MAX15002_12 Datasheet, PDF (19/29 Pages) Maxim Integrated Products – Dual-Output Buck Controller with Tracking/Sequencing

MAX15002
Dual-Output Buck Controller with
Tracking/Sequencing
Below are equations that define the power modulator:
G_MOD(DC)
= ^VIN
VRAMP
= ^VIN
2V
f_LC
=
2π
L
×
1
COUT
×⎛⎝⎜
ROUT
ROUT
+ESR ⎞
+DCR ⎠⎟
≈
2π
L
1
× COUT
f_ESR
=
1
2π ×ESR× COUT
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 MAX15002 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 MAX15002
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 exam-
ple, 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/dec slope
and approaches 180° of phase shift, indicative of a
potentially unstable system. Together with the inherent
180° of phase delay in the negative feedback system,
this can lead to near 360° or positive feedback—an
unstable system.
The desired (compensated) roll-off follows a -20dB/dec
slope (and commensurate 90° of phase shift), and, in
this example, occurs at approximately 6x the uncom-
pensated crossover frequency, f_CO. In this example, a
Type II compensator provides for stable closed-loop
operation, leveraging the +20dB/dec slope of the
capacitor’s ESR zero (see Figure 5b).
POWER MODULATOR GAIN AND PHASE RESPONSE
WITH LOSSY BULK OUTPUT CAPACITORS
(ALUMINUM ELECTROLYTICS)
40
90 ^{MAX15002 fig05a}
f_LC
20
|G_MOD|
ASYMPTOTE
0
45
|G_MOD|
0
-20
< G_MOD
f_ESR
-45
-40
-90
-60
-135
-80
-180
10 100 1k 10k 100k 1M 10M
FREQUENCY (Hz)
Figure 5a. Power Modulator Gain and Phase Response (Large,
Bulk COUT)
Maxim Integrated
POWER MODULATOR AND TYPE II COMPENSATOR
GAIN AND PHASE RESPONSE WITH LOSSY BULK
OUTPUT CAPACITORS (ALUMINUM ELECTROLYTICS)
80
180 ^{MAX15002 fig05b}
60
< G_E/A
135
40
20
|G_E/A|
f_LC
90
f_CO
45
0
0
-20
< G_MOD
f_ESR
-45
-40
-90
-60
|G_MOD|
-135
-80
-180
10 100 1k 10k 100k 1M 10M
FREQUENCY (Hz)
Figure 5b. Power Modulator (Large, Bulk COUT) and Type II
Compensator Responses
19