Single-chip Type with Built-in FET Switching Regulators
Output 1.5A or Less
High-efficiency Step-down Switching Regulator
with Built-in Power MOSFET
BD8312HFN
●Description
BD8312HFN produces step-down output including 1.2, 1.8, 3.3, or 5 V from 4 batteries, batteries such as Li2cell or Li3cell,
etc. or a 5V/12V fixed power supply line.
This IC allows easy production of small power supply by a wide range of external constants, and is equipped with an external
coil/capacitor downsized by high frequency operation of 1.5 MHz, built-in synchronous rectification SW capable of
withstanding 15 V, and flexible phase compensation system on board.
2) Incorporates phase compensation device between input and output of Error AMP.
3) Small coils and capacitors to be used by high frequency operation of 1.5MHz
4) Input voltage 3.5 V – 14 V
Output current 1.2A(7.4V input, 3.3V output)
0.8A(4.5V input, 3.3V output)
5) Incorporates soft-start function.
6) Incorporates timer latch system short protecting function.
7) As small as 2.9mm×3 mm, SON 8-pin package HSON8
●Application
For portable equipment like DSC/DVC powered by 4 dry batteries or Li2cell and Li3cell, or general consumer-equipment
with 5 V/12 V lines
●Operating Conditions (Ta = 25℃)
Parameter Symbol Voltage circuit Unit
Power supply voltage VCC 3.5 to 14 V
Output voltage VOUT 1.2 to 12 V
●Absolute Maximum Ratings
Parameter Symbol Rating Unit
Maximum applied power voltage VCC, PVCC15 V
Maximum input current Iinmax 1.0 A
Power dissipation Pd 630 mW
Operating temperature range Topr -25 to +85 ℃
Storage temperature range Tstg -55 to +150 ℃
Junction temperature Tjmax +150 ℃
*1 When used at Ta = 25℃ or more installed on a 70×70×1.6tmm board, the rating is reduced by 5.04mW/℃.
* These specifications are subject to change without advance notice for modifications and other reasons.
[Low voltage input malfunction preventing circuit]
Detection threshold voltage VUV - 2.9 3.2 V VREG monitor
Hysteresis range ΔVUVhy 100 200 300 mV
[Oscillator]
Oscillation frequency Fosc 1.38 1.5 1.62 MHz
[Regulator]
Output voltage VREG 4.65 5.0 5.35 V
[Error AMP]
INV threshold voltage VINV 0.99 1.00 1.01 V
Input bias current IINV -50 0 50 nA VCC=12.0V , VINV=6.0V
Soft-start time Tss 3.2 5.3 7.4 msec
[PWM comparator]
LX Max Duty Dmax - - (※)100%
[Output]
PMOS ON resistance RONP - 450 600 mΩ
NMOS ON resistance RONN - 300 420 mΩ
Leak current Ileak -1 0 1 µA
[STB]
STB pin
control voltage
STB pin pull-down resistance
Operation VSTBH 2.5 - 11 V
No-operation VSTBL -0.3 - 0.3 V
250 400 700 kΩ
[Circuit current]
Standby current
VCC pin I
PVCC pin ISTB2 - - 1 µA
Circuit current at operation VCC
Circuit current at operation PVCC
(※1) 100% is MAX Duty as behavior of a PWM conparetor.
Using in region where High side PMOS is 100% on state when the same or less input voltage than output voltage is supplied as an application circuit
causes detection of SCP then DC/DC converter stops.
1. Reference
This block produces ERROR AMP standard voltage.
The standard voltage is 1.0 V.
2. 5 V Reg
5 V low saturation regulator for internal analog circuit
BD8312HFN is equipped with this regulator for the purpose of protecting the internal circuit from high voltage. Therefore,
this output is reduced when VCC is less than 5 V, then PMOS ON resistance increases and Power efficiency and
Maximum output current of DC/DC converter decreases in this region. Please see attached data (fig14,15,16,17) about
increasing of PMOS ON resistance in this region.
3 UVLO
Circuit for preventing low voltage malfunction
Prevents malfunction of the internal circuit at activation of the power supply voltage or at low power supply voltage.
Monitors VCC pin voltage to turn off all output FET and DC/DC converter output when VCC voltage is lower than 2.9 V,
and reset the timer latch of the internal SCP circuit and soft-start circuit. This threshold contains 200 mV hysteresis.
4 SCP
Timer latch system short-circuit protection circuit
When DC/DC converter is 100% High Duty , the internal SCP circuit starts counting.
The internal counter is in synch with OSC, the latch circuit is activated about 2.7 msec after the counter counts about 4000
oscillations to turn off DC/DC converter output.
