AP1538 DIODES [Diodes Incorporated], AP1538 Datasheet - Page 9

• Manufacturer Part Number: AP1538
• Description: 18V 3A 300KHz BUCK CONVERTER
• Manufacturer: DIODES [Diodes Incorporated]

Functional Description
PWM Control
The AP1538 is a DC/DC converter that employs pulse width
modulation (PWM) scheme. Its pulse width varies in the range of
0% to 99%, based on the output current loading. The output
ripple voltage caused by the PWM high frequency switching can
easily be reduced through an output filter. Therefore, this
converter provides a low ripple output supply over a broad range
of input voltage & output current loading
Under Voltage Lockout
The under voltage lockout circuit of the AP1538 assures that the
high-side MOSFET driver remains in the off state whenever the
supply voltage drops below 3.3V. Normal operation resumes
once V
Current Limit Protection
The current limit threshold is set by external resistor R
connected from V
current I
at OCSET pin. When the PWM voltage is less than the voltage at
OCSET, an over-current condition is triggered.
The current limit threshold is given by the following equation:
where,
I
resistance; F
inductor value will affect the ripple current ΔI.
The above equation is recommended for input voltage range of
5V to 18V. For input voltage lower than 5V or ambient
temperature over 100°C, higher R
The recommended minimum R
AP1538 Rev. 2
PEAK
V
(V)
0.8
1.0
1.2
1.8
2.5
3.3
5.0
OUT
is the output peak current; R
CC
OCSET
rises above 3.5V.
I
S
PEAK
(90uA typical) across this resistor sets the voltage
is the PWM frequency (300KHz typical). Also, the
4.7K
5.6K
5.6K
5.6K
5.6K
5.6K
N/A
5V
I
Δ
PEAK
CC
×
I
R
=
supply to OCSET pin. The internal sink
DS(ON)
>
V
IN
I
F
OUT
−
S
V
=
OCSET
×
V
IN
3.0K
3.0K
3.0K
3.0K
3.0K
3.0K
3.0K
(MAX)
12V
I
OUT
L
OCSET
(V)
OCSET
DS (ON)
value is summarized below:
×
+
is recommended.
V
×
V
OUT
(
IN
R
Δ
2
is the MOSFET ON
I)
OCSET
3.3K
3.9K
3.9K
3.9K
3.9K
3.9K
4.7K
18V
www.diodes.com
OCSET
9 of 14
Inductor Selection
For most designs, the operates with inductors of 22µH to
33µH. The inductor value can be derived from the following
equation:
Where ΔI
ripple current and small value inductors result in high ripple
current. Choose inductor ripple current approximately 15% of the
maximum load current 3A, ∆I
the inductor should be at least equal to the maximum load current
plus half the ripple current to prevent core saturation
(3A+0.225A).
Input Capacitor Selection
This capacitor should be located close to the IC using short leads
and the voltage rating should be approximately 1.5 times the
maximum input voltage. The RMS current rating requirement for
the input capacitor of a buck regulator is approximately 1⁄2 the
DC load current. A low ESR input capacitor sized for maximum
RMS current must be used. A 470µF low ESR capacitor for most
applications is sufficient.
Output Capacitor Selection
The output capacitor is required to filter the output voltage and
provides regulator loop stability. The important capacitor
parameters are the 100KHz Equivalent Series Resistance (ESR),
the RMS ripples current rating, voltage rating and capacitance
value. For the output capacitor, the ESR value is the most
important parameter. The output ripple can be calculated from
the following formula.
The bulk capacitor’s ESR will determine the output ripple voltage
and the initial voltage drop after a high slew-rate transient.
An aluminum electrolytic capacitor's ESR value is related to the
capacitance and its voltage rating. In most case, higher voltage
electrolytic capacitors have lower ESR values. Most of the time,
capacitors with much higher voltage ratings may be needed to
provide the low ESR values required for low output ripple voltage.
PCB Layout Guide
If you need low T
SW pins(5& 6) and Vss pins(7& 8)on the SOP-8L package are
internally connected to die pad, The evaluation board should be
allowed for maximum copper area at output (SW) pins.
1. Connect FB circuits (R
2. Connect C3 to Vcc and Vss pin as closely as possible to get
3. Connect R4 to Vcc and OCSET pin as closely as possible.
4. Connect ground side of the C2 & D1 & C4 as closely as
keep away from inductor flux for pure V
good power filter effect.
possible and use ground plane for best performance.
18V 3A 300KHz BUCK CONVERTER
L
is inductor Ripple Current. Large value inductors lower
C
& T
L
V
=
J
RIPPLE
or large PD (Power Dissipation), The dual
(
V
1
2
, R
IN
×
=
L
2
−
=0.45A. The DC current rating of
Δ
, C
Δ
V
I
I
L
L
1
OUT
) as closely as possible and
×
×
ESR
f
)
T
osc
FB
ON
.
©
AP1538
Diodes Incorporated
JUNE 2008