MAX1514 Maxim Integrated Products, MAX1514 Datasheet - Page 14

• Manufacturer Part Number: MAX1514
• Description: (MAX1513 / MAX1514) TFT-LCD Power-Supply Controllers
• Manufacturer: Maxim Integrated Products

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input PWM comparator, the flip-flop is reset and the
MOSFET turns off. Since the inductor current is continu-
ous, a transverse potential develops across the inductor
that turns on the diode (D1). The voltage across the
inductor then becomes the difference between the out-
put voltage and the input voltage. This discharge condi-
tion forces the current through the inductor to ramp
down, transferring the energy stored in the magnetic
field to the output capacitor and the load. The N-channel
MOSFET is kept off for the rest of the clock cycle.
The internal current-limit circuit resets the PWM flip-flop
and turns off the external power MOSFET whenever the
voltage difference between CS+ and CS- exceeds
125mV (typ). The tolerance on this current limit is
±20%. Use the minimum value of the current limit to
select components of the current-sense network.
The lossless current-sense method uses the DC resis-
tance (DCR) of the inductor as the sense element.
Figure 5 shows a simplified step-up regulator using the
basic lossless current-sensing method. An RC network
is connected in parallel with the step-up inductor (L).
The voltage across the sense capacitor (C
TFT-LCD Power-Supply Controllers
Figure 4. Step-Up Regulator-Controller Functional Diagram
14
______________________________________________________________________________________
CLOCK
FAULT LOGIC
TO
Σ
Current-Sense Amplifier (CS+, CS-)
COMPARATOR
ILIM
RESET DOMINANT
S
R
Lossless Current Sense
Current Limiting and
Q
125mV
1.0V
SLOPE_COMP
SOFT-START
LEVEL
BLOCK
SHIFT
REF
S
) is the
GATE
CS+
CS-
FB
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input to the current-sense amplifier. To prevent the
sense amplifier from seeing large common-mode
switching voltages, the sense capacitor should always
be connected to the nonswitching end of the inductor
(i.e., the input of the step-up regulator).
Lossless current sense can be easily understood using
complex frequency domain analysis. The voltage
across the inductor is given by:
where L is the inductance, R
tor, and I
the sense capacitor is given by:
where R
C
rewritten as:
Therefore, the sense capacitor voltage is directly pro-
portional to the inductor current if the time constant of
the RC sense network matches the time constant of the
inductor/DCR. The sense method is equivalent to using
a current-sense resistor that has the same value as the
inductor DCR.
Figure 5. Step-Up Regulator Using Lossless Current Sensing
If
S
R
V
is the sense capacitor. The above equation can be
V
L
IN
L
S
=
=
S
L
is the series resistor in the sense network and
C
R C
1
S+
is the inductor current. The voltage across
S S
sL
+
+
sR C
+
R
,
L
MAX1513
MAX1514
V
then the equation becomes
INDUCTOR
S S
R
V
S
V
C
V
L
S
L
S
S
=
I
=
=
L
L
1
R I
-
I sL
=
L
+
L L
(
1
1
sR C
L
C
1
GATE
+
GND
S-
+
is the DCR of the induc-
FB
+
S S
R
sR C
S
sL R
R
L
S S
/
V
)
L
L
R I
L L
:
V
MAIN
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