APW8725A Datasheet, PDF (10/20 Pages) Anpec Electronics Coropration – 5V to 12V Synchronous Buck Controller

APW8725A
Function Description
Power-On-Reset (POR)
The Power-On-Reset (POR) function of APW8725A con-
tinually monitors the input supply voltage (VCC) and en-
sures that the IC has sufficient supply voltage and can
work well. The POR function initiates a soft-start process
while the VCC voltage exceeds the POR threshold; the
POR function also inhibits the operations of the IC while
the VCC voltage falls below the POR threshold.
Soft-Start
The APW8725A builds in a 40-steps digital soft-start to
control the output voltage rise as well as limit the current
surge at the start-up. During soft-start, the internal step
voltage connected to the one of the positive inputs of the
error amplifier replaces the reference voltage (0.8V typical)
until the step voltage reaches the reference voltage. The
digital soft-start circuit interval (shown as figure 1) de-
pends on the switching frequency.
T_SS
=
(t₃
−
t₂ ) =
1
F_OSC
⋅ 512
Voltage(V)
V_VCC
POR
OCSET count completed
OCSET count start
(OCSET duration, t₂ - t₁, less than 0.9ms)
V_OUT
t₀
t₁ t₂ t₃
Time
Figure 1. Soft-Start Interval
Over-Current Protection
The over-current function protects the switching converter
against over-current or short-circuit conditions. The con-
troller senses the inductor current by detecting the drain-
to-source voltage which is the product of the inductor’s
current and the on-resistance of the low-side MOSFET
during on-state.
A resistor (R_OCSET), connected from the LGATE to the GND,
programs the over-current trip level. Before the IC ini-
tiates a soft-start process, an internal current source, I
OCSET
(21.5µA
typical),
flowing
through
the
R
OCSET
develops
a
voltage (V_ROCSET) across the R_OCSET. During the normal
operation, the device holds V_ROCSET and stops the current
source, I_OCSET. When the voltage across the low-side
MOSFET exceeds the double V_ROCSET (2 x V_ROCSET), the IC
shuts off the converter and then initiates a new soft-start
process. After 2 over-current events are counted, the de-
vice is shut down and all the gate drivers (UGATE, LGATE,
and DRIVE) are off. Both the output of the PWM converter
and linear controller are latched to be floating.
The APW8725A has an internal OCP voltage, V_{OCP_MAX},
and the value is 0.3V minimum. When the R x I
OCSET OCSET
exceeds 0.3V or the R is floating or not connected,
OCSET
the V_ROCSET will be the default value 0.3V. The over current
threshold would be 0.7V across low-side MOSFET. The
threshold of the valley inductor current-limit is therefore
given by:
I_LIMIT
=
2 × I_OCSET × R_OCSET
R_DS(ON)(low − side)
For the over-current is never occurred in the normal oper-
ating load range; the variation of all parameters in the
above equation should be considered:
- The R of low-side MOSFET is varied by tempera-
DS(ON)
ture and gates to source voltage. Users should deter-
mine the maximum R_DS(ON) by using the manufacturer’s
datasheet.
- The minimum I_OCSET (19.5µA) and minimum R_OCSET
should be used in the above equation.
- Note that the I_LIMIT is the current flow through the low-
side MOSFET; I_LIMIT must be greater than valley
inductor current which is output current minus the half
of inductor ripple current.
I_LIMIT
> I_OUT(MAX) −
∆I
2
Where ∆I = output inductor ripple current
- The overshoot and transient peak current also should
be considered.
Copyright © ANPEC Electronics Corp.
10
Rev. A.1 - Aug., 2010
www.anpec.com.tw