ns
PGND1, PGND2
−0.3 V to 0.3 V
All other pin
−0.3 V to 6.0 V, −1.0 V for T < 100 ns
Operating Temperature Range, TA
0°C to +85°C
Junction Temperature, TJ
−40°C to +150°C
Storage Temperature Range, TS
−55°C to +150°C
Pkg Power Dissipation (TA = +25°C), PD (Note 2)
2.45 W max
RqJA = 51°C/W
RqJ−Lead = 26°C/W
RqJ−BoardTop = 3.2°C/W
Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the
Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect
device reliability.
1. Operation at −40°C to 0°C guaranteed by design, not production tested.
2. These data are based on JEDEC JESD51.7 highly conductive PCB multiple layer PCB (2 power and/or 2 ground planes 76 mm x 76 mm
1 oz each) connected by 20 thermal vias. 100 sq mm Cu heat spreader, 2 oz.
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4
NCP81038
Table 3. ELECTRICAL CHARACTERISTICS
(VIN = 12 V, Vout = 5.0 V, TA = +25°C for typical value; 0°C < TA < 85°C for min/max values unless noted otherwise)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Units
28
V
5.15
V
POWER SUPPLY
Input Supply Voltage
VIN
5.5
INTERNAL LDO OUTPUT
5V_LDOOUT Voltage
VIN =12 V, I5V_LDOOUT = 60 mA
5V_LDOOUT Current
VIN =12 V, EN1 = EN2 = 0
5V_LDO Switch to Bypass Threshold
4.85
5.0
100
4.7
5V_LDOOUT to 5V_IDOBYP Impedance
LDOBYP = 5 V
4.95
V
1.0
W
100
200
mV
LDO2_EN = VCC, VIN = 15 V,
Load Current = 10 mA
11.4
12
12.6
V
LDO2_EN = 1/2 VCC, VIN = 15 V,
Load Current = 10 mA
3.2
3.3
3.46
V
10
15
Hysteresis
LDO2_OUT Voltage
mA
LDO2_OUT Current
mA
SUPPLY CURRENT
BSTx Quiescent Current
BSTx Shutdown Supply Current
IBST
IBST_SD
Vin Pin Supply Current
Shutdown Current
IVIN_SD
VFB = 1.5 V, EN = 5.0
(No Switching), GH and GL are open
0.3
EN = 0, BST = 5 V, SWN = 0
6.0
mA
mA
Iload = 0
2.0
mA
EN1, EN2, LDOEN, LDO2_EN = 0
5.0
mA
EN1, EN2, LDOEN = 5 V,
LDO2_EN = 5 V
1.15
mA
EN1, EN2 = 0, LDOEN = 5 V,
LDO2_EN = 2.5 V
1.57
mA
EN1, EN2 = 0, LDOEN = 5 V,
LDO2_EN = 0
1.11
mA
OSCILLATOR
Oscillator Frequency
Fsw
Rset = 1.8 k
270
300
330
KHz
Rset = 9.1 k
340
400
460
KHz
Rset = 16 k
540
600
660
KHz
±10
%
Oscillator Frequency Accuracy
ERROR AMPLIFIER
80
Open Loop DC Gain (Note 3)
Open Loop Unity Gain Bandwidth (Note 3)
F0dB,EA
10
Open Loop Phase Margin (Note 3)
dB
15
MHz
60
Input Bias Current (Note 3)
deg
−200
200
nA
−1.0
1.0
mV
Input Offset Voltage (Note 3)
