AP7173 Datasheet, PDF (11/15 Pages) Diodes Incorporated – 1.5A LOW DROPOUT LINEAR REGULATOR WITH PROGRAMMABLE SOFT-START

AP7173
1.5A LOW DROPOUT LINEAR REGULATOR WITH
PROGRAMMABLE SOFT-START
Application Note
BIAS VOLTAGE V_VCC
The AP7173 is a low V_IN, low dropout regulator that uses an
NMOS pass FET. The VCC pin must be connected to a DC
bias supply V_VCC for the internal control circuitry and the gate
drive of the pass FET to function properly and to obtain low
dropout. The V_VCC needs to be equal to or higher than the V_IN
and in the range of 2.7V-5.5V. Figure 27 illustrates the typical
application circuit for the AP7173.
V_IN
C1
+5V
C3
R3
IN
OUT
PG
FB
AP7173
VCC
SS
EN
GND
VOUT
R1 C2
R2
CSS
Figure 27. Typical Application Circuit for AP7173
ADJUSTABLE OUTPUT VOLTAGE
With an external voltage divider, the AP7173 can provide
output voltage from 0.8V to 3.3V. R1 and R2 can be calculated
for any output voltage using the following equation, where
V_REF=0.8 is the AP7173’s internal reference voltage. Refer to
Table 1 for resistor combinations for commonly used output
voltages. For maximum voltage accuracy, R2 should be ≤ 5kΩ.
V_OUT = V_REF x (1 + R1/R2)
INPUT V_IN AND BIAS V_VCC CAPACITORS
It is important to keep the IN and VCC pins clear of large
ripples, glitches and other noises by connecting capacitors to
the IN and VCC pins. The required capacitance on these pins
is strongly dependent on source and wiring impedance of the
supplies.
To provide good decoupling for the input power supply V_IN, it is
recommended that a ceramic capacitor with capacitance of at
least 1μF is connected between the IN and GND pins at a
location as close to them as possible. High quality, low ESR
capacitors should be used for better performance.
It is critical to provide good decoupling to the VCC pin for the
AP7173’s internal control circuitry to function properly. The
minimum recommended capacitance for the V_VCC is 1μF when
the V_VCC and V_IN are separate supplies. If the V_IN and V_VCC are
connected to the same supply, the recommended minimum
capacitance for V_VCC is 4.7μF. Again good quality, low ESR
capacitors should be used for optimum performance.
OUTPUT CAPACITOR
The output capacitor affects the stability and transient response
of the LDO. The AP7173 is designed to be stable for all types
of output capacitors ≥ 2.2μF, single or multiple in parallel. Using
high-quality, low ESR capacitors and placing them close to the
OUT and GND pins can improve perfomance.
DROPOUT VOLTAGE
The very low dropout makes the AP7173 well suited for
high-current, low V_IN/low V_OUT applications. To achieve the
specified low-dropout performance for such applications, the
VCC pin should be connected to a separate supply of at least
3.25V higher than V_OUT. Figure 28 shows an application circuit
where V_VCC is 5V and V_OUT is 1.2V.
VVCC (5V) VIN (1.5V)
Vref
VCC
+
_
IN
OUT
AP7173
FB
IOUT (1.5A)
VOUT (1.2V)
R1
C2
R2
Figure 28. Typical Application Circuit for AP7173
Using Separate VCC and IN Rails
For applications where low dropout is not required or a separate
V_VCC supply is not available, the IN and VCC pins can be tied
together. In this situation, a voltage difference of at least 1.7V
between the V_VCC and V_OUT has to be maintained for the V_VCC to
provide enough gate drive to the pass FET. Therefore, the V_OUT
needs to be 1.7V or more below V_IN, as shown in Figure 29.
V_IN (3.0V)
VCC
+
_
Vref
IN
OUT
IOUT (1.5A)
V_OUT (1.2V)
R1
C2
AP7173
FB
R2
Figure 29. Typical Application Circuit for AP7173
Without an Auxiliary VCC Rail
PROGRAMMABLE SOFT-START
The AP7173 features a voltage-controlled soft-start that is
programmable with an external capacitor (C_SS). The AP7173
achieves a monotonic soft-start by tracking the voltage ramp of
the external soft-start capacitor until the ramp voltage reaches
the internal reference voltage. The relationship between the
soft-start time and the soft-start charging current (I_SS), soft-start
capacitance (C_SS), and the internal reference voltage (V_REF) is
t_SS = (V_REF x C_SS) / I_SS
Refer to Table 2 for suggested soft-start capacitor values.
AP7173 Rev. 4
11 of 15
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MARCH 2008
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