AAT1185 Datasheet, PDF (9/16 Pages) Skyworks Solutions Inc. – High Voltage Step-Down Controller

converter’s response time to load transients. The peak
switch current should not exceed the inductor saturation
current of the MOSFETs. The DCR of the inductor sets the
designed current limit in the following formula:
100mV
I_LIM = _DCR
For 10A output load, the selected DCR should be less
than 10m to avoid the peak inductor current triggers
the current limit.
MOSFET Selection
The step-down (buck) converter utilizes synchronous
rectification (Q1) for constant frequency (PWM mode)
voltage mode control. The synchronous rectifier is
selected based on the desired R_DS(ON) value and Q_G (total
gate charge), these two critical parameters are weighed
against each other. To get a low R_DS(ON) value, the
MOSFET must be very large; a larger MOSFET will have
a large Q_G. Conversely, to get a low Q_G, the MOSFET
must be small and thus have a large R_DS(ON) value. In
addition to the trade off between R_DS(ON) and Q_G, the
maximum voltage rating for the external synchronous
MOSFET must exceed the maximum application input
voltage value (V_DS [max] > V_IN [max]).
The Q_G affects the turn-on/turn-off time of the synchro-
nous MOSFET; the longer the turn-on/turn-off time, the
more likely the step-down converter will have “shoot-
through” current issues. “Shoot-through” current occurs
when the high-side MOSFET and the low-side MOSFET
are conducting current at the same time. This will result
in a low impedance path to ground from the input volt-
age through the two MOSFETs, and the current may
exceed the maximum current rating of the MOSFETs.
Exceeding the maximum current ratings will lead to the
destructive derating of the MOSFETs.
The critical parameter recommendations for the external
minimum 25V MOSFET are as follows:
Q_G (Total Gate Charge): 5nC to 15nC (max)
(V_GS: 4.5V to 5V)
R_DS(ON): 10mΩ to 30mΩ (max) (V_GS: 4.5V to 5V)
Input Capacitor Selection
For low-cost applications, a 470μF/25V electrolytic
capacitor is selected to control the voltage overshoot
across the high side MOSFET. A 10μF/25V ceramic
capacitor with a voltage rating at least 1.05 times great-
er than the maximum input voltage is connected as close
DATA SHEET
AAT1185
High Voltage Step-Down Controller
as possible to the input pins (Pins 9 and 11) for high
frequency decoupling.
Feedback and Compensation Networks
C10
C11
R4
C12
R5
VOUT
COMP
R6
FB
R7
REF
Figure 1: AAT1185 Feedback and Compensation
Networks for Type III Voltage-Mode Control Loop.
The transfer function of the error amplifier is dominated
by DC gain and the L C_OUT output filter of the regulator.
This output filter and its equivalent series resistance
(ESR) create a double pole at F_LC and a zero at F_ESR in the
following equations:
Eq.
1:
F_LC
=
2
·
π
·
1
L
·
C_OUT
Eq.
2:
F_ESR
=
2
·
π
·
1
ESR
·
C_OUT
The feedback and compensation networks provide a
closed loop transfer function with the highest 0dB cross-
ing frequency and adequate phase margin for system
stability. Equations 3, 4, 5 and 6 relate the compensation
network’s poles and zeros to the components R₄, R₅, R₆,
C₁₀, C₁₁, and C₁₂:
Eq.
3:
F_Z1
=
2
·
π
1
· R₄
·
C₁₁
Eq.
4:
F_Z2
=
2
·
π
·
1
(R₅ +
R₆)
·
C₁₂
1
Eq. 5: _{FP1 =}
2 · π · R₄ ·
C₁₀ · C₁₁
C₁₀ + C₁₁
Eq. 6: F_P2 =
1
2 · π · R₅ · C₁₂
Components of the feedback, feed-forward, and com-
pensation networks need to be adjusted to maintain the
system's stability for different input and output voltages
applications as shown in Table 1.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
202001A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 31, 2012
9