HV9911_06 Datasheet, PDF (10/14 Pages) Supertex, Inc – Switch-Mode LED Driver IC with High Current Accuracy

HV9911
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0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Minimum drop in Linear Regulator (V)
-40°C
25°C
85°C
125°C
2.2 2.4
Fig. 3. Graph of input current vs minimum voltage drop
across linear regulator for different junction temperatures
Assume a maximum junction temperature of 85°C (this give a
reasonable temperature rise of 45°C at an ambient tempera-
ture of 40°C). At 2.86mA input current, the minimum voltage
drop from Fig. 3 can be approximately estimated to be V_DROP
= 0.75V. However, before the IC starts switching the current
drawn will be the total current minus the gate drive current.
In this case, that current is I_{Q_TOTAL} = 1.2mA. At this current
level, the voltage drop is approximately V_DROP1 = 0.4V. Thus,
the start/stop V_IN voltages can be computed to be:
VIN = UVLO + V
START
MAX
DROP1
= 7.2V + 0.4V
= 7.60V
VIN_STOP = UVLO_MAX - 0.5V + V_DROP
= 7.2V - 0.5V + 0.75V
= 7.45V
Note: In some cases, if the gate drive draws too much cur-
rent, VIN_START might be less than VIN_STOP. In such cases, the
control IC will oscillate between ON and OFF if the input volt-
age is between the start and stop voltages. In these circum-
stances, it is recommended that the input voltage be kept
higher than VIN_STOP.
Reference
Note: In order to avoid abnormal startup conditions, the
bypass capacitor at the REF pin should not exceed 0.22µF.
Oscillator
The oscillator can be set in two ways. Connecting the
oscillator resistor between the RT and GATE pins will
program the off-time. Connecting the resistor between RT
and GND will program the time period.
In both cases, resistor R_T sets the current, which charges
an internal oscillator capacitor. The capacitor voltage ramps
up linearly and when the voltage increases beyond the
internal set voltage, a comparator triggers the SET input of
the internal SR flip-flop. This starts the next switching cycle.
The time period of the oscillator can be computed as:
T_S ≈ R_T x 11pF
Slope Compensation
For converters operating in the constant frequency mode,
slope compensation becomes necessary to ensure stability
of the peak current mode controller, if the operating duty
cycle is greater than 0.5. Choosing a slope compensation
which is one half of the down slope of the inductor current
ensures that the converter will be stable for all duty cycles.
HV9911 includes a 2% accurate, 1.25V reference, which can
be used as the reference for the output current as well as
to set the switch current limit. This reference is also used
internally to set the over voltage protection threshold. The
reference is buffered so that it can deliver a maximum of
500µA external current to drive the external circuitry. The
reference should be bypassed with at least a 10nF low ESR
capacitor.
Slope compensation can be programmed by two resistors
R_SLOPE and R_SC. Assuming a down slope of DS (A/µs) for the
inductor current, the slope compensation resistors can be
computed as:
R_slope
=
10× R_SC
DS× 10⁶ ×T_s ×
R_cs
A typical value for R_SC is 499Ω.
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