apu3073o APEC, apu3073o Datasheet - Page 6

• Manufacturer Part Number: apu3073o
• Description: Synchronous Pwm Controller With Over-current Protection / Ldo Controller
• Manufacturer: APEC

APU3073
The feedback voltage increases linearly as the injecting
current goes down. The injecting current drops to zero
when soft-start voltage is around 2V and the output volt-
age goes into steady state.
As shown in Figure 4, the positive pin of feedback UVLO
comparator is always higher than 0.4V, therefore, feed-
back UVLO is not functional during soft-start.
From this analysis, the output start-up time is defined
as when soft-start capacitor voltage increases from 1V
to 2V. The start-up time will be dependent on the size of
the external soft-start capacitor and can be estimated
by:
For a given start up time, the soft-start capacitor can be
calculated as:
MOSFET Drivers
The driver capabilities of both high and low side drivers
are optimized to maintain fast switching transitions. They
are sized to drive a MOSFET that can deliver up to 20A
output current.
The low side MOSFET diver is supplied directly by V
while the high side driver is supplied by V
An internal dead time control is implemented to prevent
cross-conduction and allows the use of several kinds of
MOSFETs.
of Error Amp and Feedback
Voltage at negative input
20mA3T
C
SS
Figure 4 - Theoretical operation waveforms
= 20mA3T
UVLO comparator
Voltage at Fb1 pin
Output of UVLO
Current flowing
START
into Fb1 pin
Soft-Start
/C
Voltage
during Soft-Start.
START
POR
SS
= 2V-1V
/1V
64uA
1.6V
0V
0V
C
0uA
0.8V
.
0.8V
3V
1V
2V
CC
LDO Controller
The LDO section is powered directly from Vcc. The out-
put of LDO can be set as low as 0.8V and can be pro-
grammed to higher voltages by using two external resis-
tors.
Supply Voltage Under-Voltage Lockout
The under-voltage lockout circuit assures that the
MOSFET driver outputs, remain in the off state when-
ever the supply voltage drops below set parameters. Lock-
out occurs if Vcc or VcH fall below 4.0V and 3.3V re-
spectively. Normal operation resumes once these volt-
ages rise above the set values.
Shutdown
The PWM section can be shutdown by pulling the soft-
start pin below 0.4V. The control MOSFET turns off and
the synchronous MOSFET turns on during shutdown.
Over-Current Protection
Over-current protection is achieved with a cycle by cycle
scheme and it is performed by sensing current through
the R
an external resistor (R
pin and the drain of low side MOSFET (Q2) and sets the
current limit set point. The internal current source devel-
ops a voltage across R
turned on, the inductor current flows through the Q2 and
results a voltage which is given by:
When voltage V
comparator flips and disables the oscillator. The high
side MOSFET is turned off and the low side MOSFET is
turned on until the inductor current reduces to below
current set value. The critical inductor current can be
calculated by setting:
Osc
V
V
I
Figure 5 - Diagram of the over current sensing.
SET
DS(ON)
OCSET
OCSET
APU3073
= I
L(CRITICAL)
of low side MOSFET. As shown in Figure 5,
= I
= I
OCSET
OCSET
I
OCSET
OCSET
3R
3R
=
R
is below zero, the current sensing
SET
SET
SET
SET
SET
R
) is connected between OCSet
OCSet
-R
3I
DS(ON)
- R
. When the low side switch is
DS(ON)
OCSET
DS(ON)
R
SET
3i
3I
L
L
= 0
Q1
Q2
---(1)
---(2)
L1
V
OUT
6/17