V+ = V− = 0.8 V
Slew Rate
COMP pin to GND = 10 pF
Maximum Output Voltage
10 mV of overdrive, ISOURCE = 2.0 mA
Minimum Output Voltage
10 mV of overdrive, ISINK = 2.0 mA
Output Source Current
10 mV of overdrive, Vout = 3.5 V
2.0
mA
Output Sink Current
10 mV of overdrive, Vout = 1.0 V
2.0
mA
3. Guaranteed by Design
4. Parameters are for design only, not for product test.
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5
2.5
V/ms
3.3
V
0.3
V
NCP81038
Table 3. ELECTRICAL CHARACTERISTICS
(VIN = 12 V, Vout = 5.0 V, TA = +25°C for typical value; 0°C < TA < 85°C for min/max values unless noted otherwise)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Units
792
800
808
mV
0.25
%/V
5.5
V
FEEDBACK VOLTAGE
Reference Voltage
VREF
Feedback Voltage Line Regulation
5V_LDOOUT = 4.5 V ~ 5.5 V
DIFFERENTIAL CURRENT SENSE AMPLIFIER
CSP and CSN Common−mode Input
Voltage Range
Refer to AGND
Current Sense Input to Output Gain
(CSP)−(CSN) = 10 mV
Differential Input Voltage Range
−0.2
200
−60
mA/V
60
mV
45
mV
46
mV
OVER CURRENT PROTECTION
OCP Threshold Voltage
OCP Trigger Clock Tick
V(CSP)−V(CSN) @ 25°C
35
V(CSP)−V(CSN) @ 0 ~ 85°C
34
After EN, latch off after trigger # clocks
40
16
Short Circuit OCP Threshold Voltage
60
mV
GATE DRIVER
GH Pull−High Resistance
RH_GH
Source, V(BST−GH) = 0.1
2.5
W
GH Pull−Low Resistance
RL_GH
Sink, V(GH−SWN) = 0.1 V
1.5
W
GL Pull−High Resistance
RH_GL
Source, V(VCC−GL) = 0.1 V
2.0
W
GL Pull−Low Resistance
RL_GL
Sink, V(GL−PGND) = 0.1 V
1.0
W
Dead Time
GL off to GH on
10
20
30
ns
GH off to GL on
10
20
30
ns
VOLTAGE MONITOR
VCC Start Threshold
3.7
4.2
4.4
V
VCC UVLO Hysteresis
100
200
300
mV
91.5
95
97.5
%
Power Good Threshold
Power Good High Delay
PG in from lower
PG hysteresis
5.0
%
After soft start is done
500
ms
1.5
ms
mA
Power Good Low Delay
Power Good Sink Current
PG = 0.4 V
2.5
5.0
Output Overvoltage Rising Threshold
After VCC POR, with respect to VFB
106
110
Overvoltage Fault Blanking Time
Output Under-Voltage Trip Threshold
118
1.5
After soft start, with respect to VFB
45
50
%Vref
ms
55
%Vref
Under-voltage Protection Blanking Time
tss
ms
Under−voltage Protection Delay
2.0
ms
PWM
Minimum Controllable ON Time
50
Minimum OFF Time
100
150
ns
0.4
0.44
V
PWM Ramp Offset
PWM Ramp Amplitude
PWM Comparator Propagation Delay
0.36
ns
VIN = 5 V
1.25
V
VIN = 12 V
3.0
V
10 mV to 20 mV overdrive
25
3. Guaranteed by Design
4. Parameters are for design only, not for product test.
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6
30
ns
NCP81038
Table 3. ELECTRICAL CHARACTERISTICS
(VIN = 12 V, Vout = 5.0 V, TA = +25°C for typical value; 0°C < TA < 85°C for min/max values unless noted otherwise)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Units
INTERNAL BST DIODE
Forward Voltage Drop
IF = 10 mA, TA = 25°C
0.3
V
Reverse−bias Leakage Current
VBST = 34 V, VSW = 28 V, TA = 25°C
0.1
6.0
mA
EN = 0, Vout = 0.5 V
20
30
W
SOFT−STOP
Output Discharge On−Resistance
Discharge Threshold in Vcc
0.7
V
SOFT−START
Soft−Start Ramp Time
tss
From EN assertion to Vout ready
6.0
HI Threshold
1.4
12
18
ms
EN
EN1/EN2 Threshold
V
LO Threshold
5V_LDOEN Threshold
0.4
Hysteresis
200
mV
Source Current, pull high to 5 V internally
0.75
mA
HI Threshold
1.4
V
LO Threshold
0.4
Hysteresis
LDO2_EN
V
200
Vout = 3.3 V
1.5
Vout = 12 V
4.95
2.5
Vout = 0
V
mV
3.5
V
5.5
V
0.4
V
THERMAL SHUTDOWN
Thermal Shutdown Threshold (Note 3)
150
°C
Thermal Shutdown Hysteresis (Note 3)
40
°C
3. Guaranteed by Design
4. Parameters are for design only, not for product test.
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NCP81038
DETAILED DESCRIPTION
Overview
temperature range of the controller. The reference voltage is
trimmed using a test configuration that accounts for error
amplifier offset and bias currents.
