A8600 Datasheet, PDF (41/52 Pages) Allegro MicroSystems – Designed to provide the power supply requirements of next generation car audio and infotainment systems, the A8600 provides all the control and protection circuitry to produce...

A8600
Quadruple Output Regulator with Two High-Side Switches,
BU/ACC Voltage Detectors, and Mute Delay
is higher than the pulse-by-pulse current threshold, as shown in
figure 24. This phenomenon is more pronounced when using high
value, electrolytic-type output capacitors.
To avoid prematurely triggering hiccup mode, the soft start
capacitor, CSSx, should be calculated using the following for-
mula:
The upper and lower MOSFETs must support the SW4 peak
output current according to the following equations:
Upper MOSFET:
I_DHS4 ≥ I_SW4(peak) _×
V_SW4
V_IN4(min)
(26)
C_SSx ≥ ^{ISSSSUx × VSWx × CSWx}
0.8 (V) _× I_COx
(22)
where V_SWx is the output voltage, C_SWx is the output capacitance,
I_COx is the amount of current allowed to charge the output capaci-
tance during soft start (Allegro recommends an I_COx between
0.1 and 0.3 A).
Lower MOSFET:
I_DHS4 ≥ I_SW4(peak) _×
1 – ^VSW4
V_IN4(max)
(27)
Examples of several 40 V MOSFETs with less than 12 nC of gate
charge are shown in table 4.
Higher values of I_CO result in faster soft start times. Howewer,
lower values of I_CO ensure that hiccup mode is not falsely trig-
gered. Allegro recommends starting the design with an I_CO of
0.1 A and increasing it only if the soft start time is too slow. If
a non-standard capacitor value for CSSx is calculated, the next
larger value should be used.
The output voltage ramp time, t_SSRAMPx, can be calculated by
using either of the following methods:
or
t_SSx = V_{SWx ×}
C_SWx
I_COx
(24)
t_{SSx =}
0.8
(V)
×
C_SSx
I_SSSUx
(25)
When the A8600 is in hiccup mode, the CSSx capacitor is used
as a timing capacitor and sets the hiccup period. The SSx pin
charges the CSSx capacitor with I_SSSUx during a startup attempt,
and discharges the CSSx capacitor with I_SSHICx between startup
attempts. Because the ratio of the SSx pin currents is 2:1, the time
between hiccups will be at least twice as long as the startup time.
Therefore, the effective duty-cycle of the A8600 will be very low
when the output is shorted to ground, and the junction tempera-
ture will be kept low.
SW4 External MOSFET Selections
The external MOSFETs for SW4 must withstand the maximum
expected input voltage. In an automotive environment this is usu-
ally the 40 V load dump situation. The BOOT4 regulator shown
in the Typical Application Circuit diagram, internal gate drivers,
and protection circuits were optimized for MOSFETs with less
than12 nC of gate charge at V_GS = 5 V.
SW4 Current Sense Resistor
The current limit of SW4 at its minimum on-time (t_ON(min)) is
determined by the value of the external sense resistor according
to the following equation:
I_SW4(peak) at t_ON(min) ₌
I_LIM4
R_SENSE4
75 (mV) (typ)
= _RSENSE4
(28)
Notice that this sets the current limit at t_ON(min) only. The
actual current limit will depend on the duty cycle and switching
frequency as shown in equations 5 and 6. Therefore, the sense
resistor should be chosen to support the required load current
(plus some margin) at a relatively high duty cycle and minimum
switching frequency.
Compensation Components (RZx, CZx, CPx)
To compensate the system, it is important to understand where
the buck power stage, load resistance, and output capacitance
form their poles and zeros in frequency. Also, its important to
understand that the compensated error amplifier introduces a zero
Table 4: Possible 40 V MOSFETs for SW4
Part Number
Manufacturer
Typical at
V_GS= 4.5 V
(A) (mΩ) nC
FDS8449
Fairchild
6.8 26
8
Si4446DY
Vishay
4.9 37
8
DMN4034SSS
Diodes, Inc.
5.5 39
5
NTMS5838NL
ON Semi
7
25
9
Allegro MicroSystems, Inc.
41
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com