AAT1276 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – Boost Converter with USB Power Switch

SwitchReg^TM
Setting the Output Voltage
Program the output voltage through a resistive divider
located from the output to the FB pin to ground. The
internal error amplifier reference voltage is 0.6V. A
59.0kΩ programming resistor value from VFB to GND
with a 432kΩ resistor from FB to the output will set the
output voltage to 5V.
V_OUT = V_REF ·
⎛⎝_{1 +}
R2_⎞
R3⎠
=
0.6V
·
⎛⎝₁
+
432kΩ _⎞
59.0kΩ⎠
= 5.0V
USB Load Switch Application
Information
Setting the Load Switch Current Limit
In most applications, the variation in I_LIM must be taken
into account when determining R_SET. The I_LIM variation is
due to processing variations from part to part, as well as
variations in the voltages at OUT1 and OUT2, plus the
operating temperature. The typical R_SET value for a
300mA load is in the range of 20 to 22kΩ.
R_SET (kΩ)
11
12
14
16
18
20
30
40
I_OUT (mA)
740
690
590
550
450
420
160
100
Table 1: Current Limit vs.R_SET.
Operation in Current Limit
When a heavy load is applied to OUT2 of the AAT1276,
the load current is limited to the value of I_LIM (deter-
mined by R_SET) causing a drop in the output voltage. This
increases the AAT1276 power dissipation and die tem-
perature. When the die temperature exceeds the over-
temperature limit, the AAT1276 shuts down until it has
cooled sufficiently, at which point it will start up again.
The AAT1276 will continue to cycle on and off until the
PRODUCT DATASHEET
AAT1276
Boost Converter with USB Power Switch
load is removed, power is removed, or until a logic low
level is applied to the EN pin.
A fault flag indicates when the OUT2 pin load current has
exceeded the current limit level set by R_SET. The fault flag
is an active low, open-drain pin that requires 10kΩ pull-
up to V_IN. The fault signal has a 4ms blanking time to
prevent false over current indicator during the charging
of the USB bus capacitor.
Steady-State Maximum Power Dissipation
The maximum power dissipation for the AAT1276 occurs
at the minimum input voltage, where it operates in con-
tinuous conduction mode (CCM). The total power dissi-
pation at full load is dominated by the R_DS(ON) losses of
the power MOSFET. The dissipation includes the losses in
the input and output switch, as well as both synchronous
switches.
Due to the magnitude of the inductor ripple current, it
cannot be neglected when analyzing the R_DS(ON) power
dissipation. Once the ripple current has been deter-
mined, the RMS current during the on and the off period
can be calculated.
D_MAX
=
V_O
-
V_IN(MIN)
V_O
I_PP =
V_IN(MIN) · D_MAX
L · F_S
I_P
=
I_O
1-D
I_PK = I_P +
I_PP
2
I_V = I_P - I_PP
I_RMS(ON) =
(I_P2 + I_PK · I_V + I_V2) · D_MAX
3
I_RMS(OFF) =
(I_P2 + I_PK · I_V + I_V2) · (1 - D_MAX)
3
P_TOTAL = I_RMS(ON)2 · (R_DS(ON)IN + R_DS(ON)N)
+ I_RMS(OFF)2 · (R_DS(ON)IN + R_DS(ON)P + R_DS(ON))
T_J(MAX) = P_TOTAL · θ_JA + T_AMB
12
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1276.2007.11.1.1