MAX15003 Datasheet, PDF (21/32 Pages) Maxim Integrated Products – Triple-Output Buck Controller with Tracking/Sequencing

Triple-Output Buck Controller with
Tracking/Sequencing
Below are equations that define the power modulator:
G_MOD(DC)
= ^VIN
VRAMP
f_LC
=
2π×
1
L × COUT
f_ZERO,ESR
=
1
2π ×ESR× COUT
f_ZERO,ESL
= ^ESR
2π ×ESL
The switching frequency is programmable between
200kHz and 2.2MHz using an external resistor at RT.
Typically, the crossover frequency (f_CO), which is the
frequency when the system’s closed-loop gain is equal
to unity crosses the 0dB axis—should be set at or
below one-tenth the switching frequency (f_SW/10) for
stable, closed-loop response.
The MAX15003 provides an internal transconductance
amplifier with its inverting input and its output available
to the user for external frequency compensation. The
flexibility of external compensation for each converter
offers wide selection of output filtering components,
especially the output capacitor. For cost-sensitive appli-
cations, use aluminum electrolytic capacitors and for
space-sensitive applications, use low-ESR tantalum or
multilayer ceramic chip (MLCC) capacitors at the out-
put. The higher switching frequencies of the MAX15003
allow the use of MLCC as the primary filter capacitor(s).
First, select the passive and active power components
that meet the application’s output ripple, component
size, and component cost requirements. Second,
choose the small-signal compensation components to
achieve the desired closed-loop frequency response
and phase margin as outlined below.
Closed-Loop Response and Compensation
of Voltage-Mode Regulators
The power modulator’s LC lowpass filter exhibits a vari-
ety of responses, depending on the value of the L and
C (and their parasitics).
One such response is shown in Figure 5a. In this example
the power modulator’s uncompensated crossover is
approximately 1/6^th the desired crossover frequency,
f_CO. Note also, the uncompensated roll-off through the
0dB plane follows the double-pole, -40dB/decade
slope and approaches 180° of phase shift, indicative of
a potentially unstable system. Together with the inher-
ent 180° of phase delay in the negative feedback
system, this may lead to near 360° or positive feed-
back—an unstable system.
The desired (compensated) roll-off follows a
-20dB/decade slope (and commensurate 90° of phase
shift), and, in this example, occurs at approximately 6x
the uncompensated crossover frequency, f_CO. In this
example, a Type II compensator provides for stable
closed-loop operation, leveraging the +20dB/decade
slope of the capacitor’s ESR zero (see Figure 5b).
POWER MODULATOR (LARGE, BULK OUTPUT
CAPACITOR(S)) GAIN (REAL, ASYMPTOTIC/
PHASE RESPONSE vs. FREQUENCY
40
90 ^{MAX15003 fig05a}
f_LC
20
45
|G_MOD|
|G_MOD|
0
0
-20
< G_MOD
f_ZERO,ESR
-45
-40
-90
f_ZERO,ESL
-60
-135
-80
-180
10 100 1k 10k 100k 1M 10M
FREQUENCY (Hz)
Figure 5a. Power Modulator Gain and Phase Response (Large,
Bulk COUT)
POWER MODULATOR (LARGE, BULK OUTPUT
CAPACITOR(S)) AND TYPE II COMPENSATION GAIN
(ASYMPTOTIC)/PHASE RESPONSE vs. FREQUENCY
80
<G_EA
180 ^{MAX15003 fig05b}
60
135
40
90
20
|G_EA|
f_ZERO,ESR
45
0
-20
-40
-60
-80
10
f_LC
< G_MOD
f_ZERO,ESL
f_CO
100 1k 10k 100k 1M
FREQUENCY (Hz)
0
-45
-90
-135
-180
10M
Figure 5b. Power Modulator (Large, Bulk COUT) and Type II
Compensator Responses
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