AAT2749 Datasheet, PDF (12/17 Pages) Skyworks Solutions Inc. – 3.0MHz Step-Down Converter and Low-V in LDO

DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-V_IN LDO
The maximum output capacitor RMS ripple current is
given by:
I = _RMS(MAX)
1
2·
·
3
V_OUT · (V_IN(MAX) - V_OUT)
L · F_S · V_IN(MAX)
Dissipation due to the RMS current in the ceramic output
capacitor ESR is typically minimal, resulting in less than
a few degrees rise in hot-spot temperature.
LDO Input Capacitor
Typically, a 1μF or larger capacitor is recommended for
C_IN in most applications. A C_IN capacitor is not required
for basic LDO regulator operation. However, if the
AAT2749 is physically located more than three centime-
ters from an input power source, a C_IN capacitor will be
needed for stable operation.
C_IN should be located as close to the device V_IN pin as
possible. C_IN values greater than 1μF will offer superior
input line transient response and will assist in maximiz-
ing the highest possible power supply ripple rejection.
Ceramic, tantalum, or aluminum electrolytic capacitors
may be selected for C_IN. There is no specific capacitor
ESR requirement for C_IN. However, for 300mA LDO regu-
lator output operation, ceramic capacitors are recom-
mended for C_IN due to their inherent capability over
tantalum capacitors to withstand input current surges
from low impedance sources, such as batteries in por-
table devices.
LDO Output Capacitor
For proper load voltage regulation and operational stabil-
ity, a capacitor is required between the VOUT and GND
pins. The C_OUT capacitor connection to the LDO regulator
ground pin should be as close as possible for maximum
device performance. The AAT2749 LDO has been spe-
cifically designed to function with very low ESR ceramic
capacitors. For best performance, ceramic capacitors are
recommended.
Typical output capacitor values for maximum output cur-
rent conditions range from 1μF to 10μF. Applications
utilizing the exceptionally low output noise and optimum
power supply ripple rejection characteristics of the chan-
nel 2 should use 2.2μF or greater for C_OUT. If desired, C_OUT
may be increased without limit. In low output current
applications where output load is less than 10mA, the
minimum value for C_OUT can be as low as 0.47μF.
LDO Enable Function
The AAT2749 features an LDO regulator enable/disable
function. This pin (EN) is active high and is compatible
with CMOS logic. To assure the LDO regulator will switch
on, the EN turn-on control level must be greater than
1.5V. The LDO regulator will go into the disable shut-
down mode when the voltage on the EN pin falls below
0.6V. If the enable function is not needed in a specific
application, it may be tied to V_IN to keep the LDO regula-
tor in a continuously on state. When the LDO regulator
is in shutdown mode, an internal 1.5kΩ resistor is con-
nected between VOUT and GND. This is intended to dis-
charge C_OUT when the LDO regulator is disabled. The
internal 1.5kΩ has no adverse effect on device turn-on
time.
LDO Short-Circuit Protection
The AAT2749 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.
LDO Thermal Protection
The AAT2749 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 temperature
damage. The LDO regulator output will remain in a shut-
down state until the internal die temperature falls back
below the 150°C trip point. The combination and inter-
action between the short circuit and thermal protection
systems allows the LDO regulator to withstand indefinite
short-circuit conditions without sustaining permanent
damage.
Thermal Calculations
There are three types of losses associated with the
AAT2749 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-
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
12
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012