SM72445 Datasheet, PDF (8/19 Pages) Texas Instruments – Programmable Maximum Power Point Tracking Controller With Adjustable PWM Frequency

STARTUP
SM72445 has a soft start feature that will ramp its output volt-
age for a time of 250ms if the bridge is configured to run at
215kHz and up to 500ms if the bridge is configured for
110kHz.
If no output current is detected during soft-start time, the de-
vice will then enter Panel Mode for 60 seconds. A counter will
start once the minimum output current threshold is met (set
by ADC input channel 4, pin A4). During these 60 seconds,
any variation on the output power will not cause the chip to
enter MPPT mode. Once 60 seconds have elapsed, the unit
will enter operational PM mode and the pre-determined power
level variation at the output will engage the chip in MPPT
mode.
If the output current is greater than the current threshold set
at A/D Channel 6 (A6) during soft-start, the chip will then en-
gage in MPPT mode and will not be subject to the start-up
delay.
30176105
FIGURE 5. Startup Sequence
MAXIMUM OUTPUT VOLTAGE
The maximum output voltage on the SM72445 is set by the
resistor divider ratio on pin A0. (Please refer to Figure 2 Typ-
ical Application Circuit).
The value of the voltage on pin A0 is sampled and stored by
the ADC of the SM72445 at start-up and after reset events.
While voltage on pin AVOUT is above the voltage set at pin
A0, the duty cycle of the converter will be reduced every
MPPT cycle (1ms-2ms depending on the set switching fre-
quency). This is true when the converter is running in MPPT
state or during Soft-Start. When the unit is in Panel Mode (PM)
or in Startup Panel Mode (PM_Startup) there is no control on
the output voltage and the device will not react to the presence
of a voltage on AVOUT higher than the A0 setpoint. See Fig-
ure 4 for more details on the different states of operation.
This means that the voltage limit setting cannot be used to
ensure overall maximum output voltage for the system: there
will be times during Panel Mode operation and Stand-by
mode operation when the output will increase above the pro-
grammed output voltage if the input (solar panel) gets over
that voltage limit. Therefore, the maximum output voltage
threshold set by programming A0 is only valid if its value is
higher than the maximum input voltage (solar panel in open
circuit at coldest operating point). If over-voltage protection
needs to be implemented, it must be done using external
components. For exampe, a voltage comparator with its out-
put connected to the reset pin of the SM72445 is one possible
implementation.
The maximum output voltage is always enforced during
MPPT operation of the IC.
The following equation sets the maximum output voltage:
Where RT1 and RB1 are the resistor divider on the ADC pin
A0 and RFB1 and RFB2 are the output voltage sense resis-
tors. A typical value for RFB2 is about 2 kΩ
CURRENT LIMIT SETTING
Maximum output current can be set by changing the resistor
divider on A4 (pin 18). Refer to Figure 2. Overcurrent at the
output is detected when the voltage on AIOUT (pin 21) equals
the voltage on A4 (pin 18). The voltage on A4 can be set by
a resistor divider connected to 5V whereas the voltage on
AIOUT can be set by a current sense amplifier.
AVIN PIN
AVIN is an A/D input to sense the input voltage of the
SM72445. A resistor divider can be used to scale max voltage
to about 4V, which is 80% of the full scale of the A/D input.
CONFIGURABLE SETTINGS
A/D pins A0, A2, A4, and A6 are used to configure the be-
havior of the SM72445 by adjusting the voltage applied to
them through resistor dividers as shown in Figure 2, where
RT1 to RT4 should be in the range of 20 kΩ.
The voltages of the configuration pins are read and the oper-
ating mode is then set at start-up and after each reset of the
device.
Three different frequencies for the PWM operation of the H-
bridge as well as two different implementations of the Panel
Mode switch can be set on the ADC input channel 2 (pin A2).
The table below lists the different conditions that a user can
select on pin A2. Each frequency has a different associated
dead time for the operation of the synchronous switches.
When dedicated PM switch modes are used, the unit will stop
switching the converter upon entering PM mode and the
PM_OUT pin will switch at a high frequency to provide acti-
vation of a dedicated Panel Mode switch. When the H-bridge
modes are used, the unit will keep the H-bridge switching at
half the operating frequency (to reduce switching losses) and
with a total input to output ratio of 1. The dead times are un-
changed during this phase.
TABLE 1 Programmable Settings on Pin A2
A2
PWM Frequency Panel Mode Operation
setting
4.69 V
HIGH
Uses dedicated PM
switch
4.06 V
HIGH
Uses dedicated PM
switch
3.44 V
LOW
Uses H-bridge for PM
operation
2.81 V
MED.
Uses H-bridge for PM
operation
2.19 V
HIGH
Uses H-bridge for PM
operation
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