AMC3202 Datasheet, PDF (8/11 Pages) ADDtek Corp – 1.5A 280kHz BOOST REGULATORS

AMC3202
APPLICATION INFORMATION
Operation:
The AMC3202 incorporates a current mode control scheme, in which the duty cycle of the switch is directly
controlled by switch current rather than by output voltage. The output of the oscillator turns on the power switch at a
frequency of 280kHz as shown in the block diagram. The power switch is turned off by the output of the PWM
comparator.
A TTL low voltage will shut down the chip and high voltage enable the chip through EN pin. This pin may also be
used to synchronize the part to nearly twice the base oscillator frequency. In order to synchronize to a higher frequency,
a positive transition turns on the power switch before the output of the oscillator goes high, thereby resetting the
oscillator. The synchronization operation allows multiple power supplies to operate at the same frequency. If
synchronization is not used, this pin should be either tied high or left floating for normal operation.
Component Selection:
2 .7 V DC ~ 3 0 V DC
V IN
C IN
10uF
Enable
D SS
C SS
C P2
200pF
L
D_F
22uH
VCC
VSW
EN
FB
AMC3202
SS
COMP
AGND PGND
C P1
0.1uF
RP
5K
VOUT
C OUT
22uF
R2
R1
The AMC3202 develops a 1.276V reference from the FB pin to ground. Output voltage is set by connecting the
FB pin to an output resistor divider and the maximum output voltage is determined by the VSW pin maximum voltage
minus the output diode forward voltage. Referring to typical application circuit, the output voltage is set by the below
formula (1):
V_OUT
= 1.276V ⎜⎛1 +
⎝
R2 _⎟⎞
R1 _⎠
2.7V≦V_OUT≦43V－V_F
(1)
where, V_F is the output diode D_F forward voltage.
When choosing the inductor, one must consider factors such as peak current, core and ferrite material, output
voltage ripple, EMI, temperature range, physical size, and cost. Lower values are chosen to reduce physical size of the
inductor, and higher values reduce ripple voltage and core loss. In continuous conduction mode, the peak inductor
current is equal to average current plus half of the ripple current, which should not cause inductor saturation. Based on
the tolerance of the ripple current in the circuits, the following formula (2) can be referenced:
( ) _{I Ripple}
= ^VIN
V_OUT − V_IN
fLV _OUT
where, f = 280kHz.
(2)
In Boost circuits, the inductor becomes part of the input filter. In continuous mode, the input current waveform is
triangular and does not contain a large pulsed current. This reduces the requirements imposed on the input capacitor
selection. Capacitors in the range of 10uF to 100uF with an ESR less than 0.3Ω work well up to full 1.5A switch
current.
Copyright © 2006 ADDtek Corp.
8
DD074_B -- AUGUST 2006