Rainbow Electronics MAX17008 User Manual
Page 20
MAX17007A/MAX17008
Switching Frequency
The MAX17007A/MAX17008 feature independent resis-
tor-programmable switching frequencies for each
SMPS, providing flexibility for applications where one
SMPS operates at a lower switching frequency when
connected to a high-voltage input rail while the other
SMPS operates at a higher switching frequency when
connected to a lower voltage rail as a second-stage
regulator. Connect a resistor (R
TON
) between TON and
V
IN
to set the switching period T
SW
= 1/f
SW
:
T
SW1
= C
TON
(R
TON1
+ 6.5k
Ω)
T
SW2
= C
TON
(R
TON2
+ 6.5k
Ω)
where C
TON
= 16.26pF. A 97.5k
Ω to 302.5kΩ corre-
sponds to switching periods of 1.67μs (600kHz) to 5μs
(200kHz) for SMPS1 and SMPS2. High-frequency
(600kHz) operation optimizes the application for the
smallest component size, trading off efficiency due to
higher switching losses. This may be acceptable in
ultra-portable devices where the load currents are
lower and the controller is powered from a lower volt-
age supply. Low-frequency (200kHz) operation offers
the best overall efficiency at the expense of component
size and board space.
For continuous conduction operation, the actual switching
frequency can be estimated by:
where V
DIS
is the sum of the parasitic voltage drops in
the inductor discharge path, including synchronous
rectifier, inductor, and printed-circuit board (PCB) resis-
tances; V
CHG
is the sum of the resistances in the
charging path, including the high-side switch, inductor,
and PCB resistances; and t
ON
is the on-time calculated
by the on-time block.
When operating in separate mode, it is recommended
that both SMPS switching frequencies be set apart by
10% to 30% to prevent the two sides from beating
against each other.
Combined-Mode On-Time One-Shot
In combined mode (FB2 = V
CC
), TON1 sets the on-
time, and hence the switching frequency, for both SMPS.
The on-time is programmed using the TON1 equation,
which sets the switching frequency per phase. The effec-
tive switching frequency as seen on the input and output
capacitors is twice the per-phase frequency.
Combined-Mode Current Balance
In combined mode, the one-shot for SMPS2 varies the
on-time in response to the input voltage and the differ-
ence between the SMPS1 and SMPS2 inductor cur-
rents. The SMPS1 one-shot in combined mode behaves
the same way as it does in separate mode. As such,
SMPS2 regulates the current balance, while SMPS1
regulates the voltage.
Two identical transconductance amplifiers integrate the
difference between SMPS1 and SMPS2 current-sense
signals. The summed output is internally connected to
CCI, allowing adjustment of the integration time con-
stant with a compensation network (usually a capacitor)
connected between CCI and the output.
The resulting compensation current and voltage are
determined by the following equations:
I
CCI
= G
m
[(V
CSH1
- V
CSL1
) - (V
CSH2
- V
CSL2
)]
V
CCI
= V
OUT
+ I
CCI
Z
CCI
where Z
CCI
is the impedance at the CCI output. The
SMPS2 on-time one-shot uses this integrated signal
(V
CCI
) to set the SMPS2 high-side MOSFETs on-time.
When SMPS1 and SMPS2 current-sense signals (V
CSH1
- V
CSL1
and V
CSH2
- V
CSL2
) become unbalanced, the
transconductance amplifiers adjust the SMPS2 on-time,
which increases or decreases the SMPS2 inductor cur-
rent until the current-sense signals are properly bal-
anced. In combined mode, the SMPS2 on-time is given
by:
SMPS2 On-Time t
ON2
= T
SW2
(V
CCI
/V
IN
)
SMPS Enable Controls (EN1, EN2)
EN1 and EN2 provide independent control of output
soft-start and soft-shutdown. This allows flexible control
of startup and shutdown sequencing. The outputs can
be started simultaneously, sequentially, or indepen-
dently. To provide sequential startup, connect EN of
one regulator to PGOOD of the other. For example, with
EN1 connected to PGOOD2, OUT1 soft-starts after
OUT2 is in regulation.
When configured in separate mode, the two outputs are
independent. A fault at one output does not trigger
shutdown of the other.
When configured in combined mode (FB2 = V
CC
), EN1
is the master control input that enables/disables the
combined output, while EN2 has no function and must
be connected to GND. The startup slew rate follows
that of SMPS1.
Toggle EN low to clear the overvoltage, undervoltage,
and thermal-fault latches.
f
V
V
t
V
V
SW
OUT
DIS
ON
IN
CHG
=
+
+
(
)
Dual and Combinable QPWM Graphics
Core Controllers for Notebook Computers
20
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