Datasheet
MAX1513/MAX1514
TFT-LCD Power-Supply Controllers
14 ______________________________________________________________________________________
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.
Current Limiting and
Current-Sense Amplifier (CS+, CS-)
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.
Lossless Current Sense
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
S
) is the
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
L
is the DCR of the induc-
tor, and I
L
is the inductor current. The voltage across
the sense capacitor is given by:
where R
S
is the series resistor in the sense network and
C
S
is the sense capacitor. The above equation can be
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.
V
sL R
sR C
I
sL R
sR C
RI
If
L
R
R C then the equation becomes
VRI
S
L
SS
L
L
SS
LL
L
SS
SLL
/
, :
=
+
+
=
+
+
=
=
1
1
1
V
sR C
V
S
SS
L
=
+
1
1
V I sL R
LL L
=+
()
Σ
CLOCK
RESET DOMINANT
ILIM
COMPARATOR
S
RQ
125mV
LEVEL
SHIFT
SLOPE_COMP
TO
FAULT LOGIC
1.0V
SOFT-START
BLOCK
REF
FB
CS-
CS+
GATE
MAX1513
MAX1514
R
L
L
INDUCTOR
R
S
C
S
+ V
S
-
V
IN
V
MAIN
C
S+
C
S-
GATE
GND
FB
Figure 4. Step-Up Regulator-Controller Functional Diagram
Figure 5. Step-Up Regulator Using Lossless Current Sensing