To reset the latch circuit, turn off the STB pin once. Then, turn it on again or turn on the power supply voltage again.
5 OSC
Circuit for oscillating sawtooth waves with an operation frequency fixed at 1.5 MHz
6 ERROR AMP
Error amplifier for detecting output signals and output PWM control signals
The internal standard voltage is set at 1.0 V.
A primary phase compensation device of 200 pF, 62 kΩ is built in-between the inverting input terminal and the output
terminal of this ERROR AMP.
7 PWM COMP
Voltage-pulse width converter for controlling output voltage corresponding to input voltage
Comparing the internal SLOPE waveform with the ERROR AMP output voltage, PWM COMP
controls the pulse width to the output to the driver.
8 SOFT START
Circuit for preventing in-rush current at startup by bringing the output voltage of the DC/DC converter into a soft-start
Soft-start time is in synch with the internal OSC, and the output voltage of the DC/DC converter reaches the set voltage
after about 8000 oscillations.
9 PRE DRIVER/TIMING CONTROL
CMOS inverter circuit for driving the built-in synchronous rectification SW
The synchronous rectification OFF time for preventing feedthrough is about 25 nsec.
10 STBY_IO
Voltage applied on STB pin (7 pin) to control ON/OFF of IC
Turned ON when a voltage of 2.5 V or higher is applied and turned OFF when the terminal is open or 0 V is applied.
Incorporates approximately 400 kΩ pull-down resistance.
11 Pch/Nch FET SW
Built-in synchronous rectification SW for switching the coil current of the DC/DC converter
Incorporates a 450 mΩ PchFET SW capable of withstanding 15 V.and 300 mΩ SW capable of withstanding 15 V.
Since the current rating of this FET is 1.0A, it should be used within 1.0A including the DC current and ripple current of the coil.
・The radiation plate on the rear should be a GND flat surface of low impedance in common with the PGND flat surface.
・It is recommended to install a GND pin in another system as shown in the drawing without connecting it directly to this PNGD.
・Produce as wide a pattern as possible for the VBAT, Lx and PGND lines in which large current flows.
●Selection of Part for Applications
(1) Inductor
A shielded inductor that satisfies the current rating (current value,
Ipecac as shown in the drawing below) and has a low DCR
(direct resistance component) is recommended.
ΔIL
Inductor values affect inductor ripple current, which will cause output
ripple.
Ripple current can be reduced as the coil L value becomes larger
and the switching frequency becomes higher.
As a guide, inductor ripple current should be set at about 20 to 50% of the maximum input current.
*Current over the coil rating flowing in the coil brings the coil into magnetic saturation, which may lead to lower efficiency
or output oscillation. Select an inductor with an adequate margin so that the peak current does not exceed the rated
current of the coil.
(2) Output capacitor
A ceramic capacitor with low ESR is recommended for output in order to reduce output ripple.
There must be an adequate margin between the maximum rating and output voltage of the capacitor, taking the DC bias
property into consideration.
Output ripple voltage is acquired by the following equation.
Vpp=⊿IL× + ⊿IL×R
1
ESR
2π×f×Co
[V] ・・・ (3)
Setting must be performed so that output ripple is within the allowable ripple voltage.
The internal standard voltage of the ERROR AMP is 1.0 V. Output voltage is acquired by Equation (4).
VOUT
R1
R2
INV
ERROR AMP
(R1+R2)
Vo= ×1.0 [V] ・・・ (4)
R2
VREF
1.0 V
Fig.33 Setting of voltage feedback resistance
(4) DC/DC converter frequency response adjustment system
Condition for stable application
The condition for feedback system stability under negative feedback is that the phase delay is 135 °or less when gain is 1
(0dB).
Since DC/DC converter application is sampled according to the switching frequency, the bandwidth GBW of the whole
system (frequency at which gain is 0 dB) must be controlled to be equal to or lower than 1/10 of the switching frequency.
In summary, the conditions necessary for the DC/DC converter are:
- Phase delay must be 135°or lower when gain is 1 (0 dB).
- Bandwidth GBW (frequency when gain is 0 dB) must be equal to or lower than 1/10 of the switching frequency.
To satisfy those two points, R
, R2, R3, DS and RS in Fig. 34 should be set as follows.
1
, R2, R3
[1] R
1
BD8313HFN incorporates phase compensation devices of
R4=62kΩ and C2=200pF. These C2 and R
, R2, and R
1
3
valuesdecide the primary pole that determines the bandwidth
of DC/DC converter.