The NCP81038 is a cost effective dual output controllers
with three selectable LDO outputs suitable for desktop and
server application. It provides one independent LDO which
is 5 V/100 mA, two selectable LDOs which is 12 V or
3.3 V/10 mA, and two synchronous PWM controllers that
incorporate all the control and protection circuitry necessary
to satisfy a wide range of applications. The NCP81038
PWM switchers employ adaptive−ripple control to provide
seamless transition between CCM and DCM while maintain
high efficiency during light load. It also provides fast
transient response and excellent stability. The features of the
NCP81038 include a precision reference, selectable
switching frequency, an error amplifier, adaptive gate
driver, programmable soft−start, and very low shutdown
current. The protection features of the NCP81038 include
fixed/programmable soft−start, over−current protection,
wide input voltage range, power good monitor, over voltage
and under voltage protection, built in output discharge and
thermal shutdown.
Oscillator Frequency
A fixed precision oscillator is provided. The actual
switching frequency is set at 300 KHz, 400 KHz or 600 KHz
by the resistor on GL1/FSET pin. The resistor and frequency
can be referred to the table below.
FSET resistor
1.8 K
9.1 K
16 K
Switching Frequency
300 KHz
400 KHz
600 KHz
Error Amplifier
The error amplifier’s primary function is to regulate the
converter’s output voltage using a resistor divider connected
from the converter’s output to the FB pin of the controller,
as shown in the Applications Schematic. A type III
compensation network must be connected around the error
amplifier to stabilize the converter. It has a bandwidth of
greater than 15 MHz, with open loop gain of at least 80 dB.
The COMP output voltage is clamped to a level above the
oscillator ramp in order to improve large−scale transient
response.
5V LDO and Switchover (5V_LDOOUT)
The NCP81038 includes a high−current (100 mA) linear
regulator that is configured for 5 V operation, which is bias
supply necessary to power up the main analog supply rail for
the IC and provides the current for the gate drivers. When the
3.3 V switching regulator is running and the 5 V switching
regulator is still off (EN2 = 0), the 5 V linear regulator can
provide about 80 mA to external load, while the remaining
20 mA is consumed by the 3.3 V regulator’s MOSFETS’
switching, giving typical switching frequency and
MOSFETS’ gate capaciatance. Once the 5 V switching
regulator is enabled, this 5V_LDO may be bypassed using
5V_LDOBYP input. Typically, a capacitor with 10−mF or
higher is needed to keep 5V_LDO stable. Additionally, if
VOUT2 voltage exceeds 4.75 V, the 5V_LDO is switched
off and VOUT2 (5V buck output) is connected to
5V_LDOOUT through a bypass FET (typical 1 ohm) to
provide 5 V rail. With this bypass function, the whole system
efficiency is improving. The 5V_LDOEN pin is high
voltage and can be connected to VIN voltage. However,
5V_LDOEN is not allowed to go beyond VIN pin voltage.
Soft−Start
To limit the start−up inrush current, an internal soft start
circuit is used to ramp up the reference voltage from 0 V to
its final value linearly. The internal soft start time is 13 ms
typically, from EN assertion to Vout ready. It includes a
delay of 240 ms from EN assertion to the Vout ramp starting.
500 ms after both channel Vout ready, the PG (Power Good)
is asserted.
Soft−Stop
Soft−Stop or discharge mode is always on during faults or
disable. In this mode, a fault (UVP, OCP, TSD) or disable
(EN) causes the output to be discharged through an internal
20−ohm transistor inside of VO terminal. The time constant
of soft−stop is a function of output capacitance and the
resistance of the discharge transistor.
Adaptive Non−Overlap Gate Driver
In a synchronous buck converter, a certain dead time is
required between the low side drive signal and high side
drive signal to avoid shoot through. During the dead time,
the body diode of the low side FET free-wheels the current.
The body diode has much higher voltage drop than that of
the MOSFET, which reduces the efficiency significantly.
The longer the body diode conducts, the lower the
efficiency. NCP81038 implements adaptive dead time
control to minimize the dead time, as well as preventing
shoot through from happening.
LDO2_OUT
The NCP81038 includes 10 mA linear regulators that can
be programmed for 12 V or 3.3 V operations. LDO2 can be
enabled only when VCC is present. When LDO2_EN is
connected to VCC, LDO2_OUT is programmed at 12 V.
When LDO2_EN is connected to 1/2VCC, LDO2_OUT is
set at 3.3 V. Typically, a minimum capacitor with 1.0−mF or
higher is needed to keep LDO2_OUT stable.
