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LA5735M-TLM-E

P1-P3P4-P6P7-P7
LA5735M
No.A0588-4/7
Choke coil L1
• Note that choke coil heating due to overload or load shorting may be a problem.
The inductance value can be determined from the following equation once the input voltage, output voltage, and
current ripple conditions are known. ∆IR indicates the ripple current value.
Reference example : V
IN
= 12V, V
OUT
= 5V, ∆IR = 150mA
L =
V
IN
- V
OUT
- Vsat
∆IR
× Ton
=
12 - 5.0 - 1.0
0.15
× 1.58 × 10
-6
≈ 68
µ
H
Ton =
T
((V
IN
- V
OUT
- Vsat)/(V
OUT
+ VF)) + 1
Toff = T - Ton
t : Switching repetition period················· 3.33µs is assumed for the calculation
VF : Schottky diode forward voltage ······ 0.4V is assumed for the calculation
• Inductance current : peak value
The ripple current peak value must be held within the rated current values for the inductor used.
Here, IRP is the peak value of the ripple current. IRP can be determined from the following equation.
Reference example : V
IN
= 12V, V
OUT
= 5V, I
OUT
= 0.5A, L = 68µH
IRP = I
OUT
+
V
IN
- V
OUT
- Vsat
2L
× Ton
= 0.5 +
12 - 5.0 - 1.0
2 × 68 × 10
-6
× 1.58 × 10
-6
≈ 0.57A
• Inductance current : ripple current value
Here ∆IR is the ripple current. ∆IR can be determined from the following equation.
If the load current becomes less than one half the ripple current, the inductor current will become discontinuous.
∆IR =
V
IN
- V
OUT
- Vsat
L
× Ton
=
12 - 5.0 - 1.0
68 × 10
-6
× 1.58 × 10
-6
≈ 0.15A
Diode D1
• A Schottky barrier diode must be used for this diode.
If a fast recovery diode is used, it is possible that the IC could be destroyed by the applied reverse voltage due to the
recovery and the on-state voltage.
• Diode current (peak current)
Applications must be designed so that the peak value of the diode current remains within the rated current of the
diode.
The peak value of the diode current will be the same current as the peak value of the inductor current.
• Repetitive peak reverse voltage
Applications must be designed so that the repetitive peak reverse voltage remains within the voltage rating of the
diode.
Here, V
RRM
is the repetitive peak reverse voltage. V
RRM
can be determined from the following equation.
V
RRM
≥ V
CC
Since noise voltage and other terms will be added in actual operation, the voltage handling capacity of the device
should be about 1.5 times that given by the above calculation.
LA5735M
No.A0588-5/7
Diode D1
• Low-impedance, high-frequency components designed for use in switching power supplies must be used for C1 and
C2 since large ripple currents flow in these capacitors. Note that if a ceramic capacitor, tantalum capacitor, or other
capacitor with an extremely low equivalent series resistance (ESR) is used for C2, abnormal oscillations may occur in
the output waveform. Do not use such a capacitor for C2.
The following equation gives the stability condition for the C2 capacitor and ESR values.
1
2 × π × C2 × ESR
≤ 20kHz··················· Equation 5
• Repetitive peak reverse voltage
The AC ripple currents flowing in the input capacitor will be larger currents than those flowing in the output
capacitors.
The RMS current is given by the following equation. Note that capacitors must only be used with currents that are
within their current ratings.
IC1=
V
OUT
V
IN
I
OUT
2
1-
V
OUT
V
IN
+
1
12
× ∆IR
2
[Arms]
• Output capacitor RMS current
Since the ripple current, an AC current, flowing in the output capacitors has a sawtooth waveform, the RMS value is
given by the following equation. Select output capacitors whose allowable ripple current rating is not exceeded by
this value.
IC2 =
1
2
√
3
×
V
OUT
(V
IN
- V
OUT
)
L × fsw × V
IN
[Arms]
fsw = Switching frequency··················· 300kHz
Resistors R1 and R2
• The resistors R1 and R2 set the output voltage. The error in the V
OS
pin voltage increases as the values used for these
resistors increase. Note also that the output voltage may rise due to leakage current in the switching transistors during
light load operation. For these reasons, R1 and R2 should be selected so that the current flowing in them is on the
order of 500µA.
R1=
1.23V
500µA
≈ 2.4kΩ We recommend values in the range 2.0 to 2.4kΩ
R2=
V
OUT
1.23V
-1 × R1
The following equation gives the output voltage set by R1 and R2.
V
O
= 1+
R2
R1
× 1.23V (typ)
LA5735M
No.A0588-6/7
Protection Circuit Functional Descriptions
1. Overcurrent protection function
The overcurrent protection function detects, on a cycle-by-cycle basis, the output transistor current and turns off that
output transistor current if it exceeds 0.7A in a cycle-by-cycle manner.
2. Short circuit protection function
This IC prevents the current from increasing when the outputs are shorted by setting the switching frequency to 15kHz
if the V
OS
pin voltage falls below 0.8V.
Note : At startup, since the switching frequency will be 15kHz while the V
OS
pin voltage is 0.8V or lower, the current
capacity is reduced. If the load is applied at startup and the applications has trouble starting, increase the value of
the inductor to resolve this problem.
Timing Chart
Limit current
Inductor current
SW
OUT
voltage
0V
0.8V
1.23V
15kHz
V
IN
voltage
SW
OUT
voltage
V
OS
voltage
300kHz
P1-P3P4-P6P7-P7

LA5735M-TLM-E

Mfr. #:
Buy LA5735M-TLM-E
Manufacturer:
ON Semiconductor
Description:
Switching Voltage Regulators
Lifecycle:
New from this manufacturer.
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