Diodes AP1534 - Manual (Page 6)

AP1534
PWM CONTROL 2A STEP-DOWN CONVERTER
AP1534
Document number: DS31314 Rev. 6 - 2
6 of 9
www.diodes.com
April 2011
© Diodes Incorporated
Functional Description
PWM Control
The AP1534 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 AP1534 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
CC
rises above 3.5V.
Current Limit Protection
The current limit threshold is set by external resistor
R
OCSET
connected from V
CC
supply to OCSET pin. The
internal sink current I
OCSET
(90uA typical) across this
resistor sets the voltage 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:
R I R I
OCSET OCSET DS(ON) PEAK
Ч = Ч
2
I) (
I I
(MAX) OUT PEAK
Δ
+ >
where,
IN
OUT
S
OUT IN
V
V
L F
V V
I
Ч
Ч
−
= Δ
I
PEAK
is the output peak current; R
DS (ON)
is the MOSFET
ON resistance; F
S
is the PWM frequency (300KHz
typical). Also, the 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
OCSET
is
recommended.
The recommended minimum R
OCSET
value is
summarized below:
V
IN
(V) V
OUT
(V) R
OCSET
( Ω)
4 1 6.8K
5 3.3 5.6K
12 5 3.9K
18 12 4.7K
Inductor Selection
For most designs, the operates with inductors of 22µH to
33µH. The inductor value can be derived from the
following equation:
IN
V
OUT V
I
S
f
OUT V IN V
L ×
Δ ×
−
=
Where
ΔI
L
is inductor Ripple Current. Large value
inductors lower ripple current and small value inductors
result in high ripple current. Choose inductor ripple
current approximately 15% of the maximum load current
2A,
∆I
L
=0.3A. The DC current rating of the inductor
should be at least equal to the maximum load current
plus half the ripple current to prevent core saturation
(2A+0.15A).
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.
ESR L I RIPPLE V
× Δ =
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.