Detailed description – Rainbow Electronics MAX17062 User Manual
Page 7
Detailed Description
The MAX17062 is a highly efficient power supply that
employs a current-mode, fixed-frequency, PWM archi-
tecture for fast-transient response and low-noise opera-
tion. The device regulates the output voltage through a
combination of an error amplifier, two comparators, and
several signal generators (Figure 2). The error amplifier
compares the signal at FB to 1.24V and varies the
COMP output. The voltage at COMP determines the
current trip point each time the internal MOSFET turns
on. As the load changes, the error amplifier sources or
sinks current to the COMP output to command the
inductor peak current necessary to service the load. To
maintain stability at high duty cycles, a slope-compen-
sation signal is summed with the current-sense signal.
At light loads, this architecture allows the MAX17062 to
“skip” cycles to prevent overcharging the output voltage.
In this region of operation, the inductor ramps up to a
peak value of approximately 50mA, discharges to the
output, and waits until another pulse is needed again.
Output Current Capability
The output current capability of the MAX17062 is a
function of current limit, input voltage, operating fre-
quency, and inductor value. Because of the slope com-
pensation used to stabilize the feedback loop, the
inductor current limit depends on the duty cycle. The
current limit is determined by the following equation:
I
LIM
= (1.26 - 0.35 x D) x I
LIM_EC
where I
LIM
_
EC
is the current limit specified at 75% duty
cycle (see the
Electrical Characteristics
table) and D is
the duty cycle.
The output current capability depends on the current-
limit value and is governed by the following equation:
where I
LIM
is the current limit calculated above,
η is the
regulator efficiency (85% nominal), and D is the duty
cycle. The duty cycle when operating at the current
limit is:
where V
DIODE
is the rectifier diode forward voltage and
R
ON
is the on-resistance of the internal MOSFET.
D
V
V
V
V
I
R
V
OUT
IN
DIODE
OUT
LIM
ON
DIODE
=
−
+
−
×
+
I
I
D
V
f
L
V
V
OUT MAX
LIM
IN
OSC
IN
OUT
(
)
.
=
−
Ч
Ч
Ч
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
Ч
Ч
0 5
η
MAX17062
TFT-LCD Step-Up DC-DC Converter
_______________________________________________________________________________________
7
PGND
LX
IN
FREQ
FB
COMP
4
μA
6
μA
N
ERROR
COMPARATOR
ERROR
AMPLIFIER
SKIP
COMPARATOR
SS
CLOCK
SKIP
BIAS
SHDN
MAX17062
Σ
CURRENT
SENSE
CONTROL
AND DRIVER
LOGIC
SOFT-
START
SLOPE
COMPEN-
SATION
OSCILLATOR
∞
1.24V
Figure 2. MAX17062 Functional Diagram