600mA Step-Down Converter
Functional Block Diagram
OSC
SLOPE
COMP
+
I
SENSE
COMP
-
4
R
V
IN
AAT1106
BLANKING
-
COMP
+
+
OVDET
-
S
Q
+
-
V
IN
2.7 - 5.5V
FB/OUT
5
0.6V
+
EA
-
RS LATCH
R
Q
PWM
LOGIC
DRV
3
SW
R1
R2
NON-OV ERLA P
CONTROL
V
OUT
R1
R2*
0.65V
V
IN
0.6V
+
I
ZERO
COMP
-
C
OUT
GND
2
EN
1
REF
SHUTDOWN
* For adjustable output R1 + R2 are external
The AAT1106 is a high performance 600mA,
1.5MHz fixed frequency monolithic switch-mode
step-down converter which uses a current mode
architecture with an adaptive slope compensation
scheme. It minimizes external component size and
optimizes efficiency over the complete load range.
The adaptive slope compensation allows the device
to remain stable over a wider range of inductor val-
ues so that smaller values (1µH to 4.7µH) with
associated lower DCR can be used to achieve high-
er efficiency. Apart from the small bypass input
capacitor, only a small L-C filter is required at the
output. The fixed output version requires only three
external power components (C
IN
, C
OUT
, and L). The
adjustable version can be programmed with exter-
nal feedback to any voltage, ranging from 0.6V to
the input voltage. It uses internal MOSFETs to
achieve high efficiency and can generate very low
output voltage by using an internal reference of
0.6V. At dropout, the converter duty cycle increases
to100% and the output voltage tracks the input volt-
age minus the low R
DS(ON)
drop of the P-channel
high-side MOSFET. The input voltage range is 2.5V
Functional Description
to 5.5V. The converter efficiency has been opti-
mized for all load conditions, ranging from no load
to 600mA at V
IN
= 3V. The internal error amplifier
and compensation provides excellent transient
response, load, and line regulation.
Slope compensated current mode PWM control
provides stable switching and cycle-by-cycle cur-
rent limit for excellent load and line response and
protection of the internal main switch (P-channel
MOSFET) and synchronous rectifier (N-channel
MOSFET). During normal operation, the internal P-
channel MOSFET is turned on for a specified time
to ramp the inductor current at each rising edge of
the internal oscillator, and is switched off when the
feedback voltage is above the 0.6V reference volt-
age. The current comparator, I
COMP
, limits the peak
inductor current. When the main switch is off, the
synchronous rectifier turns on immediately and
stays on until either the inductor current starts to
reverse, as indicated by the current reversal com-
parator, I
ZERO
, or the beginning of the next clock
cycle.
Current Mode PWM Control
1106.2007.07.1.0
9
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
