30015673
FIGURE 13. RRAMP to VCC for VOUT > 7.5V
Km is the effective DC gain of the modulating comparator. The
duty cycle D = VOUT / VIN. KSL is the proportional slope com-
pensation term. VSL is the fixed slope compensation term.
Slope compensation is set by mc, which is the ratio of the ex-
ternal ramp to the natural ramp. The switching frequency
sampling gain is characterized by ωn and Q, which accounts
for the high frequency inductor pole.
For VOUT < 7.5V, a negative VCC is required. This can be
made with a simple charge pump from the LO gate output.
Install a resistor from the RAMP pin to the negative VCC.
ERROR AMPLIFIER TRANSFER FUNCTION
The following equations are used to calculate the error am-
plifier transfer function:
30015675
FIGURE 14. RRAMP to -VCC for VOUT < 7.5V
If a large variation is expected in VCC, say for VIN < 11V, a
Zener regulator may be added to supply a constant voltage
for RRAMP
.
MODULATOR TRANSFER FUNCTION
The following equations can be used to calculate the control-
to-output transfer function:
Where AOL = 10,000 (80dB) and ωBW = 2π x fBW. GEA(S) is the
ideal error amplifier gain, which is modified at DC and high
frequency by the open loop gain of the amplifier and the feed-
back divider ratio.
21
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