AAT2687 Datasheet, PDF (13/20 Pages) Advanced Analogic Technologies – PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters

DATA SHEET
AAT2687
PMIC Solution for 24V Systems
with 2 High Performance Step-Down Converters
down mode, an internal 1.5kΩ resistor is connected
between V_OUT and GND. This is intended to discharge C_OUT
when the LDO regulator is disabled. The internal 1.5kΩ
has no adverse effect on device turn-on time.
Channel 2 Short-Circuit Protection
The AAT2687 LDO contains an internal short-circuit pro-
tection circuit that will trigger when the output load cur-
rent exceeds the internal threshold limit. Under short-
circuit conditions, the output of the LDO regulator will be
current limited until the short-circuit condition is removed
from the output or LDO regulator package power dissi-
pation exceeds the device thermal limit.
Channel 2 Thermal Protection
The AAT2687 LDO has an internal thermal protection
circuit which will turn on when the device die tempera-
ture exceeds 150°C. The internal thermal protection
circuit will actively turn off the LDO regulator output
pass device to prevent the possibility of over tempera-
ture damage. The LDO regulator output will remain in a
shutdown state until the internal die temperature falls
back below the 150°C trip point. The combination and
interaction between the short circuit and thermal protec-
tion systems allows the LDO regulator to withstand
indefinite short-circuit conditions without sustaining per-
manent damage.
Channel 2 No-Load Stability
The AAT2687 is designed to maintain output voltage
regulation and stability under operational no load condi-
tions. This is an important characteristic for applications
where the output current may drop to zero.
highly recommended. A larger value of C_IN with respect
to C_OUT will effect a slower C_IN decay rate during shut-
down, thus preventing V_OUT from exceeding V_IN. In appli-
cations where there is a greater danger of V_OUT exceed-
ing V_IN for extended periods of time, it is recommended
to place a Schottky diode across V_IN to V_OUT (connecting
the cathode to V_IN and anode to V_OUT). The Schottky
diode forward voltage should be less than 0.45V.
Thermal Calculations
There are three types of losses associated with the
AAT2687 step-down converter: switching losses, con-
duction losses, and quiescent current losses. Conduction
losses are associated with the R_DS(ON) characteristics of
the power output switching devices. Switching losses are
dominated by the gate charge of the power output
switching devices. At full load, assuming continuous con-
duction mode (CCM), a simplified form of the synchro-
nous step-down converter and LDO losses is given by:
P_TOTAL
= ^{IOUT12 · (RDS(ON)H · VOUT1 + RDS(ON)L · [VIN1 - VOUT1])}
V_IN1
+ (t_SW · F_S · I_OUT1 + I_Q1) · V_IN1 + (V_IN2 - V_OUT2) · I_OUT2
I_Q1 and I_Q2 are the step-down converter and LDO quies-
cent currents respectively. The term t_SW is used to esti-
mate the full load step-down converter switching losses.
For asynchronous Step-Down converter, the power dis-
sipation is only in the internal high side MOSFET during
the on time. When the switch is off, the power dissipates
on the external Schottky diode. The total package losses
for the AAT2687 reduce to the following equation:
Channel 2 Reverse Output-to-Input
Voltage Conditions and Protection
Under normal operating conditions, a parasitic diode
exists between the output and input of the LDO regula-
tor. The input voltage should always remain greater than
the output load voltage, maintaining a reverse bias on
the internal parasitic diode. Conditions where V_OUT might
exceed V_IN should be avoided since this would forward
bias the internal parasitic diode and allow excessive cur-
rent flow into the V_OUT pin, possibly damaging the LDO
regulator. In applications where there is a possibility of
V_OUT exceeding V_IN for brief amounts of time during nor-
mal operation, the use of a larger value C_IN capacitor is
P_TOTAL = I_OUT12 · R_DS(ON)H · D + (t_SW · F_S · I_OUT1 + I_Q) · V_IN + (V_IN2 - V_OUT2) · I_OUT2
Where: D = ^VOUT is the duty cycle.
V_IN
Since R_DS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Given the total losses, the maximum junction tempera-
ture can be derived from the θ_JA for the TQFN45-24
package, which is 33°C/W.
T_J(MAX) = P_TOTAL · θ_JA + T_AMB
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
202033A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
13