lm26003mhx National Semiconductor Corporation, lm26003mhx Datasheet - Page 13

• Manufacturer Part Number: lm26003mhx
• Description: 3a Switching Regulator With High Efficiency Sleep Mode
• Manufacturer: National Semiconductor Corporation

BOOTSTRAP
The drive voltage for the internal switch is supplied via the
BOOT pin. This pin must be connected to a ceramic capacitor,
Cboot, from the switch node, shown as C6 in the typical ap-
plication. The LM26003 provides the VDD voltage internally,
so no external diode is needed. A minimum value of 0.1 µF is
recommended for Cboot. Smaller values may result in insuf-
ficient hold up time for the drive voltage and increased power
dissipation.
During low Vin operation, when the on-time is extended, the
bootstrap capacitor is at risk of discharging. If the Cboot ca-
pacitor is discharged below approximately 2.5V, the LM26003
enters a high frequency re-charge mode. The Cboot cap is
re-charged via the synchronous FET shown in the block dia-
gram. Switching returns to normal when the Cboot cap has
been recharged.
CATCH DIODE
When the internal switch is off, output current flows through
the catch diode. Alternately, when the switch is on, the diode
sees a reverse voltage equal to Vin. Therefore, the important
parameters for selecting the catch diode are peak current and
peak inverse voltage. The average current through the diode
is given by:
Where D is the duty-cycle, defined as Vout/Vin. The catch
diode conducts the largest currents during the lowest duty-
cycle. Therefore ID
mum input voltage. The diode should be rated to handle this
current continuously. For over-current or short circuit condi-
tions, the catch diode should be rated to handle peak currents
equal to the peak current limit.
The peak inverse voltage rating of the diode must be greater
than maximum input voltage.
A Schottky diode must be used. It's low forward voltage max-
imizes efficiency and BOOT voltage, while also protecting the
SW pin against large negative voltage spikes.
When selecting the catch diode for high efficiency low output
load applications, select a Schottky diode with low reverse
leakage current. Also keep in mind that the reverse leakage
current of a Schottky diode increases with temperature and
with reverse voltage. Reverse voltage equals roughly the in-
put voltage in a buck converter. At hot, the diode reverse
leakage current may be larger than the current consumption
of the LM26003.
COMPENSATION
The purpose of loop compensation is to ensure stable oper-
ation while maximizing dynamic performance. Stability can be
analyzed with loop gain measurements, while dynamic per-
formance is analyzed with both loop gain and load transient
response. Loop gain is equal to the product of control-output
transfer function (power stage) and the feedback transfer
function (the compensation network).
For stability purposes, our target is to have a loop gain slope
that is -20dB /decade from a very low frequency to beyond
the crossover frequency. Also, the crossover frequency
should not exceed one-fifth of the switching frequency, i.e. 60
kHz in the case of 300 kHz switching frequency.
For dynamic purposes, the higher the bandwidth, the faster
the load transient response. The downside to high bandwidth
is that it increases the regulators susceptibility to board noise
which ultimately leads to excessive falling edge jitter of the
switch node voltage.
AVE
ID
AVE
should be calculated assuming maxi-
= Iload x (1-D)
13
A large DC gain means high DC regulation accuracy (i.e. DC
voltage changes little with load or line variations).
To achieve this loop gain, the compensation components
should be set according to the shape of the control-output
bode plot. A typical plot is shown in Figure 8 below.
The control-output transfer function consists of one pole (fp),
one zero (fz), and a double pole at fn (half the switching fre-
quency).
Referring to Figure 8, the following should be done to create
a -20dB /decade roll-off of the loop gain:
1. Place a pole at 0Hz (fpc)
2. Place a zero at fp (fzc)
3. Place a second pole at fz (fpc1)
The resulting feedback (compensation) bode plot is shown
below in Figure 9. Adding the control-output response to the
feedback response will then result in a nearly continuous
-20db/decade slope.
The control-output corner frequencies can be determined ap-
proximately by the following equations:
FIGURE 8. Control-Output Transfer Function
FIGURE 9. Feedback Transfer Function
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