HV9906LG Ver la hoja de datos (PDF) - Supertex Inc
Número de pieza
componentes Descripción
Fabricante
HV9906LG Datasheet PDF : 10 Pages
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Design Information
Programming On Time
The instantaneous voltage applied to the VON pin determines the
gate drive output on time for the VCO cycle. The on time is
inversely proportional to the voltage applied to the VON pin and may
be calculated using the following equation:
( ) P VON
≈
0.085
+
0.65
VON
×
10 −6
Where the effective control range of VON is limited between 0.2V
and 6V. For VON = 0V P(VON) defaults to a nominal maximum of
17.8µS.
Depending on the converter topology and worst case operating
conditions the minimum on time and thus the duty cycle may be
programmed.
Programming the Sense Inputs
The PS and NS sense pins are regulated at +1V and each needs
to be programmed to source the same current at the converter
output regulation set point.
In order to calculate the values of RNS and RPS, the maximum
sense current, which will avoid integrator saturation, must be
determined. Since by design the circuit will inherently soft start
from its lowest frequency, the designer only needs to establish the
lowest operating frequency (fMIN) for the design, which corresponds
to minimum converter output power under closed loop control.
Once this frequency is established the maximum PS pin sense
current IPS(MAX), which occurs during start up when VPSENSE =
VNSENSE, can be calculated using the following equation.
IPS(MAX) = CMIN × VSAT × fMIN
Where CMIN is the minimum value of the integrator capacitors, VSAT
is the minimum saturation level of the integrators and fMIN is the
minimum operating frequency of the converter. Inserting these
values the above equation can be simplified.
IPS(MAX) = (0.95) × (20 × 10−12 ) × 6 × fMIN
IPS(MAX) = 1.14 × 10−10 × fMIN
For the general case, where at regulation neither sensed node
might be at ground potential, the following equation may be used
to calculate the required RPS resistor value where VPSENSE(MIN) is the
most negative value that the node will see during starting or normal
operation.
RPS
=
1V − VPSENSE(MIN)
IPS(MAX)
Once the value of RPS has been determined the IPS and INS sense
currents at the regulation point can be calculated and the value of
RNS can be determined as follows.
IPS
= INS
=
1V
− VPSENSE
RPS
RNS
=
1V − VNSENSE
INS
HV9906
Where INS = IPS = average current in the NS and PS pins at stable
output regulation, VNSENSE is the most negative sensed node
voltage with respect to +1V and VPSENSE is the least negative
sensed node voltage with respect +1V. VNSENSE and VPSENSE must
be less than +1V and VNSENSE is always more negative than
VPSENSE.
Example 1.
For a converter operating at a minimum frequency of 50KHz and
sensing a –1V feedback node voltage with respect to ground, the
resistors connected in series with the PS and NS pins will be
determined as follows.
IPS(MAX) = 1.14 × 10−10 × 5 × 104 = 5.7µA
To provide a margin of safety let IPS(MAX) = 5µA. Since in this
configuration the resistor in series with the PS pin is connected to
ground, the sense node voltage VPSENSE(MIN) = 0V.
RPS
= 1V − VPSENSE(MIN)
IPS(MAX)
=
1− (0)
5 × 10−6
= 200kΩ
IPS
= INS
= 1V − VPSENSE
RPS
=
1− 0
200kΩ
= 5µA
RNS
= 1V − VNSENSE
INS
=
1− (−1)
5 × 10−6
= 400kΩ
Example 2.
For a converter operating at a minimum frequency of 100KHz and
sensing a +0.5V feedback node voltage with respect to ground, the
resistors connected in series with the PS and NS pins will be
determined as follows.
IPS(MAX) = 1.14 × 10−10 × 1× 105 = 11.4µA
To provide a margin of safety let IPS(MAX) = 10µA. In this
configuration the most negative value of VPSENSE(MIN) occurs during
startup at which time it is 0V.
RPS
= 1V − VPSENSE(MIN)
IPS(MAX)
=
1− 0
10 × 10−6
= 100kΩ
IPS
= INS
= 1V − VPSENSE
RPS
= 1− (+0.5)
100kΩ
= 5µA
RNS
= 1V − VNSENSE
INS
=
1− 0
5 × 10−6
= 200kΩ
Protection
The HV9906 used as a current source is inherently protected in the
case of an output short. Over voltage protection is easily
accomplished, in the flyback-buck application for example, with no
more than two diodes. Simple protection for voltage mode
applications, and other topologies is easy to accomplish. Call for
more information.
6
07/23/02
Supertex, Inc. 1235 Bordeaux Drive, Sunnyvale, CA 94089 TEL: (408) 744-0100 FAX: (408) 222-4895 www.supertex.com
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