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MAX1513 Datasheet, PDF (14/28 Pages) Maxim Integrated Products – TFT-LCD Power-Supply Controllers
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TFT-LCD Power-Supply Controllers
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 (CS) is the
RESET DOMINANT
CLOCK
S
GATE
R
ILIM
COMPARATOR
Q
125mV
Σ
TO
FAULT LOGIC
CS+
LEVEL
SHIFT
CS-
SLOPE_COMP
FB
SOFT-START REF
BLOCK
1.0V
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:
VL = IL(sL + RL )
where L is the inductance, RL is the DCR of the induc-
tor, and IL is the inductor current. The voltage across
the sense capacitor is given by:
VS
=
1+
1
sRSCS
VL
where RS is the series resistor in the sense network and
CS is the sense capacitor. The above equation can be
rewritten as:
VS
=
sL
1+
+ RL
sRSCS
IL
=
1+
1+
sL /RL
sRSCS
RLIL
If L
RL
= RSCS, then the equation becomes :
VS = RLIL
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.
INDUCTOR
RL
L
VIN
VMAIN
CS
RS
+
VS
-
CS+
MAX1513
MAX1514
CS-
GATE
GND
FB
Figure 4. Step-Up Regulator-Controller Functional Diagram
Figure 5. Step-Up Regulator Using Lossless Current Sensing
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