VOUT
R1
Cs
Rs
Inside of IC
R4
Primary pole point frequency
DC/DC converter DC Gain
fp=
2π A×( +R
DC Gain =A× ×
R
1×R2
R1+R
1
B
1
)×C2
3
2
V
IN
VO
R2
・・・・(1)
Fig.34 Example of phase compensation setting
・・・・(2)
R3
A: Error AMP Gain
About 100dB = 10
B: Oscillator amplification = 0.5
V
IN:
V
OUT
Input voltage
: Output voltage
By Equations (1) and (2), the frequency fsw of point 0 dB under limitation of the bandwidth of the DC gain at the primary
pole point is as shown below.
f
= fp×DC Gain = ××
SW
2πC2×( +R3 )
1
(R
R2)
1
・
(R1+R2)
1
B
VIN
VO
・・・・(3)
It is recommended that fsw should be approx.10 kHz. When load response is difficult, it may be set at approx. 20 kHz.
By Equation (3), R
and R2, which determine the voltage value, will be in the order of several hundred kΩ. If an
1
appropriate resistance value is not available since the resistance is so high and routing may cause noise, the use of R
enables easy setting.
[2] Cs and Rs setting
For DC/DC converter, the 2nd dimension pole point is caused by the coil and capacitor as expressed by the following
equation.
f
=
LC
1
2π√(LC)
・・・・(4)
This secondary pole causes a phase rotation of 180°. To secure the stability of the system, put a zero point in 2 places to
perform compensation.
Zero point by built-in CR f
Zero point by Cs f
Setting f
to be half to 2 times a frequency as large as f
Z2
= = 13kHz
Z1
=
Z1
1
・・・・(5)
2πR4C2
1
・・・・(6)
2π(R1+R3)CS
provides an appropriate phase margin.
LC
It is desirable to set Rs at about 1/20 of (R1+R3) to cancel any phase boosting at high frequencies.
Those pole points are summarized in the figure below. The actual frequency property is different from the ideal
calculation because of part constants. If possible, check the phase margin with a frequency analyzer or network analyzer.
Otherwise, check for the presence or absence of ringing by load response waveform and also check for the presence or
absence of oscillation under a load of an adequate margin.
(6)
(5)
(3)
(4)
Fig.35 Example of DC/DC converter frequency property
We dedicate much attention to the quality control of these products, however the possibility of deterioration or destruction
exists if the impressed voltage, operating temperature range, etc., exceed the absolute maximum ratings. In addition, it is
impossible to predict all destructive situations such as short-circuit modes, open circuit modes, etc. If a special mode
exceeding the absolute maximum rating is expected, please review matters and provide physical safety means such as
fuses, etc.
2) GND Potential
Keep the potential of the GND pin below the minimum potential at all times.
3) Thermal Design
Work out the thermal design with sufficient margin taking power dissipation (Pd) in the actual operation condition into account.
4) Short Circuit between Pins and Incorrect Mounting
Attention to IC direction or displacement is required when installing the IC on a PCB. If the IC is installed in the wrong way,
it may break. Also, the threat of destruction from short-circuits exists if foreign matter invades between outputs or the
output and GND of the power supply.
5) Operation under Strong Electromagnetic Field
Be careful of possible malfunctions under strong electromagnetic fields.
6) Common Impedance
When providing a power supply and GND wirings, show sufficient consideration for lowering common impedance and
reducing ripple (i.e., using thick short wiring, cutting ripple down by LC, etc.) as much as you can.
7) Thermal Protection Circuit (TSD Circuit)
This IC contains a thermal protection circuit (TSD circuit). The TSD circuit serves to shut off the IC from thermal runaway
and does not aim to protect or assure operation of the IC itself. Therefore, do not use the TSD circuit for continuous use or
operation after the circuit has tripped.
8) Rush Current at the Time of Power Activation
Be careful of the power supply coupling capacity and the width of the power supply and GND pattern wiring and routing since
rush current flows instantaneously at the time of power activation in the case of CMOS IC or ICs with multiple power supplies.
9
) IC Terminal Input
This is a monolithic IC and has P+ isolation and a P substrate for element isolation between each element. P-N junctions
are formed and various parasitic elements are configured using these P layers and N layers of the individual elements.
For example, if a resistor and transistor are connected to a terminal as shown on Fig.36:
○ The P-N junction operates as a parasitic diode when GND > (Terminal A) in the case of a resistor or when GND >
(Pin B) in the case of a transistor (NPN)
○ Also, a parasitic NPN transistor operates using the N layer of another element adjacent to the previous diode in
the case of a transistor (NPN) when GND > (Pin B).
The parasitic element consequently rises under the potential relationship because of the IC’s structure. The parasitic
element pulls interference that could cause malfunctions or destruction out of the circuit. Therefore, use caution to avoid
the operation of parasitic elements caused by applying voltage to an input terminal lower than the GND (P board), etc.
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