BD9528AMUV Datasheet, PDF (20/30 Pages) Rohm – High Performance Regulators for PCs Main Power Supply for Notebook PCs(With Built-in Linear Regulator)

BD9528AMUV
Technical Note
6. Setting over current protection
VIN
Detecting the ON resistance (between SW and PGND voltage) of MOSFET at
low side, it set the over current voltage protection.
Over current reference voltage (I_{LIM_ref}) is determined as in formula(12) below.
L
SW
VOUT
Co
ILIM_ref=
10k
RILIM[kΩ] ×RON[mΩ]
[A]・・・(12)
(R_ILIM: Resistance for setting of over current voltage protection value[kΩ]
RON: Low side ON resistance value of FET[mΩ])
RILIM
PGND
Coil current
Iocp
ILIM_ref
However, the value, which set the over current protection actually, is
determined by the formula (13) below.
Iocp= ILIM_ref +
1
2
⊿IL
= ILIM_ref +
1
2
×
VIN - Vo _×
L
I
f
×
Vo
VIN
・・・(13)
(⊿IL:Coil ripple current[A]、VIN:Input voltage[V]、Vo:Output voltage [V]
f:Switching frequency [H_Z]、L:Coil inductance [H])
(Example)
If load current 5A want to be realized with VIN=6～19V、VOUT=5V、f=400ｋHZ、L=2.5uH、RON=20mΩ, the formula would
be below.
Iocp=
10k
RILIM[kΩ] ×RON[mΩ]
+
1
2
×
VIN - Vo _×
L
I
f
×
Vo
VIN
＞5
When VIN=6V, Iocp will be minimum(this is because the ripple current is also minimum) so that if each condition is input,
the formula will be the following: RILIM＜109.1[kΩ].
※To design the actual board, please consider enough margin for FET ON resistor dispersion, Coil inductor dispersion, IC
over current reference value dispersion, frequency dispersion.
7. Relation between output voltage and TON time
The BD9528AMUV, both 1ch and 2ch, are high efficiency synchronous regulator controller with frequency variable.
TON time varies with Input voltage [VIN], output voltage [VOUT], and RFS of FS pin resistance.
TON time is calculated with the following formula:
Ton =k
VOUT・RFS
VIN
[nsec]・・・(14)
From TON time above, frequency on application condition is following:
Frequency =
VOUT
VIN ^×
1
Ton
[kHz]・・・(15)
However, real-life considerations (such as the external MOSFET gate capacitor and switching speed) must be factored in
as they affect the overall switching rise and fall time, so please confirm in reality by the instrument.
3.5
3
2.5
2
1.5
1
0.5
0
0
VIN=7V
VIN=12V
VIN=21V
20
40
60
80
100
120
RFS[kΩ]
RFS – ontime(VOUT=5V)
2.5
2
1.5
1
0.5
0
0
VIN=7V
VIN=12V
VIN=21V
20
40
60
80 100 120
RFS[kΩ]
RFS – ontime(VOUT=3.3V)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
VIN=7V
VIN=12V
VIN=21V
20
40
60
80 100 120
RFS[kΩ]
RFS – ontime(VOUT=1V)
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20/29
2011.11 - Rev.A