Reference Voltage
The NCP81038 incorporates an internal reference that
allows output voltages as low as 0.8 V. The tolerance of the
internal reference is guaranteed over the entire operating
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8
NCP81038
Forced Pulse Width Modulation (FPWM Mode)
Over Voltage Protection (OVP)
The device is operating as force PWM mode if SKIP is
tied to VCC. Under this mode, the low−side gate driver
signal is forced to be the complement of the high−side gate
driver signal. This mode allows reverse inductor current, in
such a way that it provides more accurate voltage regulation
and better (fast) transient response. During the soft start
operation, the NCP81038 automatically runs as FPWM
mode regardless of the SKIP setting at either FPWM or
SKIP mode to make sure to have smooth power up.
When VFB voltage is above 110% (typical) of the
nominal VFB voltage, the top gate drive is turned off and the
bottom gate drive is turned on trying to discharge the output.
It over voltage condition still exists after 1.5 ms, an OV fault
is set. The power good will go low at the same time. The
bottom gate drive will be turned off when VFB drops below
the under voltage threshold. If then over voltage condition
happens again, the high side MOSFET stays off and low side
MOSFET will turn on again till output voltage drops down
to under voltage threshold. Then low side gate will be off.
EN resets or power recycle the device can exit the fault.
Power Save Mode (Skip Mode)
If the load current decreases, the converter will enter
power save mode operation when SKIP pin is grounded.
During power save mode, the converter skips switching and
operates with reduced frequency but with minimum
switching frequency of 33 KHz, which minimizes the
quiescent current and maintains high efficiency. If SKIP pin
is open, the channel 1 will enter power saving mode with
reduced load but with minimum switching frequency of
33 KHz and channel 2 will stay in forced PWM mode.
Under Voltage Protection (UVP)
An UVP circuit monitors the VFB voltage to detect under
voltage event. The under voltage limit is 50% (typical) of the
nominal VFB voltage. If the VFB voltage is below this
threshold over 1 ms, an UV fault is set and the device is
latched off such that both top and bottom gate drives are off.
EN resets or power recycle the device can exit the fault. UVP
is delayed for soft start period (8.5 ms) after EN goes high.
Transient Response Enhancement (TRE)
EN1 and EN2
For a conventional trailing−edge PWM controller in
CCM, the minimum response delay time is one switching
period in the worst case. To further improve transient
response, a transient response enhancement circuitry is
introduced to the NCP81038. The controller continuously
monitors the COMP signal, which is the output voltage of
the error amplifier, to detect load transient events. A desired
stable close−loop system with the NCP81038 has a ripple
voltage in the COMP signal, which peak−to−peak value is
normally in a range from 200 mV to 500 mV. There is a
threshold voltage made in a way that a filtered COMP signal
pluses an offset voltage. Once a large load transient occurs,
the COMP signal is possible to exceed the threshold and then
TRE is tripped in a short period, which is typically around
one normal switching cycle. In this short period, the
controller runs at higher frequency and therefore has faster
response. After that the controller comes back to normal
operation.
EN1 and EN2 are logic level control signals to turn on or
off buck converters individually. If ENx is below 0.4 V, the
buck will be off. When ENx is above 1.8 V, the buck is
turning on. In both ENx pins, there are about 0.75 mA source
currents to pull them up to 5 V internally.
Power Good Monitor (PG)
NCP81038 provides window comparator to monitor the
output voltage. When the output voltage is above 95% of
regulation voltage, the power good pin outputs a high signal.
Otherwise, PG stays low. The PG pin is open drain 5−mA
pull down output. During startup, PG stays low until the
feedback voltage is within the specified range for 128 clocks
or about 0.5 ms. If feedback voltage falls outside the
tolerance band, the PG pin goes low within microseconds.
Over Current Protection (OCP)
The NCP81038 protects converter if over−current occurs.
The current through each channel is continuously monitored
with differential current sense. Current limit threshold
Vth_OC between CS+ and CS− is internally fixed to 40 mV.
The current limit can be programmed by inductor’s DCR
and current sensing resistor divider with Rs1 and Rs2.
PROTECTIONS
Under Voltage Lockout (UVLO)
There are two undervoltage lock out protections (UVLO)
in NCP81038. One is for VIN, which has a typical trip
threshold voltage 3.9 V and trip hysteresis 200 mV. The
other is for VCC (5V_LDOOUT serves as VCC internally),
which has a typical trip threshold voltage 4.2 V and trip
hysteresis 300 mV. If either is triggered, the device resets
and waits for the voltage to rise up over the threshold voltage
and restart the part. Please note this protection function
DOES NOT trigger the fault counter to latch off the part.
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9
NCP81038
L
DCR
Vin
I LIM + I LIM(Peak) *
Vout
C
Rs1
where Vin is the input supply voltage of the power stage, and
fsw is normal switching frequency.
Fig. X+1 shows NTC resistor network to compensate the
temperature drift of DCR.
If inductor current exceeds the current threshold, the
high−side gate driver will be turned off cycle−by−cycle. In
the mean time, an internal OC fault timer will be triggered.
If the fault still exists after 16 clocks, the part latches off,
both the high−side MOSFET and the low−side MOSFET are
turned off. If the sensed current reaches 60 mV, the part will
latch off right away. The fault remains set until the system
has shutdown and re−applied VCC and/or the enable signal
EN is toggled.
Rs2
Vc
+
−
Figure 3. X
L
DCR
Vin
Vout
C
R
V O @ (V in * V O)
2 @ V in @ f SW @ L
Pre−Bias Startup
R2
R1
RNTC
RTHE
Vc
+
In some applications the controller will be required to start
switching when its output capacitors are charged anywhere
from slightly above 0 V to just below the regulation voltage.
This situation occurs for a number of reasons: the
converter’s output capacitors may have residual charge on
them or the converter’s output may be held up by a low
current standby power supply. NCP81038 supports pre−bias
start up by holding Low side FETs off till soft start ramp
reaches the FB pin voltage.
−
Figure 4. X + 1
The Rs1, Rs2 and C can be calculated as:
C @ ǒR S1ńńR S2Ǔ +
L
DCR
Thermal Shutdown
The NCP81038 protects itself from over heating with an
internal thermal monitoring circuit. If the junction
temperature exceeds the thermal shutdown threshold the
voltage at the COMP pin will be pulled to GND and both the
upper and lower MOSFETs will be shut OFF.
The inductor peak current limit is:
I LIM(Peak) +
V th_DC
k @ DCR
, where k +
R S2
R S1 ) R S2
The DC current limit is:
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NCP81038
PACKAGE DIMENSIONS
QFN28 4x4, 0.4P
CASE 485AR
ISSUE A
PIN ONE
REFERENCE
ÇÇÇÇ
ÇÇÇÇ
ÇÇÇÇ
ÇÇÇÇ
B
A
D
L
L1
DETAIL A
ALTERNATE TERMINAL
CONSTRUCTIONS
E
NOTES:
1. DIMENSIONING AND TOLERANCING PER ASME
Y14.5M, 1994.
2. CONTROLLING DIMENSION: MILLIMETERS.
3. DIMENSION b APPLIES TO PLATED TERMINAL
AND IS MEASURED BETWEEN 0.15 AND 0.30 MM
FROM THE TERMINAL TIP.
4. COPLANARITY APPLIES TO THE EXPOSED PAD
AS WELL AS THE TERMINALS.
ÉÉ
ÉÉ
EXPOSED Cu
0.10 C
0.10 C
L
DIM
A
A1
A3
b
D
D2
E
E2
e
K
L
L1
MOLD CMPD
DETAIL B
TOP VIEW
ALTERNATE
CONSTRUCTION
A
DETAIL B
A3
0.10 C
MILLIMETERS
MIN
MAX
0.80
1.00
0.00
0.05
0.20 REF
0.15
0.25
4.00 BSC
2.50
2.70
4.00 BSC
2.50
2.70
0.40 BSC
0.30 REF
0.30
0.50
−−−
0.15
0.08 C
NOTE 4
A1
SIDE VIEW
SEATING
PLANE
C
0.10 C A B
K
8
4.30
2.71
0.10 C A B
15
28X
RECOMMENDED
MOUNTING FOOTPRINT
D2
DETAIL A
L
28X
0.62
1
E2
2.71
4.30
1
PIN 1
INDICATOR
22
e
28X
BOTTOM VIEW
PACKAGE
OUTLINE
b
0.07 C A B
0.05 C
0.40
PITCH
NOTE 3
28X
0.26
DIMENSIONS: MILLIMETERS
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NCP